A METHOD OF LIMITING THE SURVIVAL OF GENETICALLY ENGINEERED MICROORGANISMS IN THEIR ENVIRONMENT

Abstract: The survival of cells is limited by introducing into the cells a regulatably expressible gene whose expression results in the formation of a cytoplasmatically active hydrolytic enzyme. Populations of cells containing, in addition to such a regulatably expressible gene, a DNA coding for an immunologically active, pesticidally active or environmental pollutant-degrading gene product, may be useful in immunologically active compositions, pesticidally active compositions and environmental pollutant-degrading compositions, respectively.

Description: A METHOD OF LIMITING THE SURVIVAL OF GENETICALLY ENGINEERED MICROORGANISMS IN THEIR ENVIRONMENT FIELD OF INVENTION The present invention provides a novel system of biological containment of genetically engineered microorganisms. The invention relates to a cell or a population hereof containing a regulatably expressible gene whose expression results in the formation of a cytoplasmatically active hydrolytic enzyme at a rate which leads to cell function limitation; to a recombinant replicon containing such a regulatably express¬ ible gene; to methods of limiting the survival of a popula¬ tion of cells containing the expressible gene, to a method of containing an extrachromosomal recombinant replicon to a first kind of cells and of stochastically limiting the survi- val of a cell population- There is also provided an immunologically active composition, a pesticidal composition and an environmental pollutant- degrading composition, all of which contains a cell popula¬ tion as defined above wherein the cells further contains a DNA sequence coding for a gene product which is immunologi¬ cally active, pesticidally active or environmental pollutant- degrading, respectively. TECHNICAL BACKGROUND The increasing application of recombinant DNA technology to engineer novel microorganism which are industrially useful have caused concerns in both the scientific community and the general public over potential risks. These concerns are primarily related to the potential harm to humans and to undesirable and/or uncontrollable ecological consequences upon deliberate or unintentional release of such genetically engineered microorganisms (GEMs) into the environment. These concerns have led to the establishment of official guidelines for the safe handling of GEMs in laboratories and production facilities where such organisms are applied. Up till now, such guidelines have primarily been directed to measures of physically containing GEMs in laboratories and production facilities with the aim of reducing the likelihood that workers in such facilities were contaminated, or that the GEMs were to escape from their primary physical environment, such as a fermentation vessel. It is presently being recognized that the level of safety in the handling of GEMs can be increased by combining physical containment measures with biological containment measures to reduce the possibility of the survival of the genetically engineered organisms if they were to escape from their pri¬ mary environment. Lately, however, concerns have become increasingly focused on potential risks related to deliberate release of GEMs to the outer environment and to the use of GEMs as live vaccines. In this connection there is a strongly felt need to have bio¬ logical containment systems which subsequent to the environ- mental release of the GEMs or their administration as vac¬ cines to a human or an animal body, effectively kill the released organisms in a controlled way or which limit the function of the released GEMs to an extent where such GEMs are placed at a significant competitive disadvantage whereby they will eventually be ousted by the natural microflora of the environment to which they are released. The first systems of biological containment were based on the use of "safe" cloning vectors and debilitated host bacteria. As examples, it has been suggested to select vectors which lack transfer functions or which naturally have a very narrow host range. Examples of debilitated host bacteria are E. coli mutants having an obligate requirement for exogenous nutri¬ ents not present or present in low concentrations outside the primary environment of the GEMs. Other suggested biological containment systems have been based on mechanisms whereby the vector is restricted to the GEMs e.g. by using a plasmid vector with a non-sense mutation in a gene, the expression of which is indispensible for plasmid replication or a suppressor mutation in the chromo¬ some, said mutation blocking translational read-through of the message of the gene. A further approach is to maintain the rDNA stably in the host by integrating it into the chro¬ mosomes of the GEMs. Recently, an alternative biological containment strategy has been developed in which the recombinant vector is endowed with a gene encoding a cell killing function which gene is under the control of a promoter only being expressed under certain environmental conditions, such as conditions prevail- ing in an environment outside the primary environment of the GEMs, or when the vector is unintentionally transferred to a secondary host, or the expression of which is stochastically induced. By using incorporation in a GEM of such a cell killing function and selecting appropriate regulatory sequences, vectors can be constructed which are contained in the primary host cell and/or in a primary physical environ¬ ment. A cell killing function as hereindefined may also be referred to as an active biological containment factor. If a stochastically induced mechanism of expression regula- tion is selected for such a biological containment system, a population of GEMs containing the system will, upon release to the outer environment, or if used as a live vaccine be subjected to a random cell killing which will lead to an increase of the doubling time of the host cell population or eventually to the disappearance of the organisms. The above-mentioned genes encoding cell killing functions are also frequently referred to as "suicide" genes, and biologi¬ cal containment systems based upon the use of such genes, the expression of which are regulated as defined above, are commonly described as conditional lethal systems or "suicide" systems. Up till now, several cell killing functions have been found in bacterial chromosomes and in procaryotic plas¬ mids. Examples of chromosomal genes having cell killing functions are the cref (Poulsen et al., 1990) and relF (Bech et al., 1985, Embo, vol.4 no.4, 1059-1066) genes from E.coli K-12. Examples of plasmid encoded suicide genes are hok and fl A (Gerdes et al. , 1986) genes isolated from plasmids Rl and F, respectively, the snrB gene also isolated from plasmid F (Akimoto et al. , 1986) and the pnd gene isolated from plasmids R16 and R483 (Sakikawa et al. , 1989 and Ono et al. , 1987) . Common features of these genes are that they are transcribed constitutively, regulated at a post-transcript¬ ional level, and that they all encode small toxic proteins of about 50 amino acids. The application of the hok gene in a biological containment system has been disclosed in WO 87/05932. Ideally, the features of an effective biological containment system should include as a minimum requirement that the cell killing function when expressed is effective, that the con- tainment system is functional in a broad range of species of GEMs, that the risk of elimination of the cell killing function e.g. by mutations in the suicide gene or the sequences regulating the expression of the gene, is minimal and that the risk of uptake by other organisms of rDNA released when cells are killed is reduced. None of the above-mentioned known containment systems fulfill all of these ideal requirements. However, the present in¬ vention provides a novel active biological containment system which is not based on a primary cell killing function but which makes use of genes, the expression of which in a cell where the gene is inserted, results in the formation of mature forms of exoenzymes which are hydrolytically active in the cytoplasm of the cell and which can not be transported over the cell membrane. When such enzymes are expressed, the normal function of the cell becomes limited to an extent whereby the competitiveness, and hence the survival, of a population of such cells is reduced significantly. Provided the hydrolytically active enzyme is a RNA-degrading and/or DNA-degrading enzyme, a biological containment system based upon such an enzyme may have a further advantage over the known biological containment systems in that the rDNA molecules in a genetically engineered host cell is destroyed simultaneously with the genetically altered host micro¬ organisms. SUMMARY OF THE INVENTION Many cells produce hydrolytically active enzymes which are inherently translocated extracellularly, i.e. the enzymes are excreted over the cell membrane. When expressed intracellu- larly in a cell naturally producing such enzymes, the enzymes are in the form of enzvmatically inactive, immature exoenzyme molecules (proenzymes) comprising a signal peptide by means of which the proenzyme is transportable over the cell mem¬ brane. On passage of the cell membrane, the signal peptide is cleaved off the molecules which are thereby turned into the mature, enzvmatically active form. The present invention is based on the discovery that an exoenzyme which normally, while present in the cytoplasm of cells producing them, are in the form of the immature, en- zymatically inactive proenzyme may be expressed in a cell in the form of the mature enzyme being enzvmatically active in the cytoplasm of the cell and that the presence intracellu¬ larly of such active forms of exoenzymes may cause damages to the cells leading to a limitation of the normal cell func- tion. On the basis of these findings, the present invention pro¬ vides a novel approach to biological containment of GEMs aiming at limiting the survival hereof in their environment. Thus, in a first aspect, the present invention provides a cell containing a gene whose expression results in the for¬ mation of an enzyme which is present and hydrolytically active in the cytoplasm of said cell, the cell further con¬ taining a regulatory nucleotide sequence which regulates the expression of the gene, the expression of said gene leading to formation of the enzyme in the cell at a rate which re¬ sults in the hydrolysis of hydrolysable cytoplasmic substances necessary for non-limited function of the cell, to an extent whereby the function of the cell is being limited. In a further aspect, there is provided a recombinant replicon containing a regulatably expressible gene which, when ex¬ pressed in a cell encodes an enzyme which is present and hydrolytically active in the cytoplasm of the cell, the expression of said gene leading to formation of the enzyme in the cell at a rate which results in the hydrolysis of hydro- lysable cytoplasmic substances necessary for non-limited function of the cell, to an extent whereby the function of the cell is being limited, the expression of said genes being regulated by a regulatory nucleotide sequence which is contained in the recombinant replicon or in an other recombi- nant replicon present in a cell containing the replicon. In a still further aspect, the present invention relates to a population of cells consisting of a multiplicity of cells as defined above. Additionally, the present invention relates to a method of limiting the survival of a cell population in a first or a second environment which method comprises transforming the cells of said population with a recombinant replicon being replicated in the cells of the population and containing a gene whose expression results in the formation of an enzyme which is present and hydrolytically active in the cytoplasm of said cell, the cells further containing a regulatory nucleotide sequence being regulatable by an environmental factor and which regulates the expression of said gene, the expression of said gene leading to formation of the enzyme in the cells at a rate which results in the hydrolysis of hydro- lysable cytoplasmic substances necessary for non-limited function of the cell, to an extent whereby the function of the cells is being limited leading to a limitation of the survival of the cell population. In another aspect, the invention relates to a method of containing an extrachromosomal recombinant replicon to a first kind of cell, where said replicon is naturally trans¬ ferable to a second kind of cell, which method comprises providing on the recombinant extrachromosomal replicon a gene whose expression results in the formation of an enzyme which is present and hydrolytically active in the cytoplasm of a cell, the formation of said enzyme being at a rate which results in the hydrolysis of hydrolysable cytoplasmic sub- stances necessary for non-limited function of the cell, to an extent whereby the function of the cell is being limited, said first kind of cells having or being modified to have a chromosomal replicon comprising a regulatory nucleotide sequence which inhibits the expression of said gene and thereby protects said first kind of cells, said regulatory gene being lacking in said second kind of cell, whereby, if a cell of the second kind receives said extrachromosomal recom¬ binant replicon said gene is expressed and has a function- limiting effect thereon. In another further aspect, the present invention pertains to a method of stochastically limiting the survival of a cell population which comprises transforming the cells thereof with a recombinant replicon containing a regulatably expres¬ sible gene which, when expressed in a cell encodes an enzyme which is present and hydrolytically active in the cytoplasm of the cell, the expression of said gene leading to formation of the enzyme in the cell at a rate which results in the hydrolysis of hydrolysable cytoplasmic substances necessary for non-limited function of the cells, to an extent whereby the function of the cells is being limited, the expression of said genes or genes being stochastically induced as a result of recombinational excision of an excisable negatively func¬ tioning regulatory nucleotide sequence which, while present in the cells, inhibits expression of the gene coding for the enzyme, said negatively functioning regulatory nucleotide sequence being contained in the recombinant replicon or in an other recombinant replicon present in cells of the population containing the replicon. In a still further aspect, there is provided an immunologi¬ cally active composition which contains a viable function- limited cell population as defined above wherein the cells contain a further DNA sequence not naturally related to the gene coding for the hydrolytically active enzyme or to the regulatory nucleotide sequence, which further sequence is a sequence coding for an immunologically active gene product, the cells being function-limited to an extent which, when the composition is administered to a human or an animal, allows the cells to express the immunologically active gene product for a period of time and in an amount sufficient to obtain an effective immune response in said human or animal, but which does not allow the cells to persist in the human or the animal. The present invention also provides a pesticidally active composition which contains a viable cell population as defined herein wherein the cells contain a further DNA sequence not naturally related to the gene coding for the hydrolytically active enzyme or to the regulatory nucleotide sequence, which further sequence is a sequence coding for a pesticidally active gene product, the cells being function- limited to an extent which, when the composition is admini- stered to an environment containing a pest, allows the cells to express the pesticidally active gene product for a period of time and in an amount sufficient to obtain an effective pesticidal effect in said environment but which does not allow the cells to persist in the environment. Finally, there is provided an environmental pollutant-degra- ding composition which contains a viable cell population as defined herein, wherein the cells contain a further DNA sequence not naturally related to the gene coding for the hydrolytically active enzyme or to the regulatory nucleotide sequence, which further sequence is a sequence coding for an environmental pollutant-degrading gene product, the cells being function-limited to an extent which, when the composi¬ tion is administered to an environment containing a pollutant to be degraded, allows the cells to express said pollutant degrading gene product for a period of time and in an amount sufficient to obtain an effective pollutant-degrading effect in said environment but which does not allow the cells to persist in the environment. DETAILED DISCLOSURE OF THE INVENTION The concept of an active biological containment system based on the use of an intracellularly hydrolytically active exo¬ enzyme which has a cell function-limiting effect. A number of macromolecules such as proteins, lipids, phos- pholipids, DNA, RNA and polysaccharides that cannot pass cell membranes are nevertheless utilizable as substrates for cell growth. These substrates are enzymatically hydrolyzed (degra¬ ded) in the external medium of the cells by protein enzymes excreted by the cells. Such hydrolytic enzymes that mediate extracellular degradation are termed "exoenzymes". In addi¬ tion to true exoenzymes which are capable of passing the outer cell membrane and hence excreted into the external medium, certain cells produce incompletely excreted hydroly¬ tic enzymes of which some enter the cell membrane and remain there rather than passing through. Others pass through the cell membrane but not the outer membrane (periplasmic en¬ zymes) and still others remain in the outer membrane. In the present context, the term "exoenzyme" is used to designate a hydrolytic enzyme which have been excreted natu- rally by the cell natively producing the enzyme or a cell in which the genes coding for such an excretable enzyme has been inserted, by passing through the cell membrane (the cyto¬ plasmic membrane) and accordingly, this term include native true exoenzymes, cell membrane-bound enzymes, periplasmically located enzymes and outer membrane-bound enzymes. Generally, the mechanisms by which excretable exoenzymes as defined herein are excreted share common features. Thus, when first synthesized in the cytoplasm, these proteins are in the form of enzymatically inactive precursor molecules (also referred to as proenzymes) which are larger than they are after they have entered their proper extracellular location, since a portion of the amino-terminal end of the protein which is termed as the signal peptide, is removed after the protein is properly located. The function of the signal peptide is to aid in the translocation of the protein exo¬ enzyme to its place of location. When the molecule is pro¬ perly located, the signal peptide is removed whereby the protein is turned into the mature or "processed", enzymati¬ cally active hydrolytic enzyme. However, certain hydrolytically active exoenzymes including the Serratia species phospholipase as defined herein, are not provided with a signal peptide when expressed in a cell naturally producing the enzyme. In addition to the phlA gene encoding the phospholipase, the Serratia cells in which the phospholipase is naturally expressed contain a phlB gene encoding an intracellularly active protein which by inter¬ acting with the phospholipase protein render the cells refractory to damages from the phospholipase expressed in the cells. As explained above, the present invention presents a novel approach to the concept of active biological containment which is based on the finding that it is possible to obtain in the cytoplasm of a cell, i.e. intracellularly, the presence of enzymatically active mature forms of exoenzymes. This is obtained by the insertion in the cell of a nucleotide sequence which codes for the exoenzyme molecule, but which does not comprise the sequence coding for the corresponding native signal peptide herefor. By deletion from the nucleo- tide sequence of the sequence coding for the signal peptide it is also obtained that the intracellularly active enzyme molecule cannot be translocated outside the cell membrane. In accordance with the explanation above, an alternative way of obtaining an intracellularly active exoenzyme is to pro- vide in a cell according to the invention, the gene coding for a hydrolytically active exoenzyme without the presence of a gene encoding a gene product which may render the cell refractory to the intracellular activity of said enzyme. The cytoplasm of cells contains a mixture of macromolecules, the presence and function of which are necessary for the non- limited function of the cells, including basic life mani¬ festations such as growth and replication. Examples of such macromolecules include DNA, RNA, lipids, phospholipids, proteins and polymeric carbohydrates. When, in accordance with the present invention, a gene whose expression results in the formation of an exoenzyme as defined above is inserted into the cell, naturally occurring intracellular macromolecules may act as substrate for the intracellularly active hydrolytic enzyme. Provided the hydrolytically active enzyme is expressed intra¬ cellularly at a rate which results in the hydrolysis of hydrolysable cytoplasmic macromolecules to an extent whereby the life functions of the cells become limited, the compe¬ titiveness of such cells with cells of the same type, but in which an exoenzyme as defined herein is not expressed in intracellularly active forms, will be decreased. In the present context, the term "non-limited cell function" denotes that the growth of a cell as manifested i.a. by the synthesis of new cell material and the rate of replication of the cell is not decreased by an intracellular hydrolytic macromolecule degradation, not natively occurring in the cells. Accordingly, this term is used herein to describe i.a. the growth rate and the rate of replication, under any given condition, of a cell in which an intracellularly active exoenzyme as defined above is not being expressed. Conse¬ quently, the term "limited cell function" as used herein describes, in a relative manner, the state of a cell in terms of i.a. a reduced growth rate and/or reduced rate of replica- tion which results from the expression therein of an intra¬ cellularly active exoenzyme when the cell is grown under the same conditions as a cell of the same kind having a non- limited cell function as defined above. The recognizable manifestation of such limited cell function may ultimately be cell death, but it may also, relative to a cell having a non-limited cell function, be a reduced cell growth appearing as a reduced rate of replication resulting in a reduced increase of cell numbers within a certain period of time as a result of an increase of the lag phase and/or of the cell doubling time. Other manifestations may be a rela¬ tively increased requirement for one or more nutrient compo¬ nents or a relatively higher susceptibility to detrimental environmental factors such as sub-optimal temperatures or cell damaging caused by toxic substances. The actual type of manifestation or manifestations of such limited cell function and the degree hereof will depend in particular on the specific species of the exoenzyme being expressed in the cell, the rate of expression of the exo¬ enzyme, the capability of the enzyme to be hydrolytically active under the conditions prevailing in the cytoplasm, the amounts of substrate macromolecules, and the significance of such macromolecules for non-limited function of the cell, and the number of copies of the gene encoding the intracellularly active exoenzyme. The intracellularly hydrolytically active exoenzyme having cell function-limiting effect In accordance with the invention, a suitable intracellularly active exoenzyme may be any exoenzyme which, when it is expressed, is capable of hydrolytically degrading macromo- lecules present in the cytoplasm of procaryotic and euca- ryotic cells to an extent whereby the function of the cell is being limited. Interesting hydrolytic enzymes which may be useful in the present invention include nucleases, phospholi- pases, lipases, lysozyme, proteases and carbohydrases. As used herein, the term "nuclease" denotes hydrolytic exoen¬ zymes capable of degrading nucleic acids. The nucleic acids DNA and RNA are polynucleotides formed by the joining of nucleotides by phosphodiester bonds. Some nucleases are capable of degrading both DNA and RNA, whereas others (deoxy- ribonucleases or DNases) degrade only DNA and still others degrade only RNA (ribonucleases or RNases) . Nucleases may either be exonucleases which are diesterphosphate bond-hydro- lyzing enzymes cleaving off the last nucleotide residue in either of the two terminals of an oligonucleotide, or endo- nucleases which cleave phosphodiester bonds located in the interior of polynucleotides. In the art, the term "nick" is normally used to describe the lesion in a DNA strand resul¬ ting from an endonuclease cleavage of a diesterphosphate bond. It must be noted that term "exonuclease" when used in the present specific context denotes the mode of action as defined above and thus, the term is not used here to desig¬ nate a nuclease normally being excreted out of the cell. In the context of the present invention, useful nucleases include nucleases having the capability of hydrolyzing diesterphosphate bonds in DNA. Particularly useful DNA-degra- ding nucleases include endonucleases, the activity of which, when double-stranded DNA is the substrate, results in the cleavage of diesterphosphate bonds (nicks) in only one of the strands of the DNA. Accordingly, in one embodiment of the invention, the hydrolytically active enzyme is an endo¬ nuclease having such capability. Lesions in the DNA occur spontaneously in all cells with a relatively high frequency. However, in the normal cell such lesions are efficiently eliminated by an inherent DNA repair mechanism which involves that the altered portion of the damaged strand is recognized and removed by one set of en- zymes and then replaced in its original form by a DNA poly¬ merase, and finally an enzyme called DNA ligase seals the nick (the broken diesterphosphate bond) that remains in the DNA helix, to complete the restoration of an intact DNA strand. This natural cell DNA repair mechanism will also eliminate DNA lesions (nicks) which may result from the enzymatic activity in a cell of a nuclease as defined herein. Accord¬ ingly, the function of a cell in which the intracellularly active nuclease as defined above, is expressed will only be limited when the nuclease is expressed at a rate which re¬ sults in the presence of nicks in the cell nucleic acids in a number exceeding the number which can be repaired by the DNA repair mechanism, so that the number of DNA lesions (nicks) reaches a level whereby the function of the cell becomes recognizably limited. The rate of expression of the intracellularly active exo¬ enzyme as defined herein including a nuclease is also deter¬ mined by the specific activity of the enzyme under the condi¬ tions prevailing in the cytoplasm of the cell. It will be understood that these conditions may differ from those pre- vailing in the extracellular environment into which the enzyme when present in a cell where it is normally produced, is translocated. As one example, an enzyme comprising sulphur-containing amino acid residues such as cysteine and methionine will be prone to damages caused by oxidation and hence, it may be assumed that it will be at least partially enzymatically inactive when present in the cytoplasm of a cell where the E h -value is high. However, it was surprisingly found in the course of research leading to the present in- vention that an enzyme comprising sulphur-containing amino acids such as an endonuclease containing several cysteine residues may be enzymatically active intracellularly to an extent whereby the function of a cell in which the enzyme is expressed, is limited. Accordingly, in one embodiment of the invention, the hydrolytically active enzyme is an enzyme such as a nuclease, containing at least one sulphur-containing amino acid residue. Several procaryotic cells including gram-negative and gram- positive bacteria inherently produce exoenzymes which are translocated over the cell membrane, interesting examples being endonucleases produced by Serratia spp. It is known that such endonucleases have a high specific activity ex¬ tracellularly. In WO 86/06743 are disclosed such extracellu¬ lar Serratia spp. nucleases being expressed in E^ coli and their usefulness in the removal of nucleic acids from bi¬ ological materials is described. One example of a Serratia spp. nuclease is an enzyme encoded by the following DNA sequence, excluding the sequence coding for the N-terminal signal peptide: c---XffiACA--C--τc--ΛATc-aτα-A--M cα-αrrCTcc-i-cCTTACCTΛc πTt-^^ -xaα-AACC-τtπcτAτc-?rccGiCAτccrMTACcττ-^ -- 'lLUl U-ACA--ATAG O---ACT/--GAATATCCAAL-ri^ c -CTa-ca < rι ιxA--- -τrcA- π ιt---c- cΛGcτAGCGcaCTΛcrrcc--c∞- Aa ILAGGCTAC τr- ccAgχx2xcτGC- --aa ιg--u(-ATCACG-^ AArro-π- -caaz-Acca-ax-co-a-Ti^ TUIU-----(-tt--L , -T-U-U-AC«^ TOn-Go-ccicc-a. ACCCALΠI: ιu;ι i -ι---a----- cc*ag--c , π It has now been found that such a Serratia spp. nuclease when expressed intracellularly in accordance with the present invention in the form of the mature enzyme, i.e. without the signal peptide, is enzvmatically active to an extent whereby the function of the cell in which the enzyme is expressed, may be limited. Accordingly, in useful embodiments of the invention, the gene whose expression results in the formation of an intracellularly hydrolytically active enzyme is a gene coding for a Serratia spp. nuclease without its native signal peptide. Further suitable nucleases may be nucleases encoded by genes isolated from other gram-negative bacteria including Aeromonas spp. or Yersinia spp. In other embodiments of the invention, the hydrolytically active enzyme is a nuclease encoded by a gene isolated from a gram-positive bacterial species such as a Staphylococcus species, one suitable example hereof being the mature form of the exonuclease of the Foggi strain of Staphylococcus aureus. The gene encoding this enzyme has been cloned and charac- A cell containing the hydrolytically active exoenzyme As mentioned above, the present invention relates in one aspect to a cell containing a gene coding for the hydroly¬ tically active exoenzyme as defined herein. The cell may be selected from a wide variety of cells for which a need for containment exists. Thus, the cell to be contained may be a bacterial cell, a protozoan cell, a yeast or fungal cell, or a cell derived from the tissues of multicellular organisms such as plants, animal and fungi. The gene coding for the hydrolytically active enzyme may be derived from a variety of replicons contained in any organism producing an extracellular enzyme as presently defined. Thus, sources of the gene include bacterial chromosomes, bacterial plasmids, procaryotic viruses, eucaryotic viruses, eucaryotic plasmids, or eucaryotic chromosomes. The gene may also be constructed synthetically according to standard procedures. In accordance with the invention, the cell as defined herein may be obtained by methods known per se. These methods in¬ clude the steps of screening for cells expressing a suitable extracellular hydrolytic enzyme, isolating from such cells the gene encoding the enzyme, removing from the gene the nucleotide sequence coding for the signal peptide and inser¬ ting into a cell to be contained, the gene and a nucleotide sequence capable of regulating the expression hereof, growing the cell under conditions allowing the expression of the gene, and testing for the intracellular presence of the enzyme in an enzvmatically active form. The insertion of the gene coding for the hydrolytically active exoenzyme may be in the chromosome of the cell or the gene may be inserted in an extrachromosomal recombinant replicon such as a recombinant plasmid capable of replicating in the cell. The regulatory nucleotide sequence regulating the expression of the hydrolytically active exoenzyme As mentioned above, the expression of the gene coding for the hydrolytically active enzyme is regulated by a regulatory nucleotide sequence. In the present context the term "regula¬ tory nucleotide sequence" is intended to indicate a nucleo¬ tide sequence which directly or indirectly regulates the expression of the gene coding for the hydrolytically active enzyme at the level of transcription or at the level of translation. The regulatory nucleotide sequence may be one, the function of which results in a suppression or inhibition of the activity of the regulatable promoter. Such regulatory nucleotide sequences are herein referred to as "negatively functioning regulatory nucleotide sequences". One interesting example of such a negatively functioning regulatory nucleotide sequence is a sequence coding for a repressor substance which represses the expression of the gene coding for the hydrolytically active enzyme and which substance may, when a cell containing it is released to a human or an animal body or to the outer environment, undergo a decay whereby the repression of expression of the enzyme- encoding gene is gradually reduced and eventually, when the decay of the repressor is completed, the repression is remo¬ ved. In preferred embodiments of the invention, the regulatory nucleotide sequence may be contained in the cell in one or more recombinant replicons and it may be contained in the same replicon as that containing the enzyme-encoding gene or in a different recombinant replicon. One way whereby the expression of the cell function-limiting enzyme may in accordance with the invention be regulated is by providing in the cell a gene coding for the hydrolytically active enzyme, which gene is regulated at the level of trans¬ cription. The regulation at the level of transcription may be carried out in various ways including a regulation by means of a promoter, regulated by one or more factors. These fac¬ tors may either be ones which by their presence ensure ex¬ pression of the gene coding for the cell function-limiting enzyme or may, alternatively, be ones which suppress the expression of the gene so that their absence causes the enzyme to be expressed. Factors regulating the activity of the promoter as defined above may be selected from a variety of factors. Thus, the expression of the gene encoding the cell function-limiting enzyme may be determined by the environmental conditions or the physiological state of the cells, or by a cyclical or stochastic event. In the present context, the term "cyclical event" is understood to mean a cyclically recurrent event causing changes in certain factors known to be potentially useful in influencing the expression of genes such as tem¬ perature conditions, changes in light intensity or hormonal changes. The term "physiological state of the cells" denotes factors such as cell density or the growth phase of cells. In accordance with the invention, advantageous promoter regulating factors are readily regulatable factors including the presence or absence of a certain chemical substance in the environment or the physical conditions in the environment such as the prevailing temperature or other physical factors (e.g. the intensity of the light in the environment) . Thus, it is possible to envisage containment systems as presently claimed, in which the gene coding for the cell function- limiting enzyme is expressed when a certain chemical sub¬ stance present in a first environment such as the fermen- tation medium in which the cell is propagated, is not present in a second environment to which the cell is released, or when a factor required for the growth or survival of the cell is no longer present, or the factor is a factor which, when it is exhausted from an environment of the cell, has the desired effect, viz. that the gene is expressed. The promoter regulating the transcription of the gene coding for the cell function-limiting hydrolytically active enzyme may also become activated in a second environment of the cell by a chemical substance which is not present in a first environment of the cell, but which is present in the second environment in sufficient quantities to activate the pro¬ moter. Similarly, the promoter may by a promoter which is activated by a shift in temperature, such as a shift from a higher temperature in a first environment as e.g. a fermen- tation vessel, to a lower temperature prevailing in an out¬ side second environment, or the intensity of light, in that the promoter may be one which is activated in the presence of light of sufficient intensity, but is inactive in the dark¬ ness prevailing in a first environment such as a fermentation vessel. Where cells as defined herein are ones that are to be relea¬ sed to the natural environment in a controlled manner, e.g. to a restricted area of land or to the intestinal tract of a human or an animal, the regulatable promoter may be one which is regulated chemically, i.e. by the presence or absence of a certain chemical substance in the environment of the cells as it has been explained above. However, the regulatable promoter is advantageously a pro¬ moter which is activated cyclically, e.g. by changes of the temperature, or most advantageously by a stochastic event. The term "stochastic event" as used herein is intended to denote an event which occurs at random at a certain frequency per cell per generation or frequency per unit time which, in accordance with the invention may result in a limitation of the function of the cells in which the activation of expres¬ sion of the cell function-limiting intracellularly active exoenzyme occurs, optionally to an extent which leads to the death of the cells. The stochastic event may be occasioned by periodic inversions of the region carrying the promoter, but is more advantageously induced by the recombinational ex- cision of a recombinationally excisable negatively functio¬ ning regulatory nucleotide sequence as defined above. It should be noted that in order to ensure a general applica¬ bility of the present invention, the promoter used to ini- tiate transcription of the gene coding for the cell function- limiting enzyme is preferably a promoter which is capable of causing expression of said gene in a wide range of cells. In case of regulatable transcription of the hydrolytically active enzyme, the regulatory nucleotide sequence may e.g. be a promoter isolated from bacterial operons involved in the biosynthesis of amino acids or from bacterial genes, the transcription of which is activated late in the stationary growth phase or from bacterial genes involved in the syn¬ thesis of cell surface structures such as fimbriae. Examples of suitable promoters are E. coli trp which becomes activated in the absence of tryptophan, the bacteriophage λ P R and P L promoters controlled by temperature sensitive regulatory nucleotide sequences, the Bacillus subtilis sporulation gene promoters which are activated during sporulation, and the E_;_ coli and Salmonella fimbriae gene promoters which are ac¬ tivated stochastically. In case of chemically regulatable promoters, the chemical substance, the presence or absence of which determines the activation of the promoter, may suitably be selected from carbon or nitrogen sources, metabolites, amino acids, nucleo- sides, purine or pyri idine bases or metal ions. When the chemical substance is one which, when present, suppresses promoter activity, it should preferably be a substance which rarely occurs in the natural environment in such concen- trations that the promoter would not be activated when the cell is released to the natural environment. One example of a suitable promoter in e.g. an . coli cell is the trp promoter which is repressed in the presence of tryptophan in the environment of the cell, but which is derepressed in the absence of sufficient amounts of tryptophan in the environ- ment. A containment system using the trp promoter or another promoter being regulated in the same manner, might therefore comprise an amount of tryptophan in a first environment, such as a fermentation vessel, to repress the promoter which is derepressed when the cell is released from the first environ¬ ment to a second environment, e.g. the natural environment which usually contains very low amounts of tryptophan or no tryptophan at all. Another example of a regulatable promoter, the activation of which is determined by a chemical substance is the lac pro¬ moter which is inducible by e.g. isopropyl-β-D-thiogalacto- pyranoside (IPTG) . As mentioned above, the regulatable promoter may be a pro¬ moter, the activity of which is determined by the temperature prevailing in the environment of a cell containing the gene coding for the cell function-limiting enzyme and a the regu¬ latable promoter regulating the expression of the gene. In such a case, the regulation of the promoter is advantageously obtained by the presence in the cell of a temperature sensi- tive gene coding for a repressor for the promoter. As one typical example, the λ promoters including those mentioned above may be regulated by a temperature sensitive λ cl re¬ pressor. Promoters which are activated stochastically by periodic inversions of the promoter region (in the present context, such promoters are also termed as an "invertible promoter" and "inversional switch promoter") and which may be useful for the purposes of the present invention include as examples the hin, cin and gin promoters. One particularly useful invertible promoter is the fimA promoter which is one ΕX_ coli fimbriae promoter. The activation (inversional switch) of this promoter is regulated by the gene products of the two genes which for the present purposes is termed the "on" and the "off" genes, the on gene product inducing a switch from off (inactive) to on (active) , and the off gene product inducing a switch from on to off. In a wild-type E.coli cell where the fimA gene and its associated promoter is present in one copy on the chromosome, the inversional switch occurs with a switching frequency of about one cell/1000 cells/ generation. It is, however, possible to regulate the fre¬ quency of the inversional switch as required by regulating the dosage of expression of the on and off genes. This may e.g. be effected by means of suitable promoters to transcribe into the on and off genes. The frequency of transcription initiation by these promoters will then determine the rela¬ tive dosage levels of the on and off gene products being formed. As it has been explained above, the intracellular enzymatic activity of the cell function-limiting enzyme may also be regulated by the presence or absence in the cell of a gene product rendering the cell refractory to said enzymatic activity, e.g. by interacting with the enzyme so that the cell function-limiting effect hereof is not expressed. Stochastically induced regulation of the gene coding for the hydrolytically active exoenzyme by means of recombinational excision of negatively regulatory nucleotide sequences. In accordance with the invention, one particularly advanta¬ geous method of stochastically regulating the expression of the gene coding for the cell function-limiting exoenzyme is the induction of the gene expression as a result of recom¬ binational excision of an excisable negatively functioning regulatory nucleotide sequence which, while present in the cell, inhibits expression of the gene. In the present con¬ text, the term "recombinational excision" refers to the result of a naturally occurring phenomenon of genetic recom¬ bination (cross-over) whereby nucleotide sequences in repli¬ cons, in a controlled process, pair, brake and rejoin to form recombinant replicons by the sequential action of enzymes acting on the DNA. The frequency of recombinational events in a cell depends i.a. on the degree of homology between paired complementary nucleotide sequences and on the length of the complementary sequences. Thus, it has been shown that about 50 base pairs of homology may be required to obtain recombi¬ nation in a bacterial cell. When a negatively regulatory nucleotide sequence is inserted between directly repeated nucleotide sequences of a suffi¬ cient length in a recombinationally proficient cell which, in accordance with the invention contains a gene coding for a cell function-limiting enzyme, recombination between the repeats results in the recombinational excision of the nega¬ tively regulatory nucleotide sequence allowing the gene to be expressed, whereby the cell function-limiting enzyme may be produced intracellularly in amounts leading to a limitation of the cell function, optionally resulting in the death of the cell. Accordingly, the phenomenon of recombinational excision as used herein, implies that a DNA subsequence, i.e. the nega¬ tively regulatory nucleotide sequence, is excised from a longer DNA sequence through a recombination event. In es- sence, the longer DNA sequence is cleaved on either side of the subsequence and the fresh ends are joined, leaving out the subsequence. Recombination occurs between sufficient homologous flanking nucleotide subsequences. Thus, with DNA of the general structure W-X-Y-X-Z, X being a repeated sequence and Y being a negatively regulatory nucleotide sequence, this could recombine to form W-X-Z, with the Y subsequence being excised. As mentioned above, the frequency of the recombination can to some extent be determined by varying the lengths of the repeats and/or the distance between the repeats. Furthermore, the frequency may be varied by using repeat sequences of varying homologies. Thus, nucleotide sequence repeats being 100% homologous and having a size which does not impair recombination will result in a high recombination frequency and hence, in a high frequency of recombinational excision of the negatively regulatory sequence, whereas mismatches within complementary sequences will reduce the recombination fre¬ quency depending on the degree of mismatch. As an example, it has been found that 10% divergence between nucleotide se- quence repeats may reduce the recombination frequency 40- fold. Accordingly, the cell containing the gene coding for a hydro¬ lytically active exoenzyme may, in accordance with the in¬ vention, be a cell containing a regulatory nucleotide se- quence which is a recombinationally excisable negatively functioning regulatory nucleotide sequence being flanked by a first flanking nucleotide sequence and a second flanking nucleotide sequence substantially homologous with the first flanking sequence. As used herein, the term "substantially homologous with" is used to indicate that the degree of homology is sufficient to result in a desired frequency of recombination. In certain embodiments it may, in order to obtain a desirable maximum frequency of recombination, be advantageous to use direct repeats, i.e. sequences being 100% homologous, whereas it may, in other embodiments where a moderate degree of cell function limitation is desirable, be appropriate to use repeats which are more or less hetero¬ logous, but still allowing a desirable lower frequency of recombination to occur. Accordingly, in the present context, the term "sufficiently homologous" may appropriately be used to indicate a degree of homology between two flanking nucleo¬ tide sequence repeats which results in a desired frequency of recombinational events in a cell containing the gene coding for the hydrolytically active exoenzyme and a negatively regulatory nucleotide sequence. As it also has been mentioned above, the frequency of recom¬ bination depends on the lengths of the flanking sequences. In useful embodiments of the invention, flanking sequences are used which have a length being in the range of 100-5000 base pairs. In certain preferred embodiments, it may be advan¬ tageous to use flanking sequences, the length of which is in the range of 200-3000 base pairs. As the flanking sequences may be used any nucleotide repeats of sufficient lengths and homology as it has been defined above. As one useful example of flanking sequences may be mentioned the chloramphenicol resistance gene having a size of about 900 base pairs and which occurs in the plasmid pBR325 (Bolivar, 1978, Gene, 4., 121-136) . Another example of a useful nucleotide sequence which when inserted as repeats may result in recombination, is a subsequence of the rrnB gene isolated from the plasmid pKK3535 (Brosius et al., 1981, Plasmid, 6, 112-118) of the rrnB gene isolated from the plasmid pKK3535 (Brosius et al., 1981, Plasmid, .6, 112-118) having a size e.g. in the range of 500 to about 3000 base pairs, such as 598 base pairs. In one interesting embodiment of the invention, the cell containing a gene whose expression in the cell results in the formation of a hydrolytically active enzyme is a cell wherein said gene is a gene which encodes a first RNA which is a messenger RNA, and which further contains an excisable nega¬ tively regulatory nucleotide sequence operably linked to said gene encoding the hydrolytically active gene, which is a gene encoding a second RNA which forms an RNA-RNA duplex with said first messenger RNA and thereby, when it is expressed, in¬ hibits translation of said gene coding for the hydrolytically active enzyme. In another useful embodiment of the present invention, the recombinationally excisable negatively regulatory nucleotide sequence is a gene encoding a polypeptide repressor of trans¬ cription of the gene whose expression results in the intra¬ cellular formation of a hydrolytically active enzyme. Such a polypeptide repressor may, e.g. be a lac repressor. As one specific example of a useful lac repressor may be mentioned the repressor encoded by the Laclq gene. In a further useful embodiment of the invention, the excis¬ able negatively regulatory nucleotide sequence is a trans- cription termination sequence, preventing the transcription of the gene whose expression results in the formation intra¬ cellularly of a hydrolytically active enzyme. In one specific embodiment of the invention, such a suitable terminator sequence may be the rpoCt' transcription terminator isolated from the plasmid pHBA102rpoCt (Squires et al. , 1981, Nucleic Acid Res., 9., 6827-6839). Negatively regulatory nucleotide sequences which in accor¬ dance with the invention are suitable, may be isolated from nucleotide sequences derived from a virus, or a procaryotic or eucaryotic cell. Thus, sources of the nucleotide sequence include bacterial chromosomes, bacterial plasmids, proca¬ ryotic viruses, eucaryotic viruses, eucaryotic plasmids, or eucaryotic chromosomes. In preferred embodiments of the invention, the excisable negatively regulatory nucleotide sequence being operably linked to the gene coding for the hydrolytically active enzyme and the first and second flanking sequences, both as defined above, is provided in the form of a "cassette" which term is used herein to describe a readily insertable nucleo- tide sequence comprising at least the above-mentioned se¬ quences and optionally the gene coding for the hydrolytically active enzyme, and optionally further nucleotide sequences including as examples a suitable marker such as a gene coding for antibiotic resistance. In the present context, the term "insertable" denotes that the cassette as defined herein is provided with suitable restriction sites at both ends allow¬ ing for insertion in a replicon having the same restriction sites. Accordingly, such preferred restriction sites include sites which occur frequently in replicons where insertion is desirable or alternatively, restriction sites which may be easily provided in such replicons. It will be understood that, in accordance with the invention, a cassette as defined above and which does not comprise the gene coding for hydrolytically active enzyme and operably linked to the negatively regulatory nucleotide sequence, may be inserted in a replicon which is different from the repli¬ con containing said gene. Optionally, the cassette as defined above may be inserted in a first replicon such as e.g. a transposon and subsequently inserted via the transposon into the chromosome to obtain a cell as defined herein. As it has been explained above, the activation of certain invertible promoters such as the fimA promoter or functional homologues hereof is regulated by the gene products of an on gene and an off gene. It will be understood that this mechanism of promoter regulation provides the possibility of using the off gene or a functional homologue hereof as a negatively regulatory nucleotide sequence which may be inser¬ ted in the cell as defined herein, as a recombinationally excisable nucleotide sequence in the manner explained in details above. Accordingly, in one embodiment, the present invention provides a cell wherein the gene whose expression results in the formation of a hydrolytically active enzyme is stochastically expressed as a result of recombinational inversion of an invertible promoter sequence of the regula- tory nucleotide sequence said promoter being operably linked to the gene. Stochastically induced regulation of the gene coding for the hydrolytically active exoenzyme by means of site-speci ic recombinational excision of negatively regulatory nucleotide sequences. In plasmids, inherent mechanisms occur whereby multimer resolution of the plasmid during replication takes place. As exemplified by the broad host range plasmid RP4, this resolu¬ tion system may comprise (1) a gene coding for a multimer resolving enzyme, a resolvase and (2) a site for the site- specific resolvase-mediated resolution. In plasmid RP4 the gene coding for the resolvase is parA and the site for the resolution is designated mrs. If two mrs sites are placed in direct orientation, a nucleotide sequence inserted between those two sites may, if the parA gene is present in the same host cell, be deleted at a relatively high frequency whereby a site-specific recombination system is provided. In useful embodiments the ParA gene may be located in trans. It has now been found that such a site-specific recombination system provides a useful mechanism for stochastically regula¬ ting the expression of a gene such as the gene coding for the hydrolytically active enzyme as defined herein, since the site-specific recombination may be used to obtain recombi¬ national excision of a negatively regulatory nucleotide sequence as defined above. Accordingly, in one interesting embodiment, the present invention provides a cell as defined herein in which the negatively regulatory nucleotide sequence is a sequence flanked by a first site for a site-specific resolution recom- binase and a second site for site-specific resolution, the second site being recognizable by the same or a functionally equivalent multimer resolving enzyme as is the first site, whereby the regulatory sequence is recombinationally excis¬ able in the cell. In a specific embodiment, the gene coding for the multimer resolving enzyme is located in trans rela¬ tive to the sites for site-specific resolution. In the pre¬ sent context, one useful example of a suitable gene is the parA gene isolated from plasmid RP4. The gene coding for an intracellularly active exoenzyme having a cell function-limiting function. As it has been mentioned above, the gene coding for the hydrolytically active enzyme may, in accordance with the invention, be derived from several sources including a bac¬ terial replicon. In one preferred embodiment of the inven- tion, the gene is a gene derived from a gram-negative bac¬ terium such as a Serratia species, an Aeromonas species or a Yersinia species. In a specific embodiment, the Serratia species is Serratia marcescens and the hydrolytically active enzyme is an enzyme as defined herein encoded by a Serratia marcescens gene. In particularly preferred embodiments, the Serratia marcescens gene may be one encoding an endonuclease and having the DNA sequence as defined above or a gene encoding a phospholipase and having the DNA sequence as defined above. In a further useful embodiment, the invention provides a cell as defined above which contains a gene coding for a hydroly¬ tically active exoenzyme which gene is derived from a gram- positive bacterial species including as an example a gene coding for a nuclease without its signal peptide which gene is isolated from a Staphylococcus species such as Staphylo- coccus aureus and e.g. carried on a plasmid. One example of such a gene is one carried by the plasmid pFOG 408. Provision of a cell according to the invention in which the function-limiting effect of the hydrolytically active enzyme is increased bv mutation. In certain embodiments, it may be advantageous to obtain a higher enzymatic activity than is obtained by the insertion of the gene coding for the hydrolytically active enzyme and/or the regulatory nucleotide sequence in the form in which this/these sequences are primarily isolated. Such a higher activity may be the result of an increased amount of enzyme being expressed and/or of an increased specific ac¬ tivity of the enzyme, relative to the amount or the specific activity which is obtained by the insertion of the primarily isolated sequences. Such increased enzymatic activity may conveniently be ob¬ tained by subjecting the isolated sequences separately or together to a conventional in-vitro or in-vivo mutagenization treatment e.g. with a chemical mutagen, by means of a site- directed mutagenesis or by a mutagenically active radiation treatment. Cells containing thus treated nucleotide sequences in which a mutation or mutations resulting in an increased intracellular enzymatic activity have occurred, may be selected by growing a culture of the cells under conditions where the gene coding for the hydrolytically active enzyme is expressible, and isolating cells or cell clones which relative to cells or cell clones containing the primarily isolated sequences and grown under the same conditions, show an increased level of cell function limitation as defined above. Accordingly, the invention provides in one embodiment a cell as defined herein in which at least one of the gene whose expression results in the formation of a hydrolytically active enzyme and the nucleotide sequence regulating said gene, is mutated at one or more sites, whereby the cell function-limiting effect of the enzyme encoded by the gene, when expressed in the cell, is the same or increased relative to the cell function-limiting effect of the enzyme expressed in a cell containing said gene and said nucleotide sequence in non-mutated form. In one interesting embodiment, the cell in which the enzyme activity is increased by the mutation treatment is a cell containing a gene coding for an endo¬ nuclease such as e.g. a gene derived from a Serratia species. A cell according to the invention comprising a further regu¬ latably expressible gene which encodes a non-enzyme cell function-limiting function. As a means of increasing the cell function-limiting effect in a cell as defined herein it may be advantageous to insert into the cell a further regulatably expressible gene which further gene is a gene encoding a non-enzyme cell function- limiting gene product. Accordingly, the present invention provides in one specific embodiment a cell containing a further gene as defined above. Such a further regulatably expressible gene may be regulated by a regulatory nucleotide sequence of the same type as the sequence regulating the gene coding for the hydrolytically active enzyme or the gene may be one which is regulated by a regulatory nucleotide sequence of another type, said other type of regulatory sequence optionally being one which is also capable of regulating the expression of the gene coding for cell function-limiting hydrolytically active enzyme. A suitable gene coding for a further non-enzyme regulatably expressible cell function-limiting gene product may conve¬ niently be selected from the above-mentioned genes having cell killing function. In certain preferred embodiments, said further gene is selected from the hok gene of the parB region of plasmid Rl, the gef gene and a DNA sequence which is a functional equivalent of either of these genes. A cell as defined herein which further contains a DNA se- quence not naturally related to the replicon carrying the gene coding for the hydrolytically active enzyme As it has been defined above, the cell according to the present invention may, when occurring as a population com¬ prising a multiplicity of the cell which further comprises a DNA sequence coding for an immunologically active enzyme, a pesticidally active or a pollutant-degrading gene product, respectively, be useful in an immunologically active composi¬ tion, a pesticidally active or a environmental pollutant- degrading composition, respectively, to be contained in the particular environment where such a composition may be re¬ leased. Accordingly, the cell may in such specific embodiments be a cell which further comprises a DNA sequence not naturally related to the replicon carrying the gene whose expression results in the formation of a hydrolytically active enzyme and/or the replicon carrying the regulatory nucleotide se¬ quence, said DNA sequence being selected from a sequence coding for an immunologically active gene product, a sequence coding for a pesticidally active gene product and a sequence coding for a pollutant degrading gene product. In the present context, the term "immunologically active gene product" is used to describe an epitope (antigenic determi- nant) from a pathogenic organism which, when it is admini¬ stered to the body of a human or an animal is capable of stimulating the formation of antibodies therein. A cell as defined in the present invention which contains one or more genes encoding such a gene product may be utilized in the preparation of useful live vaccines. In the immunization against several pathogens it is considered advantageous to administer live vaccines as compared to killed organisms or antigenic fragments of the pathogen, since the level of immunity conferred by a live vaccine is frequently higher than that conferred by vaccines comprising killed pathogenic organisms or fragments thereof. Most known vaccines compris¬ ing viable epitope-containing organisms are either based on recombinant non-pathogenic organisms encoding the epitope or they are based on attenuated pathogenic organisms. The cell may advantageously contain a multiplicity of genes each of which coding for a specific immunologically active gene product. However, up till now the use of live vaccines has been limi¬ ted since it is often difficult to obtain the right combi- nation of attenuation, viability and adequate immune re¬ sponse. Furthermore, the deliberate release of genetically engineered microorganisms to the body and to the external environment which is a result of the use of viable recombi¬ nant organisms as vaccines, is currently not allowed in any country for reasons of public concern as to the possible long-term environmental impact, in particular the risk of permanent establishment of the GEMs in the environment. The present invention provides an advantageous means of circumventing these problems associated with the use of known GEM-based live vaccines by introducing into a viable epitope- containing cell the regulatably expressible gene coding a cell function-limiting hydrolytically active enzyme as defined above. In particularly interesting embodiments, the invention provides as a useful basis for a viable vaccine, the cell as defined above which contains a gene coding for the hydrolytically active enzyme whose expression is sto¬ chastically induced. In useful embodiments of the invention, the cell which con¬ tains the DNA sequence coding for an immunologically active gene product further comprises means for transporting the epitope, when expressed, to the outer surface of the cell, i.e. translocating it across the cell membrane. Preferably such a translocation is obtained by inserting the gene coding for the epitope into a nucleotide sequence coding for an outer cell surface polypeptide structure such as fimbriae which contains the fimbrillin protein, pili, flagellae or certain other surface proteins including as an example the OM protein found in Streptococcus species. By providing the cell with such a hybrid nucleotide sequence being expressible in the cell, the gene product hereof will be a fusion or hybrid protein comprising the epitope and the relevant cell surface structure. A cell in which a fusion protein is expressed which comprises the epitope fused to a surface structure protein by which the cell can adhere to the mucosal cells of a body to which the cell is administered is considered to be particularly useful in that the epitope will become in close contact with the mucosa. and thereby effectively stimulate a protective immune response in the form of the excretion of secretory antibodies of the IgA and IgG classes. Furthermore, the adhesion of the epitope-carrying cell will ensure that the cell is retained in the human or animal body for a period of time which is sufficient to obtain the de¬ sired immune response. It is considered that a satisfactory immunization typically may be obtained if the cell is present in sufficient numbers in a particular body environment such as the intestinal tract for a period in the range of 15-30 days, depending on the nature and the activity of the epitope expressed from the cell. As it will be understood from the above description of the gene coding for the cell function-limiting hydrolytically active enzyme and the regulatory nucleotide, the present invention may provide useful means of providing live vaccines based on recombinant organisms which are immunologically effective and which can be used without the risk of undesired spreading of recombinant genes to the microflora of humans and animals or to the outer environment. In accordance with the invention, a useful cell for the preparation of a live vaccine is one selected from a bac- terial species which inherently contains an outer surface structure as mentioned above. Such species include as exam¬ ples species of Enterobacteriaceae such as Salmonella and E. coli species, Vibrionaceae and Pseudomonadaceae. It will be understood that strains of such species which are particu- larly useful in the present invention as the basis of a live vaccine as defined above, are non-pathogenic strains or strains having a low pathogenicity. The epitope expressed by a cell as defined above may be an epitope derived from any pathogenic organism or agent the obtainment of immunity against which is desirably. Such pathogens include viruses, bacteria and eucaryotic organisms such as fungi, yeast or protozoa. In commercially important embodiments, the cell may contain a nucleotide sequence coding for a pesticidally active gene product. In this context, the term "pesticidally active gene product" is used to denote a product which when expressed in a cell being released to an environment where there is a need to reduce or eliminate the presence of pests including insect pests, vermins such as rodents or birds. Such pests may be controlled by the administration of toxic chemical pesticides to the infestated environment, but recently various naturally occurring pesticidally active organisms including viruses, bacteria and fungi have been used as biological pest control products. Prominent examples of such pesticidally active organisms include biotypes or strains of the species Bacillus thurin- giensis which produce crystalline proteins being toxic to insects, in particular to caterpillars, and several viruses being pathogenic for insects in the larval stage or in the adult stage. However, the pesticidal effect of such organisms is frequently less satisfactory and there is a strong need in farming, forestry and horticulture to provide improved pesti¬ cidally active organisms. One approach to solving this pro- blem is to construct genetically engineered organisms having an increased toxic effect or a better survival rate in the environment. Should such improved organisms be developed, their use in the environment will, as a consequence of current public concern of the potential risks involved in deliberate release of such toxic or pathogenic GEMs, only be approved by official envi¬ ronmental agencies if it can be demonstrated that the release does not lead to an undesired propagation or to an extended survival of such organisms in the environment to which they are applied. The present invention clearly provides the means of limiting the survival in the environment of genetically engineered pesticidally active organisms. As it has been explained above, the rate of expression of the cell function-limiting hydrolytically active enzyme may be regulated stochastically and thus the survival rate of pesticidally active cells may conveniently be adapted to any specific need. Also, the cell function-limiting effect may, in accordance with the present invention be adjusted by selecting a hydrolytically active enzyme which has an appropriate cell function-limiting ef¬ fect. In another useful embodiment, the invention provides a cell in which the DNA sequence not naturally related to the gene coding for the hydrolytically active enzyme, is a sequence coding for a pollutant-degrading gene product. It is known that several xenobiotic compounds polluting the outer en¬ vironment including soil and water can be degraded by micro¬ organisms having an inherent capability of degrading these compounds. Obviously, the technology of genetic engineering provides means of providing improved organisms having an increased pollutant-degrading capacity or having the capacity to degrade a broad range of compounds, in particular hydro¬ carbons. However, the public concern as mentioned above are also relevant in this context and accordingly, the present in¬ vention provides useful means of providing improved pollu¬ tant-degrading cells, the survival of which can be controlled by regulating the expression of the cell function-limiting hydrolytically active enzyme as it is defined above. In particularly preferred embodiments, the cell contains a gene coding for a pollutant-degrading gene product, the expression of which is induced by the presence of a pollutant degradable by the cell. Cell as defined herein which is a transformed cell. In a further specific embodiment of the invention, the cell according to the invention and as defined above, is a cell which is transformed with a recombinant replicon or recom¬ binant replicons containing a gene whose expression results in the formation of the cell function-limiting enzyme, the expression of said gene being regulated by a regulatory nucleotide sequence which is contained in the recombinant replicon containing the gene or in another recombinant repli¬ con present in the transformed cell. Replicon containing a gene coding for an intracellularly hydrolytically active cell function-limiting enzyme and optionally a regulatory nucleotide sequence. As mentioned above, the present invention pertains in a further aspect to a recombinant replicon containing the regulatably expressible gene encoding an intracellularly active hydrolytic enzyme as defined above, having, when it is expressed in a cell, a function-limiting effect thereon, said gene being regulated by a regulatory nucleotide sequence as also defined above and which sequence is operably linked to the expressible gene. In accordance with the invention, the regulatory nucleotide sequence may be a sequence contained in the replicon or in another replicon present in the cell containing the expressible gene. The replicon may in accordance with the invention also be a replicon wherein the gene coding for the hydrolytically active enzyme and/or the nucleotide sequence regulating the expression of said gene, when such a sequence is present in the replicon, is mutated at one or more sites, so that the cell function-limiting effect of the enzyme encoded by the gene, when it is expressed in a cell containing the replicon, is the same or increased relative to the cell function-limi¬ ting effect of the enzyme as expressed in a cell containing a replicon as defined herein not having been mutated. In one embodiment, the replicon may comprise a further regu¬ latably expressible gene which encodes a non-enzyme cell function-limiting function, the expression of which may be regulated by a regulatory nucleotide sequence of the same type as the sequence regulating the gene coding for the cell function-limiting hydrolytically active enzyme. In other embodiments, the nucleotide sequence regulating the expres¬ sion of the non-enzyme cell function-limiting function is of a different type which however, may also be capable of regu¬ lating the expression of the gene encoding the cell function- limiting hydrolytically active enzyme. Said further regulatably expressible gene may in useful embodiments be the hok gene from the parB region of plasmid Rl or a DNA sequence which is functionally homologous to the hok gene or the E_^ coli chromosomal gef gene. In other useful embodiments, the replicon may further comprise a DNA sequence as defined above which is not na¬ turally related to the replicon and which encodes a gene product selected from an immunologically active gene product, a pesticidally active gene product and a pollutant-degrading gene product. Population of cells containing a gene coding for an intra¬ cellularly hydrolytically active exoenzyme having cell func¬ tion-limiting effect As mentioned above, the present invention pertains in a further aspect to a population of cells consisting of a multiplicity of cells as they have been defined above. The cell population may preferably comprise cell having been transformed with a recombinant replicon as defined herein which is capable of replicating in the cells. Since the cell population in interesting uses may be released to the outer environment, the cells may in advantageous embodiments be bacterial cells which are selected from species whose natural habitat is a habitat selected from soil, surface water and plants, such as gram-negative bacterial species. In this context, it is interesting to note that the survival of a population of cells as presently defined and in which the expression of the gene coding for the cell function- limiting enzyme is regulatable by a repressor substance which is present in the cells, but in different amounts depending on the physiological state of the cells, may, when such a population is applied to a human or an animal body, or to the outer environment, be regulated as a result of a decay of the repressor substance to an extent whereby the repressor sub¬ stance is converted to a non-functional form. Provided the amount of the repressor substance in individual cells of a population being released, is different, it is assumed that this decay will result in a gradually increasing loss of viability of the cells of the population. Method of limiting the survival of a cell population As it has also been mentioned above, the present invention provides in one aspect a method of limiting the survival of a cell population in a first or a second environment wherein the regulatory nucleotide sequence is regulatable by an environmental factor as defined herein. The method may in preferred embodiments be related to a cell population as defined above. In specific embodiments of this method, the survival of the cell population is limited in a first environment in which the gene coding for the cell function-limiting hydrolytically active enzyme is expressed whereby the cell population is contained in the first environment. In the present context, a first environment is typically the place of primary propaga¬ tion of the cell such as a fermentation vessel. In another specific embodiment of the method, the survival of the cell population is limited in a second environment which may be a first environment changing to a chemically different second environment, e.g. by the depletion of certain chemi¬ cals or the addition of such chemicals, or to a physically different second environment which change e.g. may take place by a shift in temperature or by a change of light intensity. As it has been mentioned above, a cell population as defined herein may also by a cell population wherein the expression of the gene coding for the function-limiting enzyme is regu- latable by a repressor substance which can undergo a decay or a degradation when said cells are released to a human or animal body or the outer environment to an extent whereby the repressor substance is converted to a non-functional form, said repressor substance being present in the cells of the population in different amounts whereby as a result of said decay the function of the cells of the population will gradu¬ ally be limited. Accordingly, the present invention provides in one useful embodiment a method of limiting the survival of a cell popu¬ lation by providing the cells with a gene coding for the hydrolytically active enzyme which is operably linked to a regulatory nucleotide sequence encoding a repressor substance which can undergo a decay when said cells are released to a human or animal body or the outer environment, to an extent whereby the repressor substance is converted to a non-func¬ tional form, said repressor substance being present in the cells of the population in different amounts whereby as a result of said decay the function of the cells of the popula¬ tion will be gradually be limited. A method of containing an extrachromosomal recombinant repli¬ con As defined above, the invention relates in one aspect to a method of containing an extrachromosomal recombinant replicon to a first kind of cells where the replicon is naturally transferable to a second kind of cells, in which method the recombinant replicon is preferably a recombinant replicon as defined herein. In one preferred embodiment, this replicon is one not containing a regulatory nucleotide seguence and advantageously, such a replicon contains a gene coding for a hydrolytically active enzyme which is an endonuclease capable of hydrolyzing diesterphosphate bonds in nucleic acids in the first kind of cells containing the replicon to be contained. Method of stochastically limiting the function of cells in a cell population In a particularly interesting aspect, the invention pertains, as it has been mentioned above, to a method of stochastically limiting the survival of a cell population such as a cell population as defined above, said method comprising the transformation of the cells with a recombinant replicon as also defined herein. In one useful embodiment, the method comprises a method wherein the expression of the cell func¬ tion-limiting hydrolytically active enzyme is induced as a result of a site-specific recombinational excision of an excisable negatively functioning regulatory nucleotide se¬ quence which, while present in the cells, inhibits expression of said gene, the negatively functioning regulatory nucleo¬ tide sequence being contained in the recombinant replicon or in another recombinant replicon present in the cells. In another useful embodiment of this method, the survival of the cell population transformed with a recombinant replicon as presently defined, is being limited as a result of the above-defined recombinational inversion of an invertible promoter sequence of the negatively functioning regulatory sequence. In accordance with the invention, the negatively functioning regulatory sequence may be one contained in the recombinant replicon or it may be one present on another replicon present in the cells. As one suitable example, the promoter sequence may be a sequence carrying the fimA pro¬ moter or a functional homologue thereof. An immunologically active composition The immunologically active composition as defined above may in addition to the viable function-limited cell population comprise pharmaceutically acceptable carriers and additives. Such acceptable carriers include any vehicle which is used conventionally in vaccine production such as e.g. saline. In the present context, suitable additives include immune re¬ sponse-enhancing substances including as examples Freund's incomplete and complete adjuvant and other non-specific immunostimulating substances. It may be preferred to provide the composition in the form of lyophilized compositions, optionally in combination with a suitable aqueous vehicle such as saline. The immunologically active composition as presently defined may contain different types of cells, each of which encoding a specific epitope. Alternative, the immunologically active composition may in accordance with the invention contain a population of cells containing a multiplicity of genes coding for an immunologi¬ cally active gene product, each gene coding for a different gene product. The immunologically active compositions provided herein are useful vaccines for the immunization of both humans and animals. In preferred embodiments, the immunologically active com- position contains cells containing a sequence encoding for an immunologically active gene product which is a sequence coding for a fusion protein comprising said gene product and a polypeptide, the presence of which results in the transportation of the fusion protein to the outer surface of the cells. As suitable examples, the composition may contain cells wherein the polypeptide is one selected from a polypep¬ tide selected from a bacterial fimbrillin protein, a bac¬ terial pilus, a bacterial flagellum and a bacterial OM sur¬ face protein. Nucleotide sequences coding for such a polypeptide may con¬ veniently be isolated from a bacterium selected from Entero- bacteriaceae. Vibrionaceae and Pseudomonadaceae. A pesticidally active composition. A pesticidally active composition as defined herein may in accordance with the invention, in specific useful embodiment contain a further DNA sequence encode a gene product which i toxic for insects or their progeny. In one interesting em¬ bodiment, the further DNA sequence is one derived from a strain of Bacillus thuringiensis which codes for an insec- ticidal protein. As it has been explained above, cells as defined herein may code for a variety of pesticidally active gene products including the insecticidal protein mentioned above. Accord¬ ingly, this composition may be useful in controlling a wide range of pests and vermins. Cells which are useful in the pesticidally active composition may suitably contain a gene coding for the active gene product which are isolated from naturally occurring organisms producing or having a pestici- dal activity. The pesticidally active gene products coded for by the cells of the composition may be gene products which have a toxic effect on a pest or vermin, or the gene products may be a pest pathogenic virus which are expressed in the cells of th composition. The cells which in accordance with the present invention may be used in this composition include ce ls in which the gene coding for a desired pesticidally active gene product can be expressed. Such cells include bacterial and fungal cells and plant and animal cells, optionally grown as cell cultures. The pesticidally active composition may further comprise a suitable carrier. In this context, the term "suitable car- rier" is used to indicate that the carrier comprises com¬ pounds which enhance the spreading of the composition in the pest infestated environment such as bulking agents which do not limit the survival of the cells in the composition, and compounds which may ensure the maintenance of the viability of the cells during production and storage of the composition and optionally also after application to the environment. As an example, a carrier may contain a compound which protect against the ultraviolet light which is detrimental to many organisms. An environmental pollutant-degrading composition The environmental pollutant-degrading composition as defined herein may preferably contain cells wherein the gene coding for the cell function-limiting hydrolytically active enzyme is only expressed when the pollutant degradable by the pollu¬ tant-degrading gene product is substantially degraded. As an example, such a composition may contain cells in which the expression of the hydrolytically active enzyme is repressed in the presence of the degradable pollutant. DESCRIPTION OF THE DRAWINGS The invention is further explained below with reference to the drawing in which Fig. 1 shows a segment of plasmid pSM1088 showing an oligo- nucleotide linker comprising an EagI site inserted between the BamHl site and the Sail site of pUHE24-2. Fig. 2 shows a map of plasmid pSM1088. Fig. 3 shows a map of plasmid pSM1093. Fig. 4 shows the action of mutated nuclease ( utl) in E. coli S17.1 containing pSM1093-l: 1.5 ml culture was harvested and subjected to total DNA preparation. Lane 1, sample taken prior to IPTG addition; lanes 2-4, 30, 120 and 270 minutes after addition of IPTG; lanes 5-6, 30 and 60 minutes after IPTG addition with samples to which were added 0.5 μg of pUC18 DNA prior to DNA preparation; lane 7, lambda DNA di¬ gested with Hindlll. Fig. 5 shows a schematic representation of pCK28 which, when present in a cell with pCK24 from which the lacIO repressor gene has been deleted as the result of homologous recombi- nation, may express a killing function (gef gene product) . Fig. 6A shows the rate of recombination of pCK24 and pCK25, both contained in stationary phase E. coli CSH36, grown at 37°C on LB plates containing 100 μg/ml ampicillin. Cells were harvested at the indicated times and retransformed into E coli CKE95, a recA deficient mutant of CSH36. Plasmids from retransformants were digested with Pstl. The resulting frag¬ ments were analyzed in a gel, and the relative intensities of the resulting bands was calculated as a measure of the per¬ centage of plasmids with recombinational deletions. Fig. 6B shows the rapid progression in plasmid recombination in growing E. coli CSH36. The plasmid bearing strain was reinoculated once a day by a 1:100 inoculum in LB media containing 100 μg/ml ampicillin, and samples were withdrawn for DNA preparation with intervals of several days. The DNA was restricted with PstI and analysed in a gel and the rela¬ tive intensities were calculated as the relative percentages of plasmids with recombitional deletions. Fig. 7 shows a schematic representation of the construction of plasmid pCK79. Fig. 8 shows a schematic representation of the construction of plasmid pCK80. Fig. 9 shows a schematic representation of the plasmids pCK80, pCK81, pCK82 and pCK95. Fig. 10 shows a schematic representation of plasmids pCK72, pCK83 and plasmid pCK30 and the recombination plasmids pCK32, pCK70 and pCK71, derived from pCK30. Fig. 11 shows the growth and phenotypic expression of the pUHE24-2 derived staphylococcal nuclease gene without its signal sequence in E. coli JM109 containing pSNUC-1 with (right plate) and without (left plate) induction with IPTG. Fig. 12 shows the growth and phenotypic expression of the pUHE24-2 staphylococcal nuclease gene without its signal sequence in E. coli JM109 containing pSNUC-3 with (right plate) and without (left plate) induction with IPTG. Fig. 13 shows a polyacrylamide gel electrophoresis analysis of the pUHE24-2 nuclease gene without its signal sequence cloned in the pUHE24-2 plasmid vector. Lane 1, 123 bp DNA ladder as size standard; lane 2, lambda DNA digested with Hindlll restriction enzyme as size standard; lanes 3 and 5, pUHE24-2 digested with Hindlll; lane 4, plasmid pSNUC-1 digested with Hindlll; lane 6, PCR-amplified nuclease gene fragment from pFOG 408 using L-SNUS and R-SNUC primers. Fig. 14A shows the kinetics of induction of E. coli JM109 (pSNUC-1) in liquid culture with and without IPTG (OD 450 ) . Fig. 14B shows log viable plate counts of E. coli JM109 (pSNUC-1) with or without IPTG. Fig. 15A shows kinetics of induction of E. coli JM109 (pSNUC- 3) in liquid culture with and without IPTG (OD 450 ) . Fig. 15B shows log viable plate counts of E. coli JM109 (pSNUC-3) with or without IPTG. Fig. 16 shows SDS-PAGE analysis of whole cell protein extract from E. coli JM109 (pSNUC-1) and E. coli JM109 (pSNUC-3) cultures with or without IPTG. The gel was stained with commassie blue. Total cellular protein from E. coli AR120 (pFOF408) was used as a positive control and E. coli (pUHE24- 2) was used as a negative control. The nuclease bands are indicated with arrows. Lane 1, protein size standard; lane 2, pSNUC-1 without IPTG; lane 3, pSNUC-1 with IPTG; lane 4, pSNUC-3 without IPTG; lane 5, protein size standard; lane 6, pSNUC-3 with IPTG; lane 7, pFOG408 as positive control; lane 8, pUHE24-2 as negative control. Fig. 17 shows a Western blot analysis of cellular proteins from cultures of E. coli JM109 (pSNUC-1) and E. coli JM109 (pSNUC-3) with or without IPTG. E. coli AR120 (pFOF408) was used as a positive control and E. coli JM109 (pUHE24-2) was used as a negative control. The 17.8 kD nuclease protein bands are indicated with arrows. Lane 1, protein size stan¬ dard; lane 2, pSNUC-1 without IPTG; lane 3, pSNUC-1 with IPTG; lane 4,-pSNUC-3 without IPTG; lane 5, pSNUC-3 with IPTG; lane 6, pFOG408 as positive control; lane 7, pUHE24-2 as negative control. Fig. 18 shows a schematic representation of the construction of plasmid pMG323. Fig. 19 shows viable counts (V.C.) of E. coli CSH50 trans¬ formed with pMG317 grown in LB medium supplemented with 100 μg/ml of ampicillin under phlA inducing conditions (41°C) and non-inducing conditions (37°C) . Fig. 20 shows the construction of the plasmids pMG317, pMG323 and pMG323/33. Enzyme abbreviations: Bl BamHl, B2 Bglll, El EcoRl. H3 Hindlll, Kp Kpnl, Sm Smal. Sc Seal, Sp SphI, Ss Sstl. Genes: bla and nm encoding resistance to ampicillin and kanamycin, respectively, ori replication origin, rep replica¬ tion functions, mob conjugative transfer, CI857 temperature sensitive lambda repressor, pJR lambda promoter region, laclq lactose repressor, p_tac tac promoter. Heavy line is Serratia DNA. Fig. 21 shows a schematic representation of the resolvase- mediated recombination reaction. Plasmid pCK155 was converted to two other species, pCK155delta and pCK15omega. The resul¬ ting fragments resulting from cutting with restriction enzyme Ndel are shown for each of the plasmid configurations. Fig. 22 shows an overview of the plasmids pCK151, pCK155 and pCK168. Unique restriction sites are shown. Restriction sites in parentheses indicate filled-in (non-functional) sites. The abbreviation T indicates the rpoCt' terminator. Plasmids pCK151 and pCK155 have a replicon derived from pUC18, whereas pCK168 is derived from pACYC184. Fig. 23 shows a detailed map of pCK155. Fig. 24 shows the hybrid resolution site in pCK155delta resulting form the in vivo parA resolvase mediated recombi- nation. The sequenced part is indicated by 'seq' and the bases originating from RP4 are marked 'RP4' . Fig. 25 shows growth experiments using pCK155 and pCK 153 (control) . Viable counts are measured as plate counts from appropriate dilutions at the indicated points of time on either LB plates containing 50 μg/ml chloramphenicol and 100 μg/ml ampicillin, or 50 μg/ml chloramphenicol and 50 μg/ml kanamycin. Plates do not contain IPTG. Fig. 26 shows in vitro titration assay of resolvase reaction (0.7% agarose gel). Lane 1 contains 110 ng of pCK155delta (0) . Lanes 2-11 contain 110 ng of pCK155 treated with the indicated amounts of resolvase (5-1200 ng) for 30 minutes at 37°C . All reactions are treated with restriction enzyme Ndel for l hour prior to applying the samples to the gel. The band at 4459 bp represents the unresolved plasmid pCKl55 and the bands at 2939 bp and 1919 bp, respectively represent plas- mids, pCK155delta and pCK15omega, respectively. Fig. 27 show scanning curves corresponding to bands illu¬ strated in Fig. 26. Absorption units are arbitrary. Fig. 28 shows in vitro titration of resolvase reaction. The percentage of reacted plasmid is defined as the area of the peaks shown in Fig. 27 from scans of pCK155delta and pCK15- omega divided by the respective fragment lengths (2939 bp and 1919 bp) relative to the area of the major peak from pCK155 divided by the length of the pCK155 Ndel major fragment (4459 bp). Fig 29 shows the kinetics of resolution reaction in vitro. 110 ng of pCK155 was treated with the two indicated amounts of resolvase and the reaction was stopped by heat inactiva- tion at the indicated times. EXAMPLE 1 Construction of a gene for a cytoplasmic nuclease The gene encoding the extracellular nuclease from Serratia marcescens has been cloned and sequenced. It has been shown that this nuclease also when expressed in E. coli is partly extracellular, partly periplasmic. The sequence of the gene indicated that the transport of the protein was mediated by a signal peptide recognised in the N-terminal end of the pro¬ tein, and determination of the N-terminal amino acid sequence from the mature excreted nuclease verified that a signal peptide was removed from the proprotein during its transport. Hence, the active enzyme normally found after expression and excretion in either S. arcescens or E. coli is a processed form of the protein. In order to assess the biological activity of the S. arces¬ cens nuclease intracellularly, a plasmid carrying an indu¬ cible promoter fused to the nuclease coding sequence repre¬ senting the mature, processed part of the protein, was constructed. This construction was facilitated by the pre- sence of an EagI site at the border between the signal se¬ quence and the sequence coding for the mature nuclease. In addition, an EagI site was present just downstream of the C- -terminal nuclease coding sequence. Thus, an EagI fragment encoding the entire mature part of the nuclease could be isolated after restriction of a plasmid like pNU121-nuc+ disclosed in WO 86/06743, encoding the total nuclease precur¬ sor protein. The inducible promoter system chosen for the following con¬ struction was the plasmid, pUHE24-2, which carries a synthe- tic lac operator-promoter that is highly repressed in the absence of inducer, and extremely efficient in transcription after induction with IPTG. The plasmid also carries an effi¬ cient ribosome binding site coupled to a translational start codon enclosed in a multiple cloning site. There is no EagI site present in pUHE24-2. For the insertion of the EagI fragment carrying the nuclease gene minus its signal sequence, an oligonucleotide linker comprising an EagI site was inserted between the Ba Hl site and the Sail site of pUHE24-2, cf. Fig. 1. Subsequently, the purified EagI fragment encoding the nuclease mature protein was ligated to the new pUHE24-2 derivative restricted with EagI, and transformants of strain JM105 comprising the lacl q gene were tested for viability on plates containing IPTG. Several clones were identified which exhibited poor growth on IPTG plates compared to control colonies harbouring no nu¬ clease gene. Mapping the plasmid isolated from such clones showed that the nuclease gene had indeed been inserted in the correct orientation, and as is shown in Fig. 2 such an inser¬ tion leads to an in-frame fusion between the start codon of the expression vector and the nuclease sequence of the EagI fragment. In the following the nuclease gene with the in¬ dicated modifications will be referred to as the nuc* gene, and the resulting plasmid described here was designated PSM1088. The plasmid pSM1088 contained in E. coliK12 JM105 was deposi¬ ted on 21 August, 1991 with Deutsche Sammlung von Mikroorga- nismen und Zellkulturen GmbH (German Collection of Microor- ganisms and Cell Cultures) , Mascheroder Weg IB, 3300 Braunschweig, Germany. EXAMPLE 2 Insertion of the nuc* gene in Tn5 In order to obtain easy ways of transferring the constructed nuc* gene to the chromosomes of other bacteria, the gene with its expression promoter (from pUHE24-2) was inserted together with a gene encoding the lac repressor in Tn5 on a mobili- zable plasmid. The first vector used was the pBR325 derivative, pSUP202 (Simon et al. , 1983, Biotechnology, JL, 784-790) which has an insertion of the mob site from plasmid RP4 upstream of the tet gene. From strains of E. coli such as strain S17.1 which has RP4 integrated in the chromosome, pSUP202 may be mobi¬ lized to a broad spectrum of bacteria. The following modifi¬ cations of plasmid pSUP202 were carried out: i) The plasmid was restricted with EcoRl. the single strand ends filled in with the Klenow fragment of DNA polyme¬ rase, the DNA ligated and finally transformed to MT102. Selection was for resistance to ampicillin, and colonies were screened for loss of chloramphenicol resistance (the EcoRl site is in the cat gene) . The resulting plasmid was designated pSM865. ii) Plasmid pSM865 was restricted with the enzymes BamHl and Sail, the single stranded ends filled in with Klenow enzyme, the DNA ligated and finally transformed to EL. coliK12 MC1000 (Simon et al., supra) . Selection was for resistance to ampicillin, and colonies were screened for loss of tetracycline resistance (the chosen restriction sites are located in the tet gene) . The resulting plasmid was designated pSM878. iii) The transposon Tn5 was inserted in pSM878 through infection of MC1000 (pSM878) with lambda phage carrying Tn5. After selection of kanamycin resistant colonies, they were pooled, plasmid DNA was prepared and subse¬ quently transformed into a plasmid-free MC1000 strain, again selecting for kanamycin resistance. The trans- formants were screened for loss of ampicillin resis¬ tance, thus identifying inserts of the transposon in the bla gene. Verification of this was done by standard mapping of the resulting plasmid which was designated PSM890. iv) The lacl q gene was inserted in Tn5 in pSM890. Plasmid pTTQ19 was restricted with Eco0109, and a Sail fragment was linker-inserted. From the resulting plasmid, the lacl q gene was isolated as a Sail-Seal fragment, which was ligated to pSM890 restricted with the enzymes Sail and Smal. The resulting plasmid harbours the lacl q gene in Tn5, and it was designated pSM1014. v) Plasmid pSM1088 was restricted with the enzymes Xhol and Sail and plasmid pSM1014 was restricted with Sail. The two restricted DNAs were mixed, ligated and transformed to MC1000 with selection for kanamycin resistance. Colo¬ nies were screened for reduced growth on plates con- taining IPTG, and resulting plasmids were analyzed and mapped. Such a plasmid with insertions within Tn5 of the nuc* gene combined with the synthetic lac promoter from pUHE24-2 and the lacl g gene, was designated pSM1093. The map of the plasmid is shown in Fig. 3. EXAMPLE 3 Immunological assay for intracellular nuclease To a culture of MC1000 (pSM1093) growing exponentially in LB medium was added 1 mM IPTG at an OD 450 of 0.2. After 30 minutes of continued growth of the culture, chloramphenicol was added at a concentration of 100 μg/ml, and incubation was continued for another 60 min. Samples of 5 ml were taken before addition of IPTG (uninduced control) 30 min after IPTG addition, and at different points of time after the addition of chloramphenicol. The addition of chloramphenicol allowed an assessment of the stability of the nuclease in the cy¬ toplasm of the cells. The cells were collected from the 5 ml samples by centrifugation and resuspended in 0.5 ml TE buf- fer. These 10 fold concentrated cell samples were sonicated until clarified, and debris spun out. The amounts of nuclease present in the cell extracts were determined by the ELISA method essentially as described in Example 14 below, using preadsorbed polyclonal antibodies raised against the purified extracellular nuclease in rabbits. The data in Table 1, which represent the ratio between OD 492 ELISA measurements and OD 260 measurement data for a standard nuclease are typical for such an experiment, and they show that nuclease protein encoded by the nuc* gene of pSM1093 was expressed in significant amounts as a consequence of induc¬ tion. However, the protein was fairly unstable, and about 60 minutes after arrest of protein synthesis (achieved by the addition of chloramphenicol) there was no detectable nuclease protein left in the cells. Table 1. The inhibition of growth of E. coli strains con¬ taining pSM1093 after induction with IPTG and arrest of protein synthesis by addition of chloramphenicol Minutes MC1000 SM825 SM1067 0 1.5 1.3 1.1 30 4.1 3.6 2.5 60 0.88 3.9 1.2 90 0.85 2.2 1.2 MC1000: E. coli. SM825: Enterobacter cloacae, SM1067: Pseudo- monas fluorescens. EXAMPLE 4 Activity assay for intracellular nuclease The demonstration of immunologically reactive nuclease as described in Example 3 only showed that the protein was expressed; whether this protein had maintained its enzymatic activity intracellularly remained to be demonstrated. Induction of nuclease synthesis in a culture of cells of MC1000 (pSM1093) was accomplished by the addition of 1 mM IPTG as described above, and after 30 minutes of incubation time the experiment was terminated. Samples were taken before and after the addition of IPTG and total DNA was prepared from the collected cells using a method employing high con¬ centrations of EDTA in order to prevent or greatly reduce nuclease mediated hydrolysis of the DNA after lysis of the cells. As a control of such post-lysis nucleolytic degrada¬ tion of the DNA an aliquot of previously purified plasmid DNA was added to one of the DNA preparations before the beginning of the lysis procedure. The presence of circular plasmid DNA after preparation of total DNA was taken as a strong indica- tion that no degradation of DNA took place after harvest of the cells. The results of-such an experiment are shown in Fig. 4. In the sample taken before induction of the nuc* gene the DNA appears as homogeneous undigested bands. In contrast, the DNA prepared from the sample taken after IPTG induction displayed a very heterogeneous appearance (smear) showing a high degree of degradation, and since the plasmid DNA added to such an induced sample retained its homogeneity (no degradation) , the digestion of DNA after induction of the nuc* gene must have taken place inside the cells, intracellularly. The expressed protein thus had enzymatic activity. EXAMPLE 5 Transfer of the nuc* system to other bacterial species In plasmid pSM1093 the IPTG inducible nuc* gene was placed within the Tn5 transposon on a mobilizable pBR322 derivative plasmid. Therefore, it should be possible to introduce the transposon comprising the nuc* gene in other bacteria by conjugational mating using, E. coli as the donor strain. In bacteria allowing the pBR322 plasmid to replicate (Enterobac- teria) the transconjugants will carry the transposon on the donor plasmid just as in E. coli; in other bacteria (e.g. Pseudomonads) pBR322 plasmids do not replicate, and therefore the only possibility of obtaining transposon mediated antibi¬ otic resistance in these species is transposition to the chromosome or to resident plasmids in the recipient organism. Conjugal matings were performed by mixing suspended donor cells and recipient cells on LB plates without antibiotics followed by overnight incubation at 30°C. From the colony of mixed cells, aliquots were streaked onto selective plates (selection for kanamycin resistance of the transposon and for the antibiotic resistance marker of the recipient organism, usually rifampicin) . Colonies appearing after 1-3 days in¬ cubation were restreaked on selective plates and tested for specific recipient characteristics to ensure that transfer had actually taken place. The resulting transconjugant strains were finally tested for viability on plates containing IPTG. Induction of the nuc* gene in these various bacteria resulted in growth inhibition in several of these, but a few bacteria were apparently resistant to induction. One species, Enterobacter cloacae, showed higher sensitivity than E. coli. In order to investigate the cause of the observed differences of sensitivity to the induction in different bacteria, measurements of the induction levels of nuclease protein were performed as ELISA assays as described in Example 3. The results summarized in Table 1 above showed that there was an excellent correlation between the level of induced nuclease protein and the degree of growth inhibition for each bac- terial species. It should also be noted that in most of the bacteria the nuclease is as unstable as in E. coli; however, in Enterobacter cloacae the nuclease is more stable, and therefore higher levels of the enzyme are obtainable intra¬ cellularly in agreement with the increased growth inhibition observed in this organism. These results showed that growth inhibition after induction of the nuc* gene varied with the intracellular concentration of the nuclease. When this con¬ centration exceeds a certain level, all bacterial species seem to be equally sensitive to the presence of the enzyme, which again indicates that DNA damages occurring with rates below a threshold value are repaired with equal efficiency in all tested bacteria. EXAMPLE 6 Optimization of the nuc* activity From the characterization of the nuc* expression system as described for plasmid pSM1093 it is apparent that more effi¬ cient killing systems may be derived by mutation of the plasmid. In principle, higher nuclease activities in the cells can be obtained either by increasing the protein con- centration after induction or by increasing the specific activity of the enzyme. Increased protein concentrations may be a consequence of increased gene dosage, increased gene expression levels or increased stability of the protein; increased specific activity requires mutation(s) in the structural gene for the nuclease. A mutant screening programme was designed with the purpose of isolating any type of mutation causing increased growth inhibition after induction of the nuc* gene. The basis for the screening was the observation that E. coli cells har¬ bouring pSM1093 are able to form small colonies on IPTG containing plates due to residual growth after induction. Therefore, mutants with increased intracellular nuclease activity would be expected to exhibit more severe growth inhibition on such plates. Due to the significance of the possibility of transfer of the nuc* system to other bacteria, it was of importance to ensure that mutations leading to optimization were located in the plasmid and not in the chromosomal genes of E. coli (e.g. repair genes or genes encoding proteases responsible for the instability of the nuclease) . Therefore, mutagenization was performed directly on purified plasmid pSM1093 DNA using the hydroxyl amine method according to Humphreys et al. , 1976, Mol. Gen. Genet., 145. 101. This method comprises the initial preparation of a reaction mixture containing 20 μl of pure isolated DNA (3 μg) , 100 μl of sodium phosphate buffer (0.1M, pH 6.0) containing InM of EDTA and 80 μl of NH 2 0H/HC1 (1M, pH adjusted to 6.0 with NaOH) containing ImM of EDTA. This mixture was incubated at 70°C for 45 minutes followed by dialyzing against DNA dialysis buffer at 2°C overnight. The thus treated DNA was subsequently transformed to a non- mutagenized E. coli strain (MC1000) . After colonies had appeared on the selective plates (LB + kanamycin) they were replica plated onto plates containing also IPTG for induction of the nuc* gene. After 5-6 hours of incubation at 37°C the replicas were inspected for growth, and colonies showing no or very poor growth on the IPTG containing plates were re- streaked from the master plate (without IPTG) on selective plates and on selective plates containing IPTG and compared with similar streakings of the wild-type pSM1093 strain. From one cycle of mutagenesis 7 clones were isolated which all showed increased growth inhibition of E. coli on plates containing IPTG. A series of tests were performed in order to characterize the mutated plasmids and possibly reveal the nature of the muta¬ tion(s) . Using methods and assays already described in the previous examples, the isolated mutants were tested for expression levels of the nuclease protein (ELISA) , for stabi¬ lity of the enzyme (ELISA and nuclease activity, spot tests) , and for the levels of intracellular nuclease activity (spot tests) . In addition, plasmid DNA from the isolated mutant clones was purified and relative amounts of these in the cells and restriction enzyme profiles were determined from inspection of agarose gels. All mutants showed increased amounts of protein and increased nuclease activity, and the correlation of the two parameters indicated that increased expression of the enzyme occurred in all cases. No mutation has caused increased stability of the protein which could be detected with the employed methods. Also, there was no in¬ dication of increased plasmid copy number of any of the mutants. Thus, a preliminary analysis of the mutants pointed at higher expression levels after induction as the most likely cause of increased growth inhibition. One mutant, pSM1093-l, ' which showed the highest level of growth inhibition and the highest level of intracellular nuclease after induction, was found to have an extra EcoRV restriction site relative to pSM1093. The position of this site coincides with the location of the lacl q gene, indicating that a mutated repressor responds more optimally to IPTG induction leading to higher post-induction rates of nuc* transcription. Two mutants were selected for further characterization, both of which represent the clones with the highest levels of nuclease activity. EXAMPLE 7 Comparison of the properties of PSM1093 and two mutant plas¬ mids The induced levels of nuclease protein (after 30 min induc- tion) as measured by ELISA essentially as described in Exam¬ ple 14 below, are shown in Table 3. The values indicated represent the ratio between OD 4g2 ELISA measurements and OD 260 measurement data for a standard muclease. The inducing agent, IPTG was added at 0 minutes and chloramphenicol was added after 30 minutes.The results clearly show that pSM1093 (wild- type) produces less enzyme than pSM1093-l. Although the initial rate of degradation of the nuclease from pSM1093-l seemed very high, the enzymes from the different strains are degraded with the same kinetics after addition of chloramphe- nicol to the cells. Table 3. Level of nuclease protein after induction with IPTG and addition of chloramphenicol encoded by mutant plasmid PSM1093-1 in MC1000 as compared to the parent plasmid Minutes pSM1093 pSM1093-l 30 54.7 119.2 60 31.6 35.8 The efficiency of growth inhibition and cell killing of the induced nuclease from pSM1093-l was studied in growth ex¬ periments. The results are shown in table 4. Table 4. The effect of growth on E. coli S17.1 of intracel¬ lular nuclease encoded by pSM1093-l Hours after Colony forming units per ml on LB with addition of Kanamycin Kanamycin + IPTG 2mM IPTG 0 1,2 x 10 1.3 x 10- ' 1.5 2.7 x 10 3.0 3.0 x 10 2.1 X 10 Overnight 1.0 x 10 E 8.7 x 10' The sample at 0 hour was taken prior to IPTG addition. The parent nuc* gene and the mutant derivative are both positioned within the Tn5 transposon, and it is therefore possible to transfer them to other bacterial species as described in Example 11 below. Each of the three plasmids described in Example 10 below were conjugated from E. coli S17.1 (donor strain) to Pseudomonas putida and Pseudomonas fluorescens. respectively. After isolation of transconjugants and colony purification, a number of independent clones from each conjugation were tested for IPTG induced growth inhibition. For each specific combination of transposon and recipient there was no varia- tion, indicating that independent of the transposition to the recipient the effect of induction was the same. The general picture mimics that observed in E. coli: The mutant is more effective as growth inhibitor even in the Pseudomonads, than the wild type. EXAMPLE 8 Construction and testing of a model recombinational system A model plasmid for evaluation of the potentials for a sto¬ chastic induction system based on homologous recombination was constructed. The plasmid is based upon the well charac¬ terized plasmid pBR325 (Bolivar, 1978, Gene, 4., 121-136), where the chloramphenicol resistance gene is repeated. Between the two stretches of homologous DNA is inserted the lacIO gene from a commercial plasmid, pTTQ19, (Stark 1987, Gene, J51, 255-267) . If another gene, controlled by a promoter which is repressed by lad is residing in the same cell as is the model plasmid, mentioned above, (and the strain back¬ ground is lacl) - then an induction of that gene will occur when the last copy of the model plasmid is recombined as the Lacl repressor is lost in the process. The vector pBR325 typically exists in 50 copies per cell. A. Construction of the model plasmids pCK24 and pCK25 pBR325 was linearized with EcoRI and the sticky ends were filled in by means of the large fragment of the DNA polyme- rase I (Klenow fragment) . The plasmid was religated, resul¬ ting in pCK20, which confers ampicillin and tetracyclin resistance but not chloramphenicol resistance to the host cell. The same procedure was conducted on the plasmid pSUP202 (Simon et al., 1983, Biotechnology, l, 784-790), resulting in pSM865. The chloramphenicol gene was cloned from the central Bbvl fragment of Tn9, which was blunt ended. This fragment was inserted into the blunt ended Xbal site of pUC18 (Yanisch-Perron et al., 1985, Gene, J33., 103-109), giving pLKP46. The orientation of the fragment was determined by the internal EcoRI site which is proximal to the BamHl site of pUC18. Into the vectors pCK20 and pSM865, cut with BamHl and Sail, were inserted the BamHl/Sail fragment of pLKP46, giving the chloramphenicol resistant but tetracycline sensitive plasmids pCK22 and pCK23, respectively. The plasmid pHB103 comprises the lacIO gene from the plasmid pTTQ19. The excised LacIQ fragment is identical to the SspI/Ecoθl09I fragment with the following modifications: The ECQ0109I site was blunt ended and ligated to a Sail linker (New England Biolabs, Inc. #1027) and subsequently, to the small SalI-BamHl fragment of pKK223-3 (Brosius et al., 1984, Proc. Natl. Acad. Sci. U.S.A., . 81 / 6929-6933). Thus, the resulting lacIO fragment is residing on a approximately 1100 base pair Sspl/BamHl fragment. This fragment was inserted into pCK22 and pCK23 cut with EcoRV and BamHl. giving pCK24 and pCK25, respectively. These plasmids are the model plas¬ mids, and differs in that pCK25 additionally contains the mob gene (approximately 2000 base pairs) inserted between the two direct repeats, together with the LacIO gene. The stretches that are homologous in the two plasmids are approximately 900 base pairs, and the inter repeat stretches are approximately 1100 base pairs (pCK24) and approximately 3100 base pairs (pCK25) . An additional plasmid, containing the gef gene under the control of a Lacl repressible promoter, the trp-lac hybrid tac promoter (Amann, E. et al., 1983, Gene, .25, 167-178), was constructed. The plasmid pLKPllδ (Poulsen et al., 1989, Molec.Microbiol. , 3., 1463-1472) containing the gef gene, was digested with Hindlll and EcoRI and ligated to pKK233-3 opened in Hindlll and EcoRI. resulting in pHBlOl. The ap¬ proximately 500 base pair BamHl fragment of pHBlOl was in¬ serted into the BamHl site of pACYC177 (Chang et al., 1978, J.Bacteriol. , 134, 1141-1156), giving pCK28 (Fig. 5). B. Determination of the model plasmid physiology in station¬ ary phase and in an adapted continuous exponential growth phase The model plasmids pCK24 and pCK25 were transformed into the E. coli strain CSH36 (Cold Spring Harbour collection, USA) , which has a delta(lac-pro) deletion on the chromosome and further harbours the F-factor, containing the complete lac- operon, except for a functional lacl gene. The strain pheno- type is thus blue on plates containing 5-bromo-4-chloro-3- indolyl-β-D-galactopyranoside (X-gal) , provided no lacl gene is residing in the cell. The phenotype of the cells transformed with pCK24 or pCK25, when plated on X-gal plates were white, blue and blue sectored colonies, indicating that cells in the blue parts of the colonies had indeed deleted all the copies of the lacIQ in the model plasmids. When restreaking uniformly white colored colonies on X-gal plates, they too become sectored over night. Uniformly blue colored colonies, however, remained blue. From plates described above, several white colonies were picked and resuspended in 2 ml of isotonic NaCl solution. From this solution aliquots of 0.2 ml were distributed as one large drop onto LB-plates and incubated at 37°C. One aliquot was frozen immediately. The plates were then harvested on day one after inoculation and on the following days, with intervals of two or more days. The plates were harvested by adding 2 ml isotonic NaCl to the plates and resuspending the cells herein. The suspension was collected, spun and the resulting pellet resuspended in 0.2 ml NaCl. The harvested cells were kept frozen at -20°C until the end of the experiment. After approximately 15 days, the cells were thawed and used for a DNA mini-preparation (a.m. Birnboim, H.C. and J.Doly, 1979, Nucleic Acids Res., 1_, 1513-1523). For assaying the genotype of the entire plasmid population, a recA deficient variant of CSH36 was constructed: CSH36 was Pl transduced with a phage stock prepared from the delta- recA: :TnlO strain JC10284 (Czonka et al., 1979, Genetics, 93., 321-343), giving CKE95. Into CKE95 were transformed DNA samples from the plates described above, and the colony colour upon plating on X-gal was evaluated. The percentage of blue colonies correlated directly to the fraction of recombined plasmids in the corresponding DNA samples. When plotted (Fig. 6) a clear progression in the recombination could be recognized, in¬ dicating that the recombination system is active in statio¬ nary phase CSH36 cells. After 14 days, more than 80 per cent of the plasmids in the population were recombined. There was no significant difference between the behaviour of pCK24 and pCK25. The recombination rate was evaluated in a simulated conti¬ nuous exponential growth experiment, where CSH36+pCK24 and CSH36+pCK25 cells were grown i LB medium supplemented with 100 μg/ml ampicillin, at 37°C and shaking overnight and reinoculated in fresh LB medium every morning, using 1:100 inoculum. At the time of reinoculation, a fraction of the overnight culture was immediately used for a DNA mini-pre¬ paration. At the end of the experiment, the DNA samples was digested with PstI, which is unique in both model plasmids and located outside of the region which is deleted in the recombination process. The relative intensity of the two resulting bands from one DNA sample is a direct measure of the proportion of recombined to intact plasmid. One such experiment showed that the percentage of recombined pCK24 plasmids went from virtually nil to 90 per cent in six days. After nine days esssentially all plasmid in the culture had undergone rearrangement as assayed by gel-scanning. pCK25 behaved similarly. This indicates that the recombination system is active in exponentially growing cells (Fig 6B) . This experiment was also conducted in the common laboratory E. coli strains C600 (Bachmann, 1987, in Escherichia coli and Salmonella typhimurium ed. Neidhard, F.C. et al. ASM, pp. 1190-1219) and MC1000 (Silhavy et al., 1984, Experiments with Gene Fusions, Cold Spring Harbour Laboratory, New York, pp. xi-xii) . A progression of recombination could be observed in these strains as well, by visually inspecting DNA digest gels as described. C. Experiments using a lac-repressed gene r gef in trans. The CSH36 strains, harbouring pCK24 or pCK25 were transformed with pCK28, which contains the gef gene under control of the tac promoter on a compatible plasmid. When the last copy of lacIO is excised by recombination, the promoter is dere¬ pressed and expression of the toxic qef-protein commences, causing the host cell to die. By streaking CKE95/pCK24/pCK28 and CKE95/pCK25/pCK28 together with the appropriate controls (CKE95/pCK24/pACYC177 and CKE95/pCK25/pACYC177) on minimal plates containing either 0.2% glucose, 0.5% glycerol or 0.5% acetate as sole carbon source and supplemented with chloram- phenicol (50 μg/ml) and kanamycin (50 μg/ml) a significant growth inhibition could be observed for the strains contain¬ ing the gef-plasmid. The lesser the quality of the carbon source, i.e. the slower overall growth rate, the more severe is the inhibitory effect. This experiment indicates that the stochastic induction system can be used in the context of containment on a laboratory basis, as a setup as described confers a significant disadvantage to cells containing the gef plasmid. To test the combined strains further, the cell doubling time of CKE95/pCK24/pCK28 was compared to that of CKE95/- pCK25/pACYC177. The experiments were conducted in minimal medium containing 0.2% glucose supplemented with chloram¬ phenicol (50 μg/ml) and kanamycin (50 μg/ml) and with or without casamino acids. In minimal medium with casamino acids, the doubling time of the strain containing the gef- plasmid was 45 minutes as compared to 43 minutes for the control. In minimal medium without casamino acids the doubling times were 93 and 85 minutes, respectively. This experiment verified the tendency seen on the minimal plate assay above. EXAMPLE 9 Construction of plasmids for making up a toolbox for the construction of insertion plasmids containing a direct repeat cassette Brief overview of the components of the system: The system is based upon assembling two separately construc¬ ted plasmids: A) First half consists of a cassette with a promoter directed towards a sequence of DNA which is of variable length, typi- cally 600-1500 bps. This sequence is to be the actual repeated DNA. After this stretch of DNA is, optionally, a resistance factor, e.g. the chloramphenicol gene. Finally, hereafter, a transcription terminator is inserted 5' to the two restriction sites XhoI-SphI in that order. B) The second half consists of a sequence identical, at least in part, to the above-mentioned repeated sequence, followed, optionally, by a killing gene or an indicator gene (or nothing) . This cassette is flanked by unique Xhol and SphI sites, which enables this cassette to be inserted into the first half, described above. The complete "recombination cassette" is flanked by two Notl sites, which enables it to be excised and reinserted into a suicide delivery plasmid for integration of the cassette into the chromosome of the host cell by a Tn5 transposition mechanism. A. Construction of a series of recombination-cassette type plasmids with a 598 base pair repeat Two basic cloning vectors, pUC18Not (Herrero et al. 1990, J.Bacteriol. , 172. 6557-6567) and pUC19 (Yanisch-Perron et al. 1985, Gene, 3_3., 103-109) were modified by exchanging their polylinker sequences. Into the SacI/PstI cut pUC18Not was inserted the synthetic polylinker 5'-CTGCAGTCCCGGGTGTCGACAGATCTAGACATGCATCTCGAGTGCA (upper strand) , destroying the original PstI site but retaining the SacI site, resulting in pCK29. Likewise, the SacI/SphI cut pUC19 was ligated to the syn¬ thetic polylinker 5'-CTCGAGGATCCTCCCGGGAGATCTGCATG (upper strand) , retaining SacI as well as SphI, resulting in pCK30. pCK29 was cut with Nsil and a 250 bp PstI fragment from the plasmid pHBA102rpoCt, containing the rpoCt / transcription terminator (Squires et al. 1981, Nucleic Acid Res. , 9., 6827- 6δ39) was inserted, the internal Bgll site proximal to the Bglll site of pCK29, resulting in pCK45. The chloramphenicol resistance gene was cloned from the central Bbvl fragment of Tn9, which was blunt ended by means of the large fragment of the DNA polymerase I (Klenow frag¬ ment) . This fragment was inserted into the blunt ended Xbal site of pUClδ (Yanisch-Perron et al., supra) giving pLKP46. The orientation of the fragment was determined by the inter- nai EcoRI site which is proximal to the BamHl site of pUC18. pLKP46 was cut with Smal and Sail and the fragment containing the chloramphenicol resistance gene was inserted into the pCK45 linearised with Smal and Sail, giving pCK49. A 690 bp Bcll-Bglll fragment from the plasmid pKK3535 (Brosius et al. 1981, Plasmid, 6., 112-118) , containing part of the 16S rrnB gene, was inserted into the Bglll site of pCK45, with the Bglll site recreated proximal to the rpoCt' sequence of pCK45 described above, giving pCK68. pCK6δ was linearised with Sail and a 127 bp Xhol/Sall frag¬ ment from pUHE24-2 (Lanzer et al. 19δδ, Proc.Natl.Acad.Sci. USA, δ5, 6973-6977) containing the P A ι o 4 /o 3 promoter was inserted. The Sail site was recreated proximal to the inser¬ ted ribosomal seguence in pCK68, giving pCK75. pCK75 was cut with PstI and BG1II and the smaller fragment was inserted into pCK49 cut with PstI and BamHl, giving pCK79 (cf. Fig 7). The two plasmids pCK75 and pCK79 (cf. Fig xx) are the basic plasmids for constructing the recombination cassette (first half, 598 bp repeat) . pCK30 was cut with BamHl and Smal and a 59δ bp Bcll-Smal fragment from pKK3535 was inserted, resulting in pCK37 (Fig. δ) . Various plasmids containing genes to be expressed upon recom¬ bination were modified for use in the system. The plasmid pCB267, containing the lacZ gene (Schneider et al. 1987, Meth.Enzy ol. , 153. 452-461), was linearised with Eco0109I and blunt ended as described above, and ligated to a Ba Hl linker (New England Biolabs, Inc. #1021), resulting in pCK52. The plasmid pSM108δ, containing the nuclease gene (nuc*, nuclease gene devoid of its proprietary leader sequence (described in a previous example) , was treated likewise: the Hindlll and EcoRI sites were sequentially blunt ended and ligated to the BamHl linker described, resulting in pCK53 and pCK59, respectively. Into the Bglll site of pCK37 was inserted: a) the approximately 3000 base pair Bglll-BamHI fragment of pCK52, containing the lacZ gene, the Bglll site recreated distal to the SphI site of pCK37, giving pCK56. a) the approximately 3000 base pair Bglll-BamHI fragment of pCK52, containing the lacZ gene, the Bglll site recreated distal to the SphI site of pCK37, giving pCK56. b) the approximately 1100 bp BamHl fragment of pCK59, contai- ning the nuclease gene, the internal EcoRV site of the frag¬ ment distal to the SphI site of pCK37, giving pCK63. c) The approximately 400 base pair BamHl fragment from the plasmid pHBδ4, comprising the gef gene, the EcoRI site of the fragment distal to the SphI site of pCK37, giving pCK36 (fig. δ) . pHBδ4 was created by inserting a BamHl linker (New Eng¬ land Biolabs #1021) into the blunt ended EcoRI site of the plasmid pLKPllδ (Poulsen et al. 1969, Molec.Microbiol. , 3./ 1463-1472) . d) Nothing (i.e. pCK37 itself). These four plasmids were used as the basic plasmids in the construction of the recombination cassette (second half, 59δ bp, containing killing function, indicator gene or no mar¬ ker) . B. Assembly of the recombination cassette and making of inte- gration plasmids The XhoI/SphI fragments of the four basic plasmids described above was introduced into pCK79 cut with Xhol and SphI, resulting in the plasmids pCKδ2, pCK95, pCKδO and pCKδl, respectively (cf. Fig 9) . From these plasmids, the Notl fragments containing the recombination cassette was ligated into the unique Notl site of the suicide vectors pUT-mini- Tn5-Tc and pUT-mini-Tn5-Km (de Lorenzo et al. 1990, J.Bac- teriol. , 172, 6566-6572), giving the vectors: vector pUClδNot pUTTc, pUTKm p->DR-CmR-DR-ιgef pCK80 pCK84 pCK87 p->DR-CmR-DR- pCKδl pCK85 pCK88 p->DR-CmR-DR-LacZ pCK82 pCKδ6 pCK89 p->DR-CmR-DR-nuc* pCK95 pCK96 pCK97 The plasmid pCKδl contained in E. coliK12 MClOOOrecA was deposited on 5 July, 1991 with Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH (German Collection of Microorganisms and Cell Cultures) , Mascheroder Weg IB, 3300 Braunschweig, Germany. EXAMPLE 10 Construction of recombination plasmids with longer stretches of repeated DNA In order to provide a more versatile toolbox for future constructions, derivatives were made to facilitate easy and fast modification of the main system parameter: repeat length. A long stretch of rrnB-DNA was inserted into the vectors pCK29 and pCK75. By cutting at a desired position in the rrnB and, if necessary, blunt end the opened vector, and following cutting at the unique PstI site, the obtained fragment can be inserted into pCK45 or pCK49, opened in PstI/Smal. Likewise a series of plasmids containing the gef. nuc* and LacZ. but without a rrnB sequence was created. In these plasmids, the desired length repeat is excised directly from pKK3535 as a blunt end to Bell fragment and subsequently inserted into the plasmids. To obtain the plasmids for the first half of the system (direct repeat I) , pCK29 and pCK75 were opened in Bglll and SphI. Into pCK29 the 3196 base pair Bell/SphI fragment of pKK3535 was inserted, giving pCK72. The 2506 base pair Bglll/SphI fragment of pKK3535 was inserted into pCK75 (which already contains part of the rrnB sequence), giving pCK83. Into pCK30 linearised at the Bglll site the following frag¬ ments were inserted (cf. Fig. 10) : a) the Bglll/BamHl fragment of pCK52 containing the LacZ gene, giving pCK71, the recreated Bglll site being proximal to the Smal site of pCK30. b) the BamHl fragment of pCK59 containing the nuc* gene, giving pCK70, the internal EcoRV site proximal to the Smal site of pCK30. c) the BamHl fragment of pHB84 containing the gef gene, giving pCK32, the internal EcoRI site proximal to the Smal site of pCK30. d) nothing (i.e. pCK30 itself) . EXAMPLE 11 Construction of strains of different genera by insertion of the pUT-derivatives By using the pUT-based vectors, the constructs can be inser¬ ted into the chromosome of virtually any gram-negative bac- terium, thus obtaining a stable maintenance of the cassette as it is replicated together with the host chromosomal DNA. This integration feature makes the system independent of the requirement of compatibility of vector/host relations. The vectors pCK84-89 were successfully integrated into the chro- mosome of Pseudomonas putida and Pseudomonas fluorescens. The pUT plasmids require a pir-protein to replicate. The replica¬ tion origin of the pUT plasmids is derived from the broad host range plasmid RP4 which also encodes the pir-protein. The pUT plasmids do not encode the pir-protein and the pro- tein is thus required in trans if the plasmid is to exist extrachromosomally. A derivative of the E. coli strain S17.1 (Simon et al., 1983, Biotechnology, JL, 784-790) into the chromosome of which the lambda phage containing the pir gene has been integrated, was used for conjugal transfer of the plasmids to other species. The S17.1 strain containing the appropriated plasmid was mixed with recipient strain on an LB agar plate and the mixture was left for 1-2 days at a tem¬ perature not exceeding 30°C to prevent the lambda phage from entering the lytic phase. Exconjugants were subsequently selected on plates which are selective for the recipient strain (e.g. containing rifam- picin) and the transposon part of the integration plasmid (tetracycline or kanamycin) . The integration was verified by plasmid DNA preparation from exconjugants. EXAMPLE 12 Preliminary experiments with the pCK recombination plasmids pCK82 was transformed into MC1000 (rec-proficient, delta- lacX74) and the transformation mixture was streaked onto LB agar plates containing 100 μg/ml of ampicillin and 50 μg/ml X-gal. After a few days, several blue colonies could be detected and additionally, several blue-sectored colonies which was an indication that the recombination system was functional as expected. The plasmids pCK80-82 and pCK79 (as a control) were trans¬ formed into MC1000 and one colony of each type was streaked onto ABTG plates or ABT+0.5% glycerol plates supplemented with 20 mg/L leucine and 100 μg/ml of ampicillin. On the ABT+glycerol plates the cells containing the pCKδO (gef) grew more slowly than the three other strains. On ABTG, no dif¬ ference was recognizable. pCK86 and pCK89 were integrated into the chromosome of Pseu¬ domonas putida and when streaked onto LB plates containing 50 μg/ml X-gal the host cells gave initially white colonies but distinctly blue colonies after a few days of incubation. The 600 base pair repeat and the effect of integrating the cassette into the host cell chromosome lowers the recombi¬ nation frequency significantly as indicated by the lower ratio between blue and white colonies as compared to having the cassette located on an extrachromosomal plasmid. EXAMPLE 13 Intracellular expression and development of a conditional cell function-limiting system in Escherichia coli using a nuclease gene from Staphylococcus aureus A. Bacterial strains, plasmids, oligonucleotides used: E. coli JM109 was purchased from Invitrogen Inc., San Diego, CA. Plasmid pFOG 408 containing the nuclease gene of Staphy- lococcus aureus Foggi strain was a gift from A. Meeker. The cloning vector plasmid pUHE24-2 was obtained from S. Molin. The oligonucleotides for PCR amplification were custom- synthesized by GENOSYS Inc. , USA. B. PCR amplification of the nuclease gene The nuclease gene from the plasmid pFOG 408 without its signal sequence was PCR amplified using two oligonucleotide primers L-SNUC and R-SNUC. The primer L-SNUC was 37 nucleo- tides long (5'GATCCGGATCCGCAACTTCAACTAAAAAATTACATAA-3') with a 11 nucleotide overhang at the 5'-end consisting of BamHl, AccII, and Hpall restriction endonuclease sites. The primer R-SNUC was 33 nucleotides long (5'-GGTACCGGAATTCGTGCCACTAGCAGCAGTGAC-3') with EcoRI, Hpal. and Kpnl restriction endonuclease sites. PCR amplifications were performed in a DNA thermal cycler 480 (Perkin Elmer Cetus) using AmpliTaq DNA polymerase (2.5 units) (Perkin Elmer Cetus), 200 μM of each of.the dNTPs (Pharmacia), PCR reaction buffer (10X reaction buffer contained 500 mM Tris.HCl [pH 8.9], 500 mM KCl, and 25 mM MGC1 2 ) , and 0.5 μM of each of the primers. The PCR amplification was performed for a total of 25 cycles, each cycle consisting of 1 min denaturation at 94°C, l min primer annealing of 55°C, and 1 min primer extension at 72°C. Successful PCR amplification of a 0.507 kb DNA fragment was determined by running 1% Seakem agarose gel (FMC Bioproducts) , stained with EtBr, and visualized under a UV transilluminator. PCR amplification using the primers L-SNUC and R-SNUC pro¬ duced a single DNA band of 0.507 kb in size indicating no non-specific priming. The upstream primer L-SNUC was designed precisely so that it would prime the first nucleotide of the first amino acid residue after the signal sequence of the gene (cf. the sequence defined above) . Moreover, a 11 bp overhang at the 5'-end of the primer ensured the synthesis of the gene from the desired nucleotide; thus eliminating the possibility of frameshift mutation during PCR amplification. The downstream primer R-SNUC was located further downstream of the stop codon and the possible hairpin structure outside the coding sequence of the gene, ensuring that all the amino acid including the stop codon of the gene remain intact and functional. C. Cloning of the amplified nuclease gene The amplified DNA of the staphylococcal nuclease gene without its signal sequence was end-repaired with DNA poll Klenow fragment to create blunt ends as described by Ausubel et al (1987) . The end-repaired blunt ended amplified nuclease gene was purified by using a Centricon 100 microconcentrator (Amicon, MA) . pUHE24-2 was linearized with BamHl, end-re¬ paired to create blunt-ends and purified by following the procedures described by Ausubel et al (19δ7) . Cloning and transformation of the cloned DNA into E. coli JM109 was performed as described by Ausubel et al (1987) . The trans¬ formed colonies were screened for the appropriate clones by replica plating on DNAse test agar (Difco) plates containing methyl green as indicator dye, 40 μg ampicillin per ml, 1 mM Isopropyl-β-D-thigalactopyranoside (IPTG), 10 mM CaCl 2 , and 1 mM MgCl 2 . Colonies which produced indicator colour were further tested for clones by restriction analysis, PCR amplification, and DNA sequence analysis. Plasmid DNAs from the putative clones, designated as pSNUC-1 and pSNUC-3 were isolated and purified by alkaline lysis method as described by Ausubel et al (1987) . The isolated plasmid DNAs were digested with Hindlll restriction enzyme (US Biochemicals) according to the manu- facturer. The restriction enzyme digested plasmid DNAs were analyzed by 10% polyacrylamide gel electrophoresis, stained with EtBr, and visualized by a UV transilluminator. The nucleotide sequence analysis of the cloned fragments in both plasmids, pSNUC-1 and pSNUC-3, were determined by Sanger dideoxy method (Sanger et al, 1977) using a Sequenase sequencing kit (US Biochemicals) and the L-SNUC primer. These two clones were further characterized for induction and expression of the nuclease. Also they were tested for their ability to grow on LB agar plates containing ampicillin (40 μg per ml) , 10 mM CaCl 2 , 1 mM MgCl 2 , with or without 1 mM IPTG. The transformed cells were initially selected on LB agar plates containing 40 μg per ml ampicillin. Two putative clones which showed indicator colour change on DNAse test agar plate, were further tested on LB agar containing ampi¬ cillin, CaCl 2 , MgCl 2 , and IPTG. E. coli JM109 containing pSNUC-1 showed no growth on agar plate containing IPTG even after incubation up to 72 hours (Fig. 11) . In contrast to this, E. coli JM109 carrying pSNUC-3 showed inhibited growth on similar type of agar plate containing IPTG (Fig. 12) . In both cases, the control LB agar plates containing CaCl 2 and MgCl 2 , but no IPTG showed complete growth within an 18-24 hours' time period. In this experiment, the inoculum sizes were kept as equal as possible for both types of plates, and they showed consistent results every time when repeated for a total of 7 times. Restriction analysis of the plasmids pSNUC-1 and pSNUC-3 with Hindlll produced expected DNA bands of approximately 0.219 kb and 4.0 kb bands (Fig. 13). Since there was one Hindlll restriction site within the coding sequence of the nuclease gene and one on the vector, it was possible to determine the right orientation of the nuclease gene on the vector. Nucleotide sequence analysis of the first 40 bases of the cloned fragments in both pSNUC-1 and pSNUC-3 plasmids at their 5'-ends confirmed the presence of the nuclease gene in the right orientation. D. Induction of PSNUC-1 and pSNUC-3 The E. coli JM109 (pSNUC-l) and E. coli JM109 (pSNUC-3) were grown in LB broth containing 10 mM CaCl 2 and 1 mM MgCl 2 at 37°C for 1 hour with shaking at 100 rpm till the optical density (OD 450 ) reached 0.5-0.7 when 1 mM sterile IPTG solu¬ tion was added for induction. Before and after the addition of IPTG the OD 450 was recorded every hour, a fraction of the cultures was serially diluted in phosphate buffer (pH 7.2) and plated on LB agar plates with 10 mM CaCl 2 and 1 mM MgCl to determine viable plate counts. A control culture of each of the plasmid constructs was kept to compare the expression and killing efficiencies of the nuclease gene. E. Expression of the cloned nuclease gene following induction E. coli JM109 (pSNUC-l) and E. coli JM109 (pSNUC-3) were grown in LB broth as described earlier to mid-exponential phase (OD 450 of 0.4 to 0.5) and IPTG was added for induction. The induction was carried on for 20 min when the cells were centrifuged and prepared for the total protein analysis by SDS-PAGE as described by Ausubel et al (1987) using the Mini- Protean II gel system (BioRad) . The PAGE separated proteins were stained with commassie blue. Uninduced cultures of E^_ coli JM108 (pSNUC-l) and (pSNUC-3) , cultures of E. coli AR120 (pFOG408) , and E. coli JM109 (pUHE24-2) were used for con¬ trols. For further characterization of the cloned nuclease genes in pSNUC-l and pSNUC-3, a western blot was performed using rabbit antibody (a gift from A. Meeker) . The proteins from the PAGE were transferred to nitrocellulose membrane (Mil¬ lipore) using the Mini-Trans Blot system (BioRad). A 1:15,000 dilution of rabbit antibody was used and detected with bio¬ tinylated goat antirabbit immunoglobulin and streptavidin peroxidase (Fisher Biotech) . The color development was per¬ formed by incubating in the peroxidase substrate by 4-chloro- 1-naphthol. Appropriate controls were tested in this experi¬ ment. Induction of E. coli JM109 (pSNUC-l) with IPTG showed signi- ficant decline in cell numbers between 2-7 hours' time period as determined by the OD 450 reading and viable plate counts (Figs. 14a and 14b) . The total cell number for E. coli JM109 (pSNUC-l) per ml declined form 2xl0 6 to 1.5xl0 4 between 0-7 hours' time period as determined by viable plate counts. In contrast to this, when E. coli JM109 (pSNUC-3) was induced with IPTG, a slow increase in cell number was evidenced as compared to the uninduced control as determined by the OD 50 measurements and viable plate counts (Figs. 15a and 15b) . The total cell number increased form 3xl0 6 to 7xl0 8 per ml between 0-7 hours as compared to the uninduced control which was 3xl0 6 to 2xl0 10 within the same time period. From these results it can be predicted that the lower killing efficiency of the plasmid pSNUC-3 upon induction as compared to pSNUC-l may be due to mutation generated by misincorporation of nucleotide(s) during the PCR DNA amplification process. However, it is not obvious which of the two clones, if not both, has the altered nucleotide sequence. The phenotypic expression of the cloned nuclease genes in pSNUC-l and pSNUC-3 plasmids on the DNAse test agar contai- ning methyl green indicator dye showed expected color change. The pSNUC-l showed more rapid color change than pSNUC-3. The control plasmid vector pSM1128 did not show any change in color on the same agar plates, whereas, plasmid pFOG408 showed significant color change. The PAGE gel for total protein analysis showed faint bands of approximately 17.8 kD size range for plasmids pSNUC-l and pSNUC-3 following induction with IPTG. The uninduced controls and pUHE24-2 did not show any such band (Fig. 16) . Since nuclease is an extracellular secretory protein, the E. coli strain carrying pFOG408 plasmid showed a very faint band in the gel. To confirm the expression of the cloned nuclease gene in plasmids pSNUC-l and pSNUC-3, a western blot was performed. In the western blot, distinct protein bands of approximately 17.6 kD were evidenced from the induced E. coli JM109 strains carrying pSNUC-l and pSNUC-3 plasmids, and E. coli AR120 strain carrying pFOG408 plasmid (Fig. 17) . The uninduced cultures and control plasmid pSM1128 did not show any band. Another non-specific band of approximately 20 kD size was noticed in all samples, which seems to be the non- specific antigenic reaction against the secondary antibody. This experiment suggested that the nuclease was produced by both plasmids pSNUC-l and pSNUC-3 upon induction. The lower killing efficiency for the pSNUC-3 plasmid could be due to the altered nucleotide sequence which has changed the protein structure and its functions for the substrate. Alternatively, this could be the case for the plasmid pSNUC-l for its rela¬ tively higher killing efficiency. Complete nucleotide sequence analysis for both clones may resolve this question and also provide the information on possible sites where change of nucleotide sequence may make this enzyme more effective. EXAMPLE 14 Determination of intracellular stability of cloned staphy¬ lococcal nuclease in Escherichia coli by ELISA assay The staphylococcal nuclease gene snuc without its signal sequence was PCR-amplified and cloned in Escherichia coli JM109 as described in Example 13 using the expression vector pUHE24-2, in the BamHl restriction site (pSNUC-l and pSNUC- 3) . The nuclease was expressed intracellularly when induced with IPTG. The stability of the intracellularly expressed SNUC enzyme in EL. coli following induction was determined by ELISA assay using antibodies raised in rabbits. Two approaches were pursued to determine the stability of the intracellularly expressed SNUC enzyme: 1. Transcriptional inhibition following induction by blocking mRNA synthesis with rifampicin. 2. Removal of inducing agent (IPTG) from the culture fol¬ lowing induction. 1. Transcriptional inhibition following induction by blocking mRNA synthesis with rifampicin E. coli cells harbouring pSNUC-l or pSNUC-3 were grown in liquid broth at 37°C till the optical density (OD 450 ) was between 0.12 and 0.15 (early exponential growth phase). The cells were then induced with IPTG for 30 minutes, and rifam¬ picin was added to the culture to block transcription. The incubation was carried on for the next 3 hours. E. coli JM109 without pSNUC-l or pSNUC-3 was used as the control. Aliquots of 1 ml of the cultures were collected before induc¬ tion, 30 minutes after induction, and every 30 minutes after. the addition of rifampicin. The cells were centrifuged, washed twice with phosphate buffer (pH 7.2), and disrupted by sonication to release the total cellular proteins. The soni- cation was carried on (8 to 10 minutes with a pulse for 30 seconds at every 30 seconds interval) till the sample became clear. The debris was spun down and the supernatant (lysate) was used for ELISA assay. An aliquot of each of the samples was subjected to spectro- photometric measurement to determine the total protein con¬ centration. ELISA assay was performed using equal quantity of total cellular protein from each lysate. Typically, 10 to 55 μl of lysates (depending on the concentration of each lysate) from various samples were absorbed in a microtiter plate (Corning) for about 16 hours (overnight) at 4°C. The wells were then washed 5 times with phosphate buffer (pH 7.2). The unreacted surface of the microtiter plate was blocked with 2% bovine serum albumin (BSA) from Sigma Chemical at room tem- perature for 1 hour with gentle shaking. The excess BSA from the microtiter plate was washed off 5 times with phosphate buffer (pH 7.2). 1:15.000 fold diluted polyclonal primary antibodies raised against the purified extracellular staphy¬ lococcal nuclease in rabbits, was reacted with preabsorbed total cellular proteins for about 16 hours (overnight) at 4°C. The unreacted primary antibodies were washed off 5 times with phosphate buffer (pH 7.2). The bound primary antibodies were then treated with secondary antibodies, goat antirabbit-Ig-biotin (CloneTech) for 1 hour at room temperature with gentle shaking. After washing off excess secondary antibodies, streptavidin-horseradish peroxi¬ dase (HRP-SA) conjugate was added to the microtiter plate and conjugated with the secondary antibodies. After washing off excess HRP-SA, colour development was carried out using 0.1 ml of substrate [2,2'-azinobis (3-ethyl benzthiazoline sul- phonic acid) in a 0.1 M citrate buffer, pH 4.2 with 0.03% hydrogen peroxide] . The absorbance was read at 415 n in an ELISA microtiter plate reader (Flow Laboratories, Inc., McClean, Virginia) . 2. Removal of inducible agent (IPTG) from the culture follo¬ wing a short induction stage E. coli cells harbouring pSNUC-l or pSNUC-3 were grown to early exponential phase (OD 450 of 0.12 to 0.15) when 1 mM IPTG (final concentration) was added to the culture. The induction with IPTG was carried on for 5 minutes at 37°C and the cells were centrifuged and washed twice with phosphate buffer (pH 7.2). The cells were resuspended in broth and incubated at 37°C for another 3 hours. Aliquots of l ml each of the cultures were collected before induction and 5 minutes after induction. Also, aliquots were collected by centrifuga¬ tion, washed twice with phosphate buffer (pH 7.2) and sonicated to release total cellular proteins. Cultures of E. coli JM109 without pSNUC-l or pSNUC-3 was used as the nega- tive control. Total cellular proteins were released by disrupting the cells using a sonic disruptor as described above. ELISA assays for intracellularly expressed staphylococcal nuclease was also performed as described above. The amount of staphylococcal nuclease protein encoded by pSNUC-l and pSNUC-3 after transcriptional inhibition as defined above, was found to be significant amounts following induction with IPTG. The data for pSNUC-l and pSNUC-3 are summarized in Tables 5 and 6, respectively. The expression of staphylococcal nuclease increased about 2-fold during the first 30 minutes after induction as compared with the amount detected prior to induction. The activity of the nuclease enzyme remained during the first 2 hours following rifampicin treatment. Between 2 and 3 hours after addition of rifampicin (2.5 and 3.5 hours after induction with IPTG) the activity of the nuclease was dropped significantly. From this experiment it can be concluded that the staphylococcal nuclease is stable intracellularly for at least 1.5 hours after the inhibition of transcription. Table 5. Determination of the stability of intracellularly expressed staphylococcal nuclease expressed by pSNUC-l following induction with IPTG and treatment with rifampicin Treatment Time (h) ELISA readings 0.427 IPTG 0.5 0.584 Rifampicin 1.0 0.666 1.5 0.685 2.0 0.675 2.5 0.542 3.0 0.488 3.5 0.407 Negative control 1.0 0.276 0 0.399 IPTG 0.5 0.495 Rifampicin 1.0 0.557 1.5 0.593 2.0 0.555 2.5 0.443 3.0 0.427 3.5 0.378 Negative control 1.0 0.258 66 After induction of with IPTG for a short period of time (5 minutes) , the cells were centrifuged, washed with phosphate buffer (pH 7.2), and grown in a rich medium for 150 minutes to determine the stability of the SNUC in the cells. The summary of the data is presented in Tables 7 and 8. There was at least 3-fold increase in the amount of SNUC following induction. The SNUC protein was found to be stable in pSNUC-l for 1.5 to 2 hours (between 5 and 125 minutes) after induc¬ tion and 1 to 1.5 hours (between 5 and 95 minutes) in pSNUC-3 as it was determined by the ELISA assay. After 125 minutes for pSNUC-l and 95 minutes for pSNUC-3 following removal of the inducing agent (IPTG) , the SNUC activity decreased signi¬ ficantly. Table 7. Determination of the stability of intracellularly expressed staphylococcal nuclease expressed by pSNUC-l following removal of IPTG after 5 minutes of induction Treatment Time (min) ELISA readings 0 0.270 IPTG 0.565 Removal of IPTG 35 0.560 65 0.543 95 0.501 125 0.456 155 0.329 Negative control 35 0.204 Table 8. Determination of the stability of intracellularly expressed staphylococcal nuclease expressed by pSNUC-3 following removal of IPTG after 5 minutes of induction Treatment Time (min) ELISA readings 0.171 IPTG 0.461 Removal of IPTG 35 0.458 65 0.430 95 0.375 125 0.269 155 0.210 Negative control 35 0.164 EXAMPLE 15 Construction and testing of a model inducible containment system based on the expression of an intracellularly active phospholipase Model plasmids for evaluation of an inducible cell function limiting function based on a toxic hydrolytically active enzyme was constructed and tested in Escherichia coli. The active component of the system is an extracellular phospho¬ lipase obtained from the strain Serratia liguefaciens and encoded by the gene phlA. The plasmids are based on the vector pMG300 (Givskov and Molin 198δ, J.Bacteriol 170. 5δ55- 5862) , a derivative of pCH624 (Boros 1984 Gene, 3J3, 257-260) . pMG300 (Fig. 18) carries the temperature inducible repressor promoter cI857/p_R of the lambda phage. The phiA gene is inserted downstream from this promoter and induction of phiA can be achieved by rising the temperature in the growth medium from 30°C to 41°C. Induction results in cell death. δδ The plasmids confer resistance to ampicillin. The phiA gene is inserted downstream from the lacl q /ptac repressor/promoter system of the broad host range plasmid pVLT33. This plasmid can be transferred by conjugation to gram-negative bacteria. A. Construction of the model plasmids pMG323 and pMG317 The construction of plasmid pMG300 was described by Givskov and Molin 1968, J.Bacteriol 170. 5855-5662. The gene phlA encoding the toxic hydrolytic phospholipase was obtained from a genomic bank of Serratia liquefaciens. constructed on the vector pNU121 (Nielsson et al. 1983, Nucl. Acid Res. 11, 8019-8030) . A 3200 base pair EcoRI restriction fragment carrying the phospholipase operon genes phlA and phlB was purified and digested in-vitro with the exonuclease Bal31. Digested DNA was ligated into the Smal restriction site of pMG300. E. coli clones, where expression of phospholipase was controlled by the temperature inducible promoter system, were isolated. Two plasmids, pMG317 and pMG323 were obtained by this procedure (Fig. 18) . These two plasmids and the DNA- sequence of the phospholipase operon has been described by Givskov and Molin 1988, J.Bacteriol 170, 5655-5862. E. coli strain CSH50 harbouring plasmid pMG317 was grown in 50 ml LB medium supplemented with 100 μg/ml ampicillin and 0.2% glucose, at a temperature of 30°C in a shaking 300 ml Erlenmeyer flask. At an OD 450 of 0.2 the culture was diluted to an OD 450 of 0.05 and growth continued to obtain an OD 450 of 0.2. The culture was divided into two, of which one continued growth at 30°C whereas the other one was shifted to a growth temperature of 41°C. Samples of the cultures were taken at different times after the temperature shift, diluted in 0.9% NaCl and plated on LB agar plates supplemented with 100 μg/ml ampicillin. The viable count of the cultures were determined by counting the number of colonies (CFU/ml) on the LB plates after an overnight incubation at 30°C. The proportion of Phl+ cells in the culture was determined by replica plating to phospholipase indicative plates, viz. plates with LB agar supplemented with 1% egg-yolk and 100 μg/ml ampicillin and incubating the plates overnight at 30°C followed by two hours incubation at 41°C (Fig. 19) . The results of the experiment is summarized in the below table: Table 9. Growth and survival of CSH50 transformed with pMG317 under phlA inducing conditions (41°C) and non-inducing condi¬ tions (30°C) 41°C culture: Killing effect after 15 minutes: 4 x 10 4 , killing effect after 30 minutes: > 2 x 10 5 . 30 °C culture: time/min OD 450 CFU/ml % Phl+ 0.450 l.δxlO 7 100 120 3.00 1.2X10 8 100 The above results are also illustrated in Fig. 19 B. Construction of model plasmid PMG323/33 Plasmid pMG323 was digested with BamHl and Seal. PVLT33 was digested with Hindlll treated with Klenow polymerase to create blunt ends, digested with BamHl and ligated to puri- fied BamHl-Seal restriction fragment obtained from pMG323 giving rise to plasmid pMG323/33 (Fig. 20) where PhlA produc¬ tion is inducible by addition of IPTG. The plasmid confer resistance to kanamycin and can be transferred to other bacteria by conjugation. EXAMPLE 16 Construction of plasmids for site-specific recombination mediated stochastically regulated induction of genes The resolution system of the broad-host range plasmid RP4 is essential for multimer resolution of the plasmid during replication (Grinter et al. 1989, Plasmid, £2 . , 203-214) . The involved components, the resolvase gene (parA) and the site for this site-specific resolution (mrs) have been cloned previously (Gerlitz et al. 1990, J.Bacteriol. 172. 6194-6203; Roberts et al. , 1990, J.Bacteriol. 172, 6204-6216). If two mrs sites are placed in direct orientation, the intervening sequence can be deleted with a high frequency by supplying the parA in trans (Roberts et al.supr . The reaction is outlined in Fig. 21. A. A model system for testing the resolvase mediated recom- bination A plasmid containing two directly oriented mrs sites was constructed on the basis of pCK46 and pCK30 described in Example 9. pMRS19A (Eberl et al. , 1992a, Molec. Microbiol. , in press) was linearized with Ndel and filled in with the large subunit of DNA polymerase I (Klenow fragment) . This blunt ended fragment was subsequently cut with Sail and the smaller fragment, containing the mrs site was ligated to pCK46 cut with Smal and Sail, resulting in pCK145. pMRS19A was cut with BamHl and SphI. The small fragment was inserted into pCK30 digested with Bglll and SphI, giving pCK146. A plasmid, pSMδ91, containing the transposon Tn5 was digested with Hindlll and Smal and the about 1100 base pair fragment containing the kanamycin (npt) gene was blunt ended as described. This fragment was inserted into pCK146, opened in Smal. The resulting plasmids were named pCK153 and pCK154. pCK153 has the recreated Smal site proximal to the Xhol site. In pCK154 the Tn5 fragment is inverted. pCKl55 (cf. Fig. 22) was created by inserting the approximately 1500 base pair Xhol-Hindlll fragment of pCK153 into pCK145, linearized with Xhol and Hindlll. The resolvase was cloned from pGMA70 (Eberl et al., 1992b, manuscript in preparation) . The promoterless parA from pGMA70 was excised as an approximately 1100 base pair EcoRI-Hindlll fragment and inserted into the EcoRI-Hindlll cut pUHE24-l (Lanzer et al., 198δ, Proc. atl.Acad.Sci. USA 85 . , 8973-8977), resulting in pCK151 (Fig. 22) . This plasmid contains the parA under the control of the synthetic lac-promoter pAl/04/03. The resolvase expression can be repressed by the lac repressor, lacl, and be induced by adding IPTG (iso- propyl-β-D-thiogalactoside) to a culture containing the plasmid. This plasmid is used for overexpression of the resolvase protein for purifying purposes. The parA gene was likewise moved to pLBJ65 (Bogø Jensen et al., 1992, manuscript in preparation) as the EcoRI-Hindlll fragment described above. The resulting plasmid, pCK158 (Fig. 22) , contains the resolvase under control of the pAl/04/03 promoter and the laclql repressor (Mϋller-Hill, 1975, Prog.Biophys.Molec.Biol. 3J), 227-252) with its own promoter. From pCK158 the entire cassette containing the lac-repressor and the resolvase with promoter was moved as a filled in Notl fragment into the EcoRV site of pACYC184 (Chang et al., 1978, J.Bacteriol. 134. 1141-1156), resulting in pCK151 and pCK168 (Fig. 22) , differing by the orientation of the insert. In pCK151 the resolvase gene is located proximal to the origin of the plasmid. B. Initial testing of the double mrs-site plasmid PCK155 in vivo The homologous recombination deficient (recA) strain HB101 (Boyer et al., 1969, J.Mol.Biol. 4JL, 459) was transformed with pCK155 and plated on LB agar plates supplemented with either 50 μg/ml of kanamycin (kan) or 100 μg/ml of ampicillin (amp) . pCK155 contains a bla gene conferring ampicillin resistance outside of the two mrs sites and an npt gene conferring kanamycin resistance within the two mrs sites (Fig. 23) . Any significant homologous recombination between the two mrs sites would be identified as a lower plating efficiency on kanamycin plates than on ampicillin plates. No significant difference in plating efficiency could be detec¬ ted. pLE026 (a plasmid containing the entire par-region of RP4 (Eberl et al , 1992a, Molec. Microbiol. in press) was sub¬ sequently transformed into the strain containing pCK155 and plated on the two types of plates as defined above, but further supplemented with 50 μg of chloramphenicol (cam) to select for pLE026 containing the cat gene conferring chlor¬ amphenicol resistance. The transformation yielded about 2000 cam and amp resistant transformants and 4 cam and kan re¬ sistant transformants, indicating that recombination had occurred in the cells. pCK153 was used instead of pCK155 in a control experiment. Here no difference could be detected. Plasmids were isolated according to the method described by Birnboim et al. , 1979, Nucleic Acids Res. 1, 1513-1523) from 10 pCK155/pLE026 clones. The plasmids were separated by passing through HB101 by retransformation and subsequent plasmid preparation. The plasmids were examined by restriction mapping. In all 10 plasmids the restriction pattern resembles the expected for a plasmid having undergone a site specific recombination between the two mrs-sites. One clone was further examined by sequencing according to Sanger et al., 1977 (Proc.Natl- .Acad.Sci. USA, 21, 5463-5467), using the modification for plasmid sequencing as described by the supplier of the sequencing kit (United States Biochemical) and using the M13 universal sequencing primer. The resulting sequence (Fig. 24) shows that a perfect site specific recombination had occurred (cf. restriction map of pCK155, (Fig. 23). The pCK155 deri¬ vative which had undergone deletion in-vivo was designated pCK155delta. The strain containing pCK155 was transformed with pCK16δ and plated on four different plates: Plate type Number of transformants 50 μg cam + 100 μg amp, no IPTG app. 1000 50 μg cam + 100 μg amp, 2 mM IPTG app. 1000 50 μg cam + 50 μg kan, no IPTG app. 500 50 μg cam + 50 μg kan, 2mM IPTG 3 The drop in the uninduced cells can be ascribed to read- through of the promoter controlling the resolvase gene thus generating a background level of resolvase protein in the cells, enabling the recombination to a low extent. C. Growth experiments with PCK155 A strain containing pCK168 was transformed with pCK155 or pCK153 and kanamycin and chloramphenicol resistant colonies were picked. From a restreaked colony of each type of strain, a 10 ml LB broth culture was grown overnight in the presence of 50 μg/ml kan and 50 μg/ml cam. From these cultures 50 ml flasks were seeded to an OD 450 of 0.05 in LB broth supplemented with 50 μg/ml cam and 100 μg/ml amp. At an OD 450 of 0.5, the cultures were diluted in fresh medium of the same type to an OD 450 of 0.05. The OD was measured to ensure that the cultures were in balanced growth and at about OD 0.5 the cultures were divided and a sample was taken for determination of viable counts on each of the following plates: 50 μg/ml kan + 50 μg/ml cam and 100 μg/ml amp + 50 μg/ml cam. The counts on plates containing amp+cam represent the number of plasmid-carrying viable cells, whereas the counts on kan+cam plates represent the viable cells with (at least one copy of) an intact, i.e. not deleted by recombination, copy of npt gene. After 5 minutes, IPTG was added at a final concentration of 2 mM to one half of each culture. After further 5 minutes another viable count sample was taken. Samples were then taken at time intervals and one sample after overnight growth. The viable counts as a function of time is shown in Fig. 25. The experiment showed that following induction of the resol¬ vase gene in cells containing the plasmids pCK168 and pCK155, a 500 fold drop in viable counts of kanamycin resistant cells compared to the uninduced culture, occurs. The drop is pro- minent already after 5 minutes of induction. After further 60 minutes, the difference in viable counts between the induced and the uninduced culture is 5000 times. After this point kanamycin resistant cells in the induced culture emerges at the same rate as the kanamycin resistant cells in the unin- duced culture. This must be due to mutants either in the resolvase gene or the promoter or in the resolution (res) sites in pCK155, both rendering the cells unable to delete the kanamycin gene. At any point of time the kanamycin viable counts are only 1 percent of the viable counts of the unin- duced culture on ampicillin plates. The explanation for this could be leakiness of the promoter regulating the resolvase or homologous recombination between the two res sites. The ampicillin viable counts in the induced culture closely follows that of the uninduced, indicating that IPTG and the resolution reaction per se is not responsible for the ob¬ served drop in kanamycin viable counts. This is further confirmed by the observation that viable counts of the con¬ trol culture (pCK168+pCK153) , are essentially the same on both kanamycin and ampicillin throughout the experiment. The conclusion of this experiment is that the resolvase action is efficient and exceptionally fast in this in-vivo setup. The promoter controlling the resolvase might be responsible for a certain degree of undesired expression of the protein and concomitant deletion of the kanamycin gene in uninduced cells. A small proportion of cells in the culture was unable to delete the kanamycin gene and grew normally in the presence of IPTG and kanamycin. D. In vitro experiments using the purified resolvase on PCK155 as the DNA substrate The resolvase was purified from a culture of HB101 harboring pCK151. The cells were grown in 250 ml LB to an OD 5 ' 90 of 0.8 to 1.0 and subsequently induced by 1 mM IPTG and shaken for additional 4 hours. The cells were harvested and resuspended in 7.5 ml buffer C (25 mM Tris pH 8.0, 0.1 mM EDTA, 50 μM benzamidin, 100 μM PMSF, 1 mM 2-mercaptoethanol, 0.02 % Brij, 200 mM NaCl) . The cells were opened by sonication. The lysate was ultracentrifuged at 36 000 rpm for 45 minutes at 4 C. The supernatant was dialyzed against buffer B (buffer A with 250 mM NaCl final concentration) . The resulting pellet was washed in buffer A and subsequently dissolved in 0.5 ml buffer C (buffer A with 1 M NaCl final concentration) . The solution was concentrated to app. 50 μl. pCK155 was prepared by two rounds of cesium chloride density gradient centrifugation (Maniatis et al., 1982, Molecular cloning: A Laboratory Manual, Cold Spring Harbor Laboratory Press, New York) . The concentration of the DNA was determined by UV absorbance at 260 nm and the protein concentration was determined by the method of Bradford, 1976 (Anal.Biochem. 72., 248-254) . The resolution assay was performed with 110 ng of plasmid DNA in a reaction volume of 10 μl. The final concentration of the reaction buffer was 150 mM KC1, 10 mM MgS0 4 , 30 mM Tris-HCl pH 7.5 and 1 mM 1,4-dithioerythritol (DTE). The reactions were stopped by heat inactivation at 65°C for 10 minutes. The samples were assayed by digesting with Ndel in a final volume of 30 μl. To the above reaction mixture, enzyme buffer and enzyme was added and the resulting mixture was incubated for 1 hour at 37°C. Samples were run on a 0.7 % agarose gel at 2 V/cm overnight. The gel was dyed with EtBr and photographed on a Polaroid negative film using UV light. The negatives were scanned on a Shimatzu scanner model CS-930 V-05. The area under each peak was evaluated as an indication of the amount of DNA in the corresponding band on the gel when corrected for fragment size (cf. Fig. 26 and Fig. 27) . The unreacted pCK155 yielded two bands of 4.5 kb and 0.4 kb, while the resolved plasmid, which is converted into two species, pCK155delta and pCK155omega, yields two bands of 2.9 kb and 1.9 kb, respectively (Fig. 26). E. Titration of the resolvase To 100 ng of pCK155 was added varying amounts of resolvase: 1200, 600, 300, 150, 75, 38, 19, 9, 5 and 0 ngε. The reaction was performed at 37C for 30 minutes. The results were plotted in Fig. 28. A relatively sharp drop in ability to resolve the plasmid, expressed as the fraction of deleted plasmids rela¬ tive to intact plasmids, is observed between 150 ng and 75 ng of protein, corresponding to about 60 molecules of resolvase per res site (150 ng) . This indicates that a lower level of the resolvase concentration can be determined and below this level little recombination occurs. When the level of resol- vase is above this level, essentially all substrate is con¬ verted. The above Figure shows that even without adding resolvase, a small fraction of substrate is converted. This could be due to indigenous resolvases in the 12^ coli host used for pro¬ pagation of the plasmid. Also a small fraction cannot be converted even at very high concentrations of resolvase. This can be accounted for by noting that the resolvase requires a supercoiled substrate for reaction and that a small fraction of even a highly purified plasmid purification will be re¬ laxed. F. Kinetics of resolvase in vitro Samples with 110 ng of the above DNA substrate and either 125 ng or 250 ng of resolvase were incubated at 37°C at varying periods of time: 1, 5, 10, 30 and 60 minutes. The data shown in Fig. 29 shows that when the amount of enzyme is suffi¬ cient, the reaction is almost completed within the first minute (more than 65% conversion) and has reached the final extent of reaction within the first 5 minutes. When the amount of enzyme is less than optimal, the kinetics are slower which can be accounted for by considering the apparent reaction speed as the speed of building up the complexes needed for resolution. INDICATIONS RELATING TO A DEPOSITED MICROORGANISM (PCT Rule 13-tts) The indications made below relate to the microorganism referred to in the description on page 73 ,ii„e 9 B. IDENTIFICATION OF DEPOSIT Further deposits are identified on an additional sheet K~J Name of depositary institution DSM Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH Address of depositary institution (including postal code and country) Mascheroder Weg 1 B D-3300 Braunschweig Date of deposit Accession Number 5 July 1991 DSM 6603 C ADDITIONAL INDICATIONS (leave blank if not applicable) This infαπnation is continued on an additional sheet ^ As regards the respective Patent Offices of the respective designated states, the applicants request that a sample of the deposited microorganisms only be made available to an expert nominated by the requester until the date on which the patent is granted or the date on which the application has been re¬ fused or withdrawn or is deemed to be withdrawn. D. DESIGNATED STATES FOR WHICH INDICATIONS ARE MADE (if the indications are not for all designated States) E. SEPARATE FURNISHING OF INDICATIONS (leave blank if not αp≠cable) The indications listed below will besubmitted to the international Bureau later (sp∞fyt t£genσαinαtureoftheinώcύtionse.g^ 'Accession Number of Deposit') Forτeceιving Office use only For international Bureau use only This sheet was received with the international application I I This sheet was received by the International Bureau on: Authorized officer INDICATIONS RELATING TO A DEPOSITED MICROORGANISM (PCT Rule I3bis) A. The indications made below relate to the microorganism referred to in the description on page 53 ine 9 B. IDENTIFICATION OF DEPOSIT Further deposits are identified On an additional sheet KT " Name of depositary institution DSM Deutsche Samπ-lung von Mikroorganismen und Zellkulturen G mbH Address of depositary institution (including postal code and country) Mascheroder Weg 1 B D-3300 Braunschweig Date of deposit Accession Number 5 July 1991 DSM 6604 C. ADDITIONAL -INDICATIONS (leave blank if not applicable) This infoπnat ion is continued on an additional sheet V~ As regards the respective Patent Offices of the respective designated states, the applicants request that a sample of the deposited microorganisms only be made available to an expert nominated by the requester until the date on which the patent is granted or the date on which the application has been re¬ fused or withdrawn or is deemed to be withdrawn. D. DESIGNATED STATES FOR WHICH INDICATIONS ARE MADE (if the indications are not for all designated States) E. SEPARATE FURNISHING OF INDICATIONS (leave blank if not applicable) The indications listed below will besubmitted to the international Bureau later (speάfytf egeneralnMureofthein caύonse.g_, 'Accession Number of Deposit') For International Bureau use only I j This sheet was received by the international Bureau on: Authoπzed officer

Claims: CLAIMS 1. A cell containing a gene whose expression results in the formation of an enzyme which is present and hydrolytically active in the cytoplasm of said cell, the cell further con- taining a regulatory nucleotide sequence which regulates the expression of the gene, the expression of said gene leading to formation of the enzyme in the cell at a rate which re¬ sults in the hydrolysis of hydrolysable cytoplasmic sub¬ stances necessary for non-limited function of the cell, to an extent whereby the function of the cell is being limited. 2. A cell according to claim 1 wherein the enzyme is selected from the group consisting of a nuclease, a phospholipase, a lipase, a lysozyme, a protease and a carbohydrase. 3. A cell according to claim 2 wherein the enzyme is a phos- pholipase. 4. A cell according to claim 1 wherein the expression of the gene results in the formation of a nuclease being capable of hydrolyzing diesterphosphate bonds in nucleic acids in the cell, the expression of said gene leading to formation of the nuclease in the cell at a rate which results in the presence of nicks in one strand of the cell nucleic acids to an extent which cannot be repaired by the nucleic acid repair mechanism of the cell, thereby limiting the function of said cell. 5. A cell according to claim 1 wherein the gene whose ex- pression results in the formation of a hydrolytically active enzyme is contained in a recombinant replicon or recombinant replicons. 6. A cell according to claim 5 wherein the regulatory sequence is contained in a recombinant replicon or recom- binant replicons. 7. A cell according to claim 5 wherein the gene and the regulatory sequence are contained in the same recombinant replicon. 8. A cell according to claim 7 which contains a multiplicity of replicons each containing both the gene and the regulatory sequence. 9. A cell according to claim 1 wherein the gene whose ex¬ pression results in the formation of an hydrolytically active enzyme is a gene lacking a sequence coding for a peptide signal sequence which, if present, would have permitted the enzyme to be transported through the cell membrane. 10. A cell according to claim 1 wherein the regulatory nucleotide sequence contains a regulatable promoter operably linked to the gene encoding the hydrolytically active enzyme. 11. A cell according to claim 10 wherein the regulatable promoter is regulated by a factor selected from the environ¬ mental conditions of the cell, the physiological state of the cell and a stochastic event. 12. A cell according to claim 11 wherein the regulatable promoter is regulated by a factor selected from the physical conditions in the environment and the presence or absence of a certain chemical in the environment. 13. A cell according to claim 12 wherein the chemical is selected from a carbon source, a nitrogen source, a metabo- lite, an amino acid, a nucleoside, a pyrimidine base, a purine base and a metal ion. 14. A cell according to claim 13 wherein the chemical is isopropyl-beta-D-thiogalactopyranoside (IPTG) . 15. A cell according to claim 12 wherein the physical condi¬ tion regulating the promoter is the temperature prevailing in the environment of the cell. 16. A cell according to claim 1 wherein the expression of the gene whose expression results in the formation of a hydroly¬ tically active enzyme is stochastically induced as a result of a recombinational excision of an excisable negatively functioning regulatory nucleotide sequence being operably linked to said gene which nucleotide seguence, while present in the cell, inhibits expression of the gene whose expression results in the formation of the hydrolytically active enzyme. 17. A cell according to claim 16 wherein said excisable negatively functioning regulatory nucleotide sequence is a sequence which is flanked by a first flanking sequence and a second flanking sequence substantially homologous with said first flanking sequence whereby said regulatory element is recombinationally excisable in the cell. 18. A cell according to claim 17 wherein the flanking sequences have a length which is in the range of 100-5000 base pairs. 19. A cell according to claim 18 wherein the flanking sequences have a length which is in the range of 200-3000 base pairs. 20. A cell according to claim 17 wherein the gene whose expression results in the formation of a hydrolytically active enzyme encodes a first RNA which is a messenger RNA, and the excisable negatively functioning regulatory nucleo¬ tide sequence being operably linked to said gene, is a gene encoding a second RNA which forms an RNA-RNA duplex with said first messenger RNA and thereby, when it is expressed, in¬ hibits translation of said gene coding for the hydrolytically active enzyme. 21. A cell according to claim 17 wherein the excisable nega¬ tively functioning regulatory nucleotide sequence is a gene encoding a polypeptide repressor of transcription of the gene whose expression results in the formation of a hydrolytically active enzyme. 22. A cell according to claim 21 wherein the excisable nega¬ tively functioning regulatory nucleotide sequence is a gene encoding a lac repressor, and the gene whose expression results in the formation of a hydrolytically active enzyme is operably linked to the lac promoter, said lac promoter in¬ cluding the operator site for the lac repressor. 23. A cell according to claim 17 wherein the negatively functioning regulatory nucleotide sequence is a termination sequence preventing transcription of the gene whose expres- sion results in the formation of a hydrolytically active enzyme. 24. A cell according to claim 1 wherein the expression of the gene whose expression results in the formation of a hydroly¬ tically active enzyme is stochastically induced as a result of a site-specific recombinational excision of an excisable negatively functioning regulatory nucleotide sequence being operably linked to said gene which nucleotide sequence, while present in the cell, inhibits expression of the gene whose expression results in the formation of the hydrolytically active enzyme. 25. A cell according to claim 24 wherein said excisable negatively functioning regulatory nucleotide sequence is a sequence which is flanked by a first site for site-specific resolution and a second site for site-specific resolution, the second site being resolvable by the same or a function¬ ally equivalent multimer resolving enzyme as is the first site, whereby said regulatory element is recombinationally excisable in the cell. 26. A cell according to claim 24 wherein the first and second site for site-specific resolution is the mrs site derived from plasmid RP4. 27. A cell according to claim 25 wherein the multimer resolving enzyme is encoded by a gene which is located in trans relative to the sites for site-specific resolution. 28. A cell according to claim 27 wherein the gene coding for the multimer resolving enzyme is the parA gene of plasmid RP4. 29. A cell according to claim 24 wherein the gene whose expression results in the formation of a hydrolytically active enzyme encodes a first messenger RNA, and the ex¬ cisable negatively functioning regulatory nucleotide sequence being operably linked to said gene, is a gene encoding a second messenger RNA which forms an RNA-RNA duplex with said first messenger RNA and thereby, when it is expressed, in¬ hibits translation of said gene coding for the hydrolytically active enzyme. 30. A cell according to claim 24 wherein the excisable nega- tively functioning regulatory nucleotide sequence is a gene encoding a polypeptide repressor of transcription of the gene whose expression results in the formation of a hydrolytically active enzyme. 31. A cell according to claim 30 wherein the excisable nega- tively functioning regulatory nucleotide sequence is a gene encoding a lac repressor, and the gene whose expression results in the formation of a hydrolytically active enzyme is operably linked to the lac promoter, said lac promoter in¬ cluding the operator site for the lac repressor. 32. A cell according to claim 24 wherein the negatively functioning regulatory nucleotide sequence is a termination sequence preventing transcription of the gene whose expres- sion results in the formation of a hydrolytically active enzyme. 33. A cell according to claim 1 wherein the gene whose ex- apression results in the formation of a hydrolytically active enzyme is stochastically expressed as a result of recombi¬ national inversion of an invertible promoter sequence of said regulatory nucleotide sequence said promoter being operably linked to the gene. 34. A cell according to claim 33 wherein the promoter sequence is a sequence carrying the fimA promoter or a funct¬ ional homologue thereof. 35. A cell according to claim 1 wherein the gene whose ex¬ pression results in the formation of a hydrolytically active enzyme is derived from a replicon selected from the group consisting of a bacterial plasmid, a bacterial chromosome, a procaryotic virus, a eucaryotic plasmid, a eucaryotic virus, a eucaryotic chromosome, eucaryotic mito¬ chondria, a eucaryotic chloroplast and a synthetic sequence. 36. A cell according to claim 1 wherein the hydrolytically active enzyme is an enzyme containing at least one cysteine residue. 37. A cell according to claim 35 wherein the gene whose expression results in the formation of a hydrolytically active enzyme is a gene which is derived from a bacterial replicon. 38. A cell according to claim 37 wherein the hydrolytically active enzyme is encoded by a gene derived from a replicon isolated from a gram-negative bacterium. 39. A cell according to claim 38 wherein the hydrolytically active enzyme is encoded by a gene derived from a Serratia species. 40. A cell according to claim 39 wherein the gene is derived from Serratia marcescens. 41. A cell according to claim 40 wherein the enzyme is an endonuclease encoded by a gene having the following DNA sequence: Cra--C- CACCCTCCMTCCATCC-ACAACT^^ cc-wx-AACcrcτ v.τcCTt-α--τcAτ--^ CCTCCTICCAC-*--AT-ACCA--GCACTACGAATAT-^ -rc c------C C-tf-t» cτaa^ (~ ca ~ m ~ mfmΛ ~ ιvimX jτ cπx-CATtxα-CT-Aτc-Accc---Xcαcα3^ CACXrrAGC-XCACTACTCC--CC-g--ACC-GACCCACa- TAACCACX-CCX- T-CCCCCa- a-AAC-ATCACGA AAτrccτccc--a-GA xcccc-CGACcrrcxACTCc^^ ACGaaτATCO--CM-\ACTaXt-CGCACCCA--AAACX-(-CACACC^ • Xχ-~Cm~~ACf-_ LJl lmAaXχf^^ TAiji_rmi iVmivviι^>mAτa.<__AACA A-uax∞ AcrxCm-wciτaimΥ 42. A cell according to claim 40 wherein the enzyme is a phospholipase encoded by a gene having the following DNA sequence: ϋl 45. A cell according to claim 1 wherein at least one of the gene whose expression results in the formation of a hydroly¬ tically active enzyme and the nucleotide sequence regulating said gene is mutated at one or more sites, whereby the cell function-limiting effect of the enzyme encoded by the gene, when expressed in the cell is the same or increased relative to the cell function-limiting effect of the enzyme expressed in a cell containing said gene and said nucleotide sequence in non-mutated form. 46. A cell according to claim 45 wherein the cell contains a gene coding for an endonuclease. 47. A cell according to claim 46 wherein the gene is derived from a Serratia species. 48. A cell according to claim 1 wherein the nucleotide sequence regulating the transcription of the gene whose expression results in the formation of a hydrolytically active enzyme is derived from a replicon selected from the group consisting of a bacterial plasmid, a bacterial chrom¬ osome, a procaryotic virus, a eucaryotic plasmid, a eucary- otic virus, a eucaryotic chromosome, eucaryotic mitochondria, a eucaryotic chloroplast and a synthetic sequence. 49. A cell according to claim 1 which comprises a further regulatably expressible gene which encodes a non-enzyme cell function-limiting gene product. 50. A cell according to claim 49 wherein the further regul¬ atably expressible gene is regulated by a regulatory nucle¬ otide sequence of the same type as the sequence regulating the gene whose expression results in the formation of a hydrolytically active enzyme. 51. A cell according to claim 49 wherein the further regul¬ atably expressible gene is regulated by a regulatory nucle¬ otide sequence of another type than the sequence regulating the gene whose expression results in the formation of a hydrolytically active enzyme, said other type being a regula¬ tory nucleotide sequence which is also capable of regulating the gene whose expression results in the formation of the hydrolytically active enzyme. 52. A cell according to claim 49 wherein the further regu¬ latably expressible gene is the hok gene from the parB region of plasmid Rl or a DNA sequence which is functionally homolo¬ gous to the Rl hok gene. 53. A cell according to claim 49 wherein the further regul¬ atably expressible gene is the gef gene. 54. A cell according to claim 1 which further comprises a DNA sequence not naturally related to the replicon carrying the gene whose expression results in the formation of a hydroly- tically active enzyme and/or the replicon carrying the regu¬ latory nucleotide sequence. 55. A cell according to claim 54 wherein the DNA sequence not naturally related to the gene whose expression results in the formation of a hydrolytically active enzyme and/or the repli- con carrying the regulatory nucleotide sequence, is a sequence coding for an immunologically active gene product. 56. A cell according to claim 54 wherein the DNA sequence not naturally related to the gene whose expression results in the formation of a hydrolytically active enzyme and/or the repli- con carrying the regulatory nucleotide sequence, is a sequence coding for a pesticidally active gene product. 57. A cell according to claim 54 wherein the DNA sequence not naturally related to the gene whose expression results in the formation of a hydrolytically active enzyme and/or the repli- con carrying the regulatory nucleotide sequence, is a sequence coding for a pollutant-degrading gene product. 58. A cell according to claim 1 which is a cell transformed with a recombinant replicon or recombinant replicons con¬ taining a gene whose expression results in the formation of an enzyme which is present and hydrolytically active in the cytoplasm of said cell, the expression of said gene leading to formation of the enzyme in the cell at a rate which re¬ sults in the hydrolysis of hydrolysable cytoplasmic substances necessary for non-limited function of the cell, to an extent whereby the function of the cell is limited, the expression of said gene being regulated by a regulatory nucleotide sequence which is contained in the recombinant replicon containing the gene or in another recombinant repli¬ con present in the transformed cell. 59. A recombinant replicon containing a regulatably expres- sible gene which, when expressed in a cell encodes an enzyme which is present and hydrolytically active in the cytoplasm of the cell, the expression of said gene leading to formation of the enzyme in the cell at a rate which results in the hydrolysis of hydrolysable cytoplasmic substances necessary for non-limited function of the cell, to an extent whereby the function of the cell is being limited, the expression of said genes being regulated by a regulatory nucleotide sequence which is contained in the recombinant replicon or in another recombinant replicon present in a cell containing the replicon. 60. A recombinant replicon according to claim 59 which contains the regulatory nucleotide sequence. 61. A replicon according to claim 59 wherein the hydrolyti¬ cally active enzyme is an enzyme which, when the replicon is expressed in a cell, is retained in the cell. 62. A replicon according to claim 59 wherein the enzyme being encoded is selected from the group consisting of a nuclease, a phospholipase, a lipase, a lysozyme, a protease and a carbohydrase. 63. A replicon according to claim 62 wherein the enzyme being encoded is a phospholipase. 64. A replicon according to claim 59 wherein the expression of the gene results in the formation of a nuclease being capable of hydrolyzing diesterphosphate bonds in nucleic acids in the cell, the expression of said gene leading to formation of the nuclease in the cell at a rate which results in the presence of nicks in one strand of the cell nucleic acids to an extent which cannot be repaired by the nucleic acid repair mechanism of the cell, thereby limiting the function of said cell. 65. A replicon according to claim 59 wherein the gene en¬ coding the hydrolytically active enzyme is a gene lacking a sequence coding for a peptide signal sequence which, if present, would have permitted the enzyme to be transported through the cell membrane. 66. A replicon according to claims 59 wherein the regulatory nucleotide sequence contains a regulatable promoter operably linked to the gene encoding the hydrolytically active enzyme. 67. A replicon according to claim 66 wherein the regulatable promoter is regulated by a factor selected from the environ¬ mental conditions of a cell containing the replicon, the physiological state of the cell and a stochastic event. 68. A replicon according to claim 67 wherein the regulatable promoter is regulated by a factor selected from the physical conditions in the environment and the presence or absence of a certain chemical in the environment. 69. A replicon according to claim 68 wherein the chemical is selected from a carbon source, a nitrogen source, a metabolite, an amino acid, a nucleoside, a pyrimidine base, a purine base and a metal ion. 70. A replicon according to claim 69 wherein the chemical is isopropyl-beta-D-thiogalactopyranoside (IPTG) . 71. A replicon according to claim 68 wherein the physical condition comprises the temperature prevailing in the en- vironment. 72. A replicon according to claim 59 wherein the expression of the gene coding for the hydrolytically active enzyme is stochastically induced as a result of a recombinational excision of an excisable negatively functioning regulatory nucleotide sequence which, while present in the cell, in¬ hibits expression of the gene coding for the hydrolytically active enzyme. 73. A replicon according to claim 72 wherein said excisable negatively functioning regulatory nucleotide sequence is a sequence flanked by a first flanking sequence and a second flanking sequence substantially homologous with said first flanking sequence whereby said regulatory element is recom¬ binationally excisable in the cell. 74. A replicon according to claim 73 wherein the flanking sequences have a length which is in the range of 100-5000 base pairs. 75. A replicon according to claim 74 wherein the flanking sequences have a length which is in the range of 200-3000 base pairs. 76. A replicon according to claim 75 wherein the gene coding for the hydrolytically active enzyme encodes a first RNA which is a messenger RNA, and the excisable negatively functioning regulatory nucleotide sequence is a gene encoding a second RNA which forms an RNA-RNA duplex with said first messenger RNA and thereby inhibits translation thereof. 77. A replicon according to claim 61 wherein the negatively functioning regulatory nucleotide sequence is a gene encoding a polypeptide repressor of transcription of the gene coding for the hydrolytically active enzyme. 78. A replicon according to claim 77 wherein the negatively functioning regulatory nucleotide sequence is a gene encoding the lac repressor, and the gene coding for the hydrolytically active enzyme is operably linked to the lac promoter, said lac promoter including the operator site for said lac repres- sor. 79. A replicon according to claim 72 wherein the negatively functioning regulatory nucleotide sequence is a termination sequence preventing transcription of the gene coding for the hydrolytically active enzyme. 80. A replicon according to claim 59 wherein the expression of the gene whose expression results in the formation of a hydrolytically active enzyme is stochastically induced as a result of a site-specific recombinational excision of an excisable negatively functioning regulatory nucleotide sequence being operably linked to said gene which nucleotide sequence, while present in the cell, inhibits expression of the gene whose expression results in the formation of the hydrolytically active enzyme. 81. A replicon according to claim 80 wherein said excisable negatively functioning regulatory nucleotide sequence is a sequence which is flanked by a first site for site-specific resolution and a second site for site-specific resolution, the second site being resolvable by the same or a function¬ ally equivalent multimer resolving enzyme as is the first site, whereby said regulatory element is recombinationally excisable in the cell. 82. A replicon according to claim 80 wherein the first and second site for site-specific resolution is the mrs site derived from plasmid RP4. 83. A replicon according to claim 81 wherein the multimer resolving enzyme is encoded by a gene which is located in trans relative to the sites for site-specific resolution. 84. A replicon according to claim 83 wherein the gene coding for the multimer resolving enzyme is the parA gene of plasmid RP4. 85. A replicon according to claim 80 wherein the gene whose expression results in the formation of a hydrolytically active enzyme encodes a first RNA which is a messenger RNA, and the excisable negatively functioning regulatory nucleo¬ tide sequence being operably linked to said gene, is a gene encoding a second RNA which forms an RNA-RNA duplex with said first messenger RNA and thereby, when it is expressed, in¬ hibits translation of said gene coding for the hydrolytically active enzyme. 86. A replicon according to claim 80 wherein the excisable negatively functioning regulatory nucleotide sequence is a gene encoding a polypeptide repressor of transcription of the gene whose expression results in the formation of a hydroly¬ tically active enzyme. 87. A replicon according to claim 86 wherein the excisable negatively functioning regulatory nucleotide sequence is a gene encoding a lac repressor, and the gene whose expression results in the formation of a hydrolytically active enzyme is operably linked to the lac promoter, said lac promoter in¬ cluding the operator site for the lac repressor. 88. A replicon according to claim 80 wherein the negatively functioning regulatory nucleotide sequence is a termination sequence preventing transcription of the gene whose expres- sion results in the formation of a hydrolytically active enzyme. 89. A replicon according to claim 59 wherein the gene coding for the hydrolytically active enzyme is stochastically ex- pressed as a result of recombinational inversion of an in¬ vertible promoter sequence of said regulatory nucleotide sequence said promoter being operably linked to the gene coding for the enzyme. 90. A replicon according to claim 89 wherein the promoter sequence is a sequence carrying the fimA promoter or a functional homologue thereof. 91. A replicon according to claims 59 wherein the gene coding for the hydrolytically active enzyme is derived from a repli¬ con selected from the group consisting of a bacterial plas- mid, a bacterial chromosome, a procaryotic virus, a eucaryo¬ tic plasmid, a eucaryotic virus, a eucaryotic chromosome, eucaryotic mitochondria, a eucaryotic chloroplast and a synthetic sequence. 92. A replicon according to claim 59 wherein the hydrolyti- cally active enzyme contains at least one cysteine residue. 93. A replicon according to claims 91 wherein the gene coding for the hydrolytically active enzyme is derived from a bac¬ terial replicon. 94. A replicon according to claim 93 wherein the hydrolyti- cally active enzyme is encoded by a gene derived from a replicon isolated from a gram-negative bacterium. 95. A replicon according to claim 94 wherein the hydrolyti¬ cally active enzyme is encoded by a gene derived from a Serratia species. 96. A replicon according to claim 95 wherein the gene is derived from Serratia marcescens. 97. A replicon according to claim 96 wherein the hydrolyti¬ cally active enzyme is an endonuclease encoded by a gene having the following DNA sequence: -XX-CC£-ΛCΛαXriCGΛATC--ΛTC--A--AΛCταX^^ ccc--ccτcπt-CCAccπA rwccr--ττ--ACG-^ CCACCAACCTCTCTATCCT--CCTCATCCTTO^ ct-τccπGCA--Gaτ«aαx*CTAα^τTC^ --α-CΛC--α-C--UGAC-XCC»Λ--ra-ΛAω C-ACX-TA-X-C>--C CTAC-TCCCC--CC-C-*C-XW τr CC-aaκ r-axc χgιcc- (-ATCAgaw.-X^ ~~( XχVτ'Ummχma m ι>m*c>-f- TATCCrCCnTC--TCTrCCATCAC-AACA--CCC^ ATACGC-a-AAAC-aCAACCTAGTCTnnt-CCt-C^ τaxx_rCcax c~tχG A GΛ--xca A-»c(-cccιccτιX a;rca:Λ--a-cιuιu>ι , ι. lu-a-co-cc-Acx-cccτcA 98. A replicon according to claim 96 wherein the hydrolyti¬ cally active enzyme is a phospholipase encoded by a gene having the following DNA sequence: U1 101. A replicon according to claim 59 wherein the gene coding for the hydrolytically active enzyme or the nucleotide sequence regulating said gene is mutated at one or more sites, whereby the cell function-limiting effect of the aenzyme encoded by the gene, when expressed in the cell is the same or increased relative to the cell function-limiting effect of the enzyme expressed in a cell not containing mutated DNA. 102. A replicon according to claim 101 wherein gene coding for the hydrolytically active enzyme is a gene coding for an endonuclease. 103. A replicon according to claim 102 wherein the gene coding for an endonuclease is a gene derived from a Serratia species. 104. A replicon according to claims 59 wherein the nucleotide sequence regulating the transcription of the gene encoding the hydrolytically active enzyme is derived from a replicon selected from a bacterial plasmid, a bacterial chromosome, a procaryotic virus, a eucaryotic plasmid, a eucaryotic virus, a eucaryotic chromosome, eucaryotic mitochondria, a eucaryo¬ tic chloroplast and a synthetic sequence. 105. A replicon according to claim 59 which is a replicon comprising a further regulatably expressible gene encoding a non-enzyme cell function-limiting function. 106. A replicon according to claim 105 wherein the further regulatably expressible gene is regulated by a regulatory nucleotide sequence of the same type as the sequence regul¬ ating the gene coding for the cell function-limiting hydro¬ lytically active enzyme. 107. A replicon according to claim 105 wherein the further regulatably expressible gene is regulated by a regulatory nucleotide sequence of an other type than the sequence regu¬ lating the gene coding for the cell function-limiting hydro- lytically active enzyme, said other type being a regulatory nucleotide sequence capable of regulating said gene coding for the cell function-limiting enzyme. 108. A replicon according to claim 105 wherein the further regulatably expressible gene is the hok gene from the parB region of plasmid Rl or a DNA sequence which is functionally homologous to the Rl hok gene. 109. A replicon according to claim 105 wherein the further regulatably expressible gene is the gef gene. 110. A replicon according to claim 59 which is a replicon further comprising a DNA sequence not naturally related to the replicon carrying the gene coding for the hydrolytically active enzyme and/or the replicon carrying the regulatory nucleotide sequence. 111. A replicon according to claim 110 wherein the DNA sequence not naturally related to the gene coding for the hydrolytically active enzyme and/or the replicon carrying the regulatory nucleotide sequence, is a sequence coding for an immunologically active gene product. 112. A replicon according to claim 110 wherein the DNA sequence not naturally related to the gene coding for the enzyme and/or the replicon carrying the regulatory nucleotide sequence, is a sequence coding for a pesticidally active gene product. 113. A replicon according to claim 110 wherein the DNA sequence not naturally related to the gene coding for the enzyme and/or the replicon carrying the regulatory nucleotide sequence, is a sequence coding for a pollutant-degrading gene product. 114. A population of cells consisting of a multiplicity of cells as defined in claims 1, said cells containing a gene whose expression results in the formation of an enzyme which is present and hydrolytically active in the cytoplasm of said cells, the cells further containing a regulatory nucleotide sequence which regulates the expression of said gene, the expression of said gene leading to formation of the enzyme in the cells at a rate which results in the hydrolysis of hydro¬ lysable cytoplasmic substances necessary for non-limited function of the cells, to an extent whereby the function of *the cells is being limited. 115. A cell population according to claim 114 comprising transformable cells having been transformed with the recom¬ binant replicon as defined in claim 59, said replicon being capable of replicating in said cells. 116. A cell population according to claim 114 wherein the cells are bacterial cells. 117. A cell population according to claim 116 wherein the bacterial cells are selected from species whose natural habitat is a habitat selected from soil, surface water and plants. 118. A cell population according to claim 116 wherein the bacterial cells are gram-negative bacterial cells. 119. A cell population according to claim 114 wherein the expression of the gene coding for the function-limiting enzyme is regulatable by a repressor substance which can undergo a decay when said cells are released to a human or animal body or the outer environment to an extent whereby the repressor substance is converted to a non-functional form, said repressor substance being present in the cells of the population in different amounts whereby as a result of said linear decay, the function of the cells of the population will be gradually limited. 120. A method of limiting the survival of a cell population in a first or a second environment which method comprises transforming the cells of said population with a recombinant replicon being replicated in the cells of the population and containing a gene whose expression results in the formation of an enzyme which is present and hydrolytically active in the cytoplasm of said cell, the cells further containing a regulatory nucleotide sequence being regulatable by an en¬ vironmental factor and which regulates the expression of said gene, the expression of said gene leading to formation of the enzyme in the cells at a rate which results in the hydrolysis of hydrolysable cytoplasmic substances necessary for non- limited function of the cell, to an extent whereby the function of the cells is being limited leading to a limita¬ tion of the survival of the cell population. 121. A method according to claim 120 wherein the cell popula- tion is the cell population as defined in claims 111-119. 122. A method according to claim 120 wherein the survival of the cell population is limited in a first environment in which the gene is expressed, said cell population thereby being contained in said first environment. 123. A method according to claim 120 wherein the survival of the cell population is not limited when present in a f rst environment, which first environment could change to a second environment physically and/or chemically distinct from the first environment, in which first environment the gene whose expression results in the formation of a hydrolytically active enzyme is not expressed, but the survival of which cell population is limited when transferred to a second environment or when present in a physically and/or chemically changed first environment, where the gene is expressed. 124. A method according to claim 120 wherein the survival of a cell population is being limited by providing in the cells a gene coding for the hydrolytically active enzyme which is operably linked to a regulatory nucleotide sequence encoding a repressor substance which can undergo a decay when said cells are released to a human or animal body or the outer environment to an extent whereby the repressor substance is converted to a non-functional form, said repressor substance being present in the cells of the population in different amounts whereby as a result of said decay, the function of the cells of the population will be gradually be limited. 125. A method of containing an extrachromosomal recombinant replicon to a first kind of cell, where said replicon is naturally transferable to a second kind of cell, which method comprises providing on the recombinant extrachromosomal replicon a gene whose expression results in the formation of an enzyme which is present and hydrolytically active in the cytoplasm of a cell, the formation of said enzyme being at a rate which results in the hydrolysis of hydrolysable cyto- plasmic substances necessary for non-limited function of the cell, to an extent whereby the function of the cell is being limited, said first kind of cells having or being modified to have a chromosomal replicon comprising a regulatory nucleo¬ tide sequence which inhibits the expression of said gene and thereby protects said first kind of cells, said regulatory gene being lacking in said second kind of cell, whereby, if a cell of the second kind receives said extrachromosomal recom¬ binant replicon said gene is expressed and has a function- limiting effect thereon. 126. A method according to claim 124 wherein the recombinant replicon is a replicon as defined in claim 59 which does not contain a regulatory nucleotide sequence. 127. A method according to claim 125 wherein the expression of the gene whose expression results in the formation of a hydrolytically active enzyme, results in the formation of an endonuclease being capable of hydrolyzing diesterphosphate bonds in nucleic acids in the cell, the expression of said gene leading to formation of the endonuclease in the cell at a rate which results in the presence of nicks in the cell nucleic acids to an extent which cannot be repaired by the nucleic acid repair mechanism of the cells, thereby limiting the function of said cell. 128. A method of stochastically limiting the survival of a cell population which comprises transforming the cells thereof with a recombinant replicon containing a regulatably expressible gene which, when expressed in a cell encodes an enzyme which is present and hydrolytically active in the cytoplasm of the cell, the expression of said gene leading to formation of the enzyme in the cell at a rate which results in the hydrolysis of hydrolysable cytoplasmic substances necessary for non-limited function of the cells, to an extent whereby the function of the cells is being limited, the expression of said genes or genes being stochastically in¬ duced as a result of recombinational excision of an excisable negatively functioning regulatory nucleotide sequence which, while present in the cells, inhibits expression of the gene coding for the enzyme, said negatively functioning regulatory nucleotide sequence being contained in the recombinant repli¬ con or in an other recombinant replicon present in cells of the population containing the replicon. 129. A method according to claim 128 wherein the recombinant replicon is the replicon as defined in claim 73. 130. A method of stochastically limiting the survival of a cell population which comprises transforming the cells there- of with a recombinant replicon containing a regulatably expressible gene which, when expressed in a cell encodes an enzyme which is present and hydrolytically active in the cytoplasm of the cell, the expression of said gene leading to formation of the enzyme in the cell at a rate which results in the hydrolysis of hydrolysable cytoplasmic substances necessary for non-limited function of the cells, to an extent whereby the function of the cells is being limited, the expression of said gene being stochastically induced as a result of a site-specific recombinational excision of an excisable negatively functioning regulatory nucleotide sequence which, while present in the cells, inhibits expres¬ sion of the gene coding for the enzyme, said negatively functioning regulatory nucleotide sequence being contained in the recombinant replicon or in an other recombinant replicon present in cells of the population containing the replicon. 131. A method according to claim 130 wherein the recombinant replicon is the replicon as defined in claim 81. 132. A method of stochastically limiting the survival of a cell population which comprises transforming the cells thereof with a recombinant replicon containing a regulatably expressible gene which, when expressed in the cells of the population encodes an enzyme not being transportable over the cell membrane of said cells, and which is hydrolytically active in the cytoplasm of the cells, the expression of said gene leading to formation of the enzyme in the cells at a rate which results in the hydrolysis of hydrolysable cyto¬ plasmic substances necessary for non-limited function of the cells, to an extent whereby the function of the cells is being limited, the expression of said gene being stochasti- cally expressed as a result of recombinational inversion of an invertible promoter sequence of said regulatory nucleotide sequence, said promoter being operably linked to the gene coding for the enzyme which, while present in the cells, inhibits expression of the gene coding for the enzyme, said regulatory nucleotide sequence being contained in the recom¬ binant replicon or in an other recombinant replicon present in a cells containing the replicon. 133. A method according to claim 132 wherein the promoter sequence is a sequence carrying the fimA promoter or func- tional homologue thereof. 134. An immunologically active composition which contains a viable function-limited cell population as defined in claim 114 wherein the cells contain a further DNA sequence not naturally related to the gene coding for the hydrolytically active enzyme or to the regulatory nucleotide seguence, which further sequence is a sequence coding for an immunologically active gene product, the cells being function-limited to an extent which, when the composition is administered to a human or an animal, allows the cells to express the immunologically active gene product for a period of time and in an amount sufficient to obtain an effective immune response in said human or animal, but which does not allow the cells to per¬ sist in the human or the animal. 135. A composition according to claim 134 wherein the cells therein contain a sequence coding for an immunologically active gene product which is a sequence coding for a fusion protein comprising the immunologically active gene product and a polypeptide, the presence of which results in the transportation of said fusion protein to the outer surface of the cells. 136. A composition according to claim 135 wherein the poly¬ peptide being present in the fusion protein is a cell surface polypeptide selected from a polypeptide derived from fim- brillin protein, a pilus, a flagellum, an OM surface protein. 137. A composition according to claim 135 wherein the cell surface polypeptide is derived from a bacterial species selected from Enterobacteriaceae. Vibrionaceae and Pseudo- monadaceae. 138. A pesticidally active composition which contains a viable cell population as defined in claims 111 wherein the cells contain a further DNA sequence not naturally related to the gene coding for the hydrolytically active enzyme or to the regulatory nucleotide sequence, which further sequence is a sequence coding for a pesticidally active gene product, the cells being function-limited to an extent which, when the composition is administered to an environment " containing a pest, allows the cells to express the pesticidally active gene product for a period of time and in an amount sufficient to obtain an effective pesticidal effect in said environment but which does not allow the cells to persist in the environ¬ ment. 139. A composition according to claim 138 wherein the further DNA sequence codes for a gene product which is toxic for insects or their progeny. 140. A composition according to claim 139 wherein the further DNA sequence is a sequence derived from a strain of Bacillus thuringiensis encoding an insecticidal protein. 141. An environmental pollutant-degrading composition which contains a viable cell population as defined in claim 114 wherein the cells contains a further DNA sequence not na¬ turally related to the gene coding for the hydrolytically active enzyme or to the regulatory nucleotide sequence, which further sequence is a sequence coding for an environmental pollutant-degrading gene product, the cells being function- limited to an extent which, when the composition is administered to an environment containing a pollutant to be degraded, allows the cells to express said pollutant degra¬ ding gene product for a period of time and in an amount sufficient to obtain an effective pollutant-degrading effect in said environment but which does not allow the cells to persist in the environment. 142. A composition according to claim 141 wherein the gene or genes the expression of which results in the formation of a cell function-limiting enzyme is only expressed when the pollutant degradable by the pollutant-degrading gene product is substantially degraded.

Inventors: BEJ ASIM K, EBERL LEO, GIVSKOV MICHAEL, KRISTENSEN CLAUS STERNBERG, MOLIN SOREN, BEJ, ASIM, K., EBERL, LEO, GIVSKOV, MICHAEL, KRISTENSEN, CLAUS, STERNBERG, MOLIN, SOEREN
Applicants: BEJ ASIM K, EBERL LEO, GENEXPRESS APS, GIVSKOV MICHAEL, KRISTENSEN CLAUS STERNBERG, MOLIN SOREN
Application year: 1993
Main classification: C12N15
IPC codes: C12N5, C12N9, C12N15, C12R1