Patent Publication Number: US-7595186-B2

Title: Cytotoxin-based biological containment

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 11/558,856, entitled CYTOTOXIN-BASED BIOLOGICAL CONTAINMENT, filed Nov. 10, 2006, which is a division of U.S. patent application Ser. No. 09/700,130, entitled CYTOTOXIN-BASED BIOLOGICAL CONTAINMENT, having a §371 date of Dec. 18, 2001, now U.S. Pat. No. 7,183,097, issued Feb. 27, 2007, which is the U.S. National Phase under 35 U.S.C. §371 of International Application No. PCT/DK99/00258, filed May 7, 1999, which claims priority to Danish Application DK 0627/98 filed May 7, 1998 and U.S. Provisional Patent Application No. 60/085,067, filed May 12, 1998. The disclosure of U.S. patent application Ser. Nos. 11/558,856 and 09/700,130 are incorporated herein by reference in their entireties. 
     SEQUENCE LISTING 
     The present application is being filed along with a Sequence Listing in electronic format. The Sequence Listing is provided as a file entitled VANM247-001DV1C-SEQ.TXT, created Aug. 9, 2007, which is 35 Kb in size. The information in the electronic format of the Sequence Listing is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of biologically containing genetically modified microorganisms in a particular environment and vectors to a particular host cell. Specifically, there is provided recombinant vectors and cells containing a proteic killer system based on the  E. coli  RelE polypeptide and functional equivalents of this cytotoxin and method of containing replicons and cells, respectively to particular host cells or particular environments, respectively. 
     TECHNICAL BACKGROUND AND PRIOR ART 
     The increasing application of recombinant DNA technology to engineer novel microorganism which are industrially useful have caused concerns in the general public over the potential risks involved. 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 primary 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 biological containment systems which subsequent to the environmental release of the GEMs or their administration as vaccines to a human or an animal body, effectively kill the released organisms in a controlled manner 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 nutrients 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 nonsense mutation in a gene, the expression of which is indispensable for plasmid replication or a suppressor mutation in the chromosome, 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 chromosomes of the GEMs. 
     Recently, an alternative biological containment strategy has been developed in which a 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 prevailing 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 environment. A cell killing function as defined herein may also be referred to as an active biological containment factor. 
     If a stochastically induced mechanism of expression regulation 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 biological 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 such cell killing functions have been found in bacterial chromosomes and in prokaryotic plasmids. Examples of chromosomal genes having cell killing functions are the gef (Poulsen et al., 1990) and relF (Bech et al., 1985) genes from  E. coli   K-12 . Examples of plasmid encoded suicide genes are hok and flmA (Gerdes et al., 1986) genes isolated from plasmids R1 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-transcriptional level, and that they all encode small toxic proteins of about 50 amino acids and that their translation is controlled by antisense RNA. The application of the hok gene in a biological containment system has been disclosed in WO 87/05932. 
     An alternative biological containment system is disclosed in WO 95/10614 which is based on the 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. 
     The stable maintenance of low copy-number plasmids in bacteria is secured by a number of plasmid-borne gene systems, one of which is based on killing of plasmid-free cells (also termed post-segregational killing). This regulated killing is based on a toxin-antidote principle, i.e. a two-component system comprising a stable toxin and an unstable antidote for the toxin. One such system, which is referred to as a proteic killer gene system is based on protein toxins and protein antidotes (reviewed by Jensen and Gerdes, 1995). The natural function of such systems is to provide stable maintenance of plasmids and it has not been suggested previously to utilize the systems as the basis for confining GEMs to a particular environment. 
     The  E. coli  relB operon encodes three genes, relB, relE and relF (Bech et al., 1985). It has now been found that relE encodes a cytotoxin whose overproduction is lethal to host cells and that the relB gene encodes an antitoxin that prevents the lethal action of RelE. When present on a plasmid, the relBE operon was able to stabilize the inheritance of a mini-R1 test plasmid. It was also found that relBE homologous gene systems are found in a wide variety of Gram-negative and Gram-positive bacteria and in Archae. 
     These results show that the relBE genes constitute a new ubiquitously occurring family of gene systems that belongs to the proteic plasmid stabilization systems. 
     These findings has opened up for an alternative, highly effective and versatile biological containment system as it is described in the following. Importantly, it has been discovered that such a system involves the significant advantage that the frequency of spontaneously occurring mutants of microorganisms that have become resistant to the lethal effect of these cytotoxins is very low. This implies that this biological containment system is very safe. 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention pertains in a first aspect to a method of conditionally controlling the survivability of a recombinant microbial cell population, the method comprising (i) providing in the cells of said population a gene coding for a cytotoxic first kind of polypeptide, the gene is selected from the group consisting of the gene coding for the  E. coli  K-12 RelE polypeptide and a gene coding for a functionally equivalent polypeptide (said genes collectively being designated herein as the relE gene family), said gene is expressible in the cells of the population and, operably linked to the gene, a regulatable regulatory DNA sequence and (ii) cultivating the cell population under conditions where the relE gene or the gene coding for a functionally equivalent polypeptide is expressed, the expression leading to an at least partial killing of the cell population. 
     In a further aspect there is provided a method of confining an extrachromosomal replicon to a microbial cell population, the method comprising the steps of
     (i) isolating a microbial cell naturally containing a gene belonging to the relE gene family coding for a first kind of polypeptide that, when it is expressed in the cell, acts as a toxin for the cell or, if the cell does not naturally contain a gene belonging to the relE gene family, introducing such a gene into the cell,   (ii) introducing into the cell the extrachromosomal replicon to be confined, said replicon containing a gene coding for a second kind of polypeptide that, by binding to the first kind of polypeptide, acts as an antitoxin for said first kind of polypeptide,   (iii) cultivating the cell under conditions where the genes coding for the first and the second kind of polypeptides are expressed, whereby a daughter cell that does not receive a copy of the extrachromosomal replicon is killed by the first kind of polypeptide being expressed in the absence of expression of the second kind of polypeptide.   

     In a still further aspect, the invention relates to a method of post-segregationally stabilizing a plasmid in a microbial host cell population, the method comprising the steps of
     (i) recombinationally inserting into the plasmid (a) a gene coding for a first kind of polypeptide selected from the group consisting of the  E. coli  K-12 RelE polypeptide and a functional equivalent thereof, said first kind of polypeptide having a toxin effect on the host cell and (b) a gene coding for a second kind of polypeptide that (1) is capable of acting as an antitoxin for first kind of polypeptide and (2) is capable of being degraded in the host cell at a higher rate than that at which the first kind of polypeptide is degraded,   (ii) cultivating the cell population under conditions where the genes coding for the first kind and second kind of polypeptides are expressed,
 
whereby a daughter cell that does not receive at least one copy of the plasmid is killed as a result of the faster degradation of the second kind of polypeptide.
   

     In yet other aspects, the invention provides a recombinant microbial cell comprising a gene coding for a first kind of polypeptide selected from the group consisting of the  E. coli  K-12 RelE polypeptide, a gene coding for a functionally equivalent polypeptide hereof or a variant or derivative of any such polypeptide, said first kind of polypeptide having a toxic effect on the cell, subject to the limitation that when the cell is  E. coli , the gene coding for the first kind of polypeptide is not derived from  E. coli , and a composition comprising such cells. 
     The invention also pertains to several methods of containing cells or replicons including
     (1) a method of limiting the survival of a cell population in a first or a second environment, which method comprises   (i) transforming the cells of said population with a gene coding for a cytotoxic polypeptide, the gene is selected from the group consisting of the gene coding for the  E. coli  K-12 RelE polypeptide, the gene coding for the plasmid F CcdB polypeptide, the gene coding for the plasmid R1 PemK polypeptide, the gene coding for plasmid RP4 ParE polypeptide, the gene coding for the prophage P1 Doc polypeptide and a gene coding for a functionally equivalent polypeptide for anyone of said polypeptides, said gene is expressible in the cells of the population, and operably linked to the gene, a regulatory DNA sequence being regulatable by an environmental factor and which regulates the expression of said gene, and   (ii) cultivating the cell population under environmental conditions where the gene coding for the cytotoxic polypeptide is expressed, the expression leading to an at least partial killing of the cell population,   (2) 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 a cytotoxic polypeptide selected from the group consisting of the  E. coli  K-12 RelE polypeptide, the plasmid F CcdB polypeptide, the plasmid R1 PemK polypeptide, the plasmid RP4 ParE polypeptide, the prophage P1 Doc polypeptide and a functionally equivalent polypeptide for anyone of said polypeptides 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 the gene product of which inhibits the expression of said gene or the cell function-limiting effect of the polypeptide 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 recombinant replicon said gene is expressed and has a function-limiting effect on said second kind of cell, and   (3) a method of stochastically limiting in an environment the survival of a cell population, the method comprising transforming the cells thereof with a recombinant replicon containing a regulatably expressible gene which, when expressed in a cell, encodes a cytotoxic polypeptide selected from the group consisting of the  E. coli  K-12 RelE polypeptide, the plasmid F CcdB polypeptide, the plasmid R1 PemK polypeptide, the plasmid RP4 ParE polypeptide, the prophage P1 Doc polypeptide and a functionally equivalent polypeptide for anyone of said polypeptides, the expression of said gene leading to formation of the polypeptide to an extent whereby the function of the cells is being limited, the expression of said gene is stochastically induced 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 polypeptide, 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.   

     DETAILED DISCLOSURE OF THE INVENTION 
     One objective of the present invention is to provide a novel approach to conditionally controlling the survivability of a recombinant microbial cell population. This approach is based on the use of what is generally referred to as proteic killer systems which have been reviewed i.a. by Jensen et al., 1995. These systems consist of two components, a cytotoxin polypeptide (also referred to herein as a first kind of polypeptide) and a corresponding antitoxin or antidote polypeptide (also referred to herein as a second kind of polypeptide) that by binding to the cytotoxic polypeptide inhibits the toxic effect hereof. A general characteristic of such proteic killer systems is that the antitoxin component in contrast to the toxin component is susceptible to protease degradation, resulting in a decay of the antitoxin polypeptide. 
     As used herein, the expression “microbial cell” includes any prokaryotic and eukaryotic cells as well as cells of Archae species. Thus this expression includes cells of bacterial species, fungal species, animal species including invertebrates, vertebrates, mammals, humans and insects, and plant cells. 
     Thus, in one aspect of the invention there is provided such a method of conditionally controlling the survivability of a recombinant microbial cell population that comprises as the first step, providing in the cells a gene coding for a cytotoxic first kind of polypeptide, which gene is selected from the gene coding for the  E. coli  K-12 RelE polypeptide and a gene coding for a structurally and functionally equivalent polypeptide and, operably linked to the gene, a regulatable regulatory DNA sequence. 
     Genes which are structurally and functionally equivalent to relE are herein collectively referred to as the relE gene family or as relE homologues. This group of genes including the  E. coli  plasmid P307 derived relE homologue encompasses genes the gene products of which have cytotoxic effects and which, relative to  E. coli  K-12 RelE, have at least 20% such as at least 30% e.g. at least 40% identical and conserved amino acids. The sequences listed in the below Table 1.5 are putative RelE homologues. 
     Whereas, in accordance with the invention, presently preferred recombinant microbial cells are prokaryotic cells such as Gram-negative and Gram-positive bacterial cells, it has been found that the survivability of other microbial cells such as Archae, yeast cells, fungal cells, animal cells including human cells and plant cells and replicons of such organisms can be conditionally controlled using the methods of the present invention. 
     In the present context, the expression “conditionally controlling” refers to a construction of the microbial cell which permits that the gene coding for the cytotoxic polypeptide can be expressed under certain pre-determined environmental conditions whereas under other such conditions, the gene is not expressed. Hence, the survivability of the microbial cells can be made dependent on certain pre-selected conditions. 
     In accordance with the invention, the survivability of microbial cells is controlled by the expression in the cells of a cytotoxic polypeptide selected from  E. coli  K-12 RelE polypeptide and a functionally equivalent polypeptide. As used herein, the term “cytotoxic” refers not only to a loss of the ability of microbial cells containing the toxin-encoding gene to remain viable as determined by the capability to propagate in media which, under identical environmental conditions, support unrestricted growth of similar cells not containing the toxin-encoding gene, but also to cells having, as a result of the expression of the polypeptide-encoding gene, a limited cell function, the latter expression denoting 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 decreased. 
     During the experimentation leading to the invention it was surprisingly found that a range of cytotoxic polypeptides according to the invention have the effect that they inhibit translation of genes. This general effect of RelE polypeptides and functionally equivalent polypeptides appears to represent a hitherto undiscovered mechanism for controlling survivability of cells and thus for containment of such cells or replicons in accordance with the methods of the present invention. 
     Whereas the recognizable manifestation of such limited cell function may ultimately be cell death, it may also 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 relatively increased requirement for one or more nutrient components or a relatively higher susceptibility to detrimental environmental factors such as sub-optimal temperatures or cell damages caused by toxic substances. 
     In the present context, the expression “a functionally equivalent polypeptide” refers to a polypeptide that has substantially the same effect on the survivability of microbial cells as the RelE polypeptide of  E. coli  K-12. As it is shown herein, a variety of Gram-positive and Gram-negative bacteria and Archae organisms comprise DNA sequences showing homology with the RelE polypeptide. To the extent gene products of structural homologues of the relE gene product show an effect on microbial cell survivability as it is defined above, they are encompassed by the present invention. It will also be appreciated that the term “functional equivalent” includes variants or derivatives of any of the above first kind of polypeptides, the sequences of which have been modified by substitution, deletion or addition of one or more amino acids and the gene product of which has retained at least part of the cytotoxic function of the gene product of the non-modified sequence. 
     In the above method, a regulatable regulatory DNA sequence is operably linked to the gene coding for the cytotoxic polypeptide. In accordance with the invention, such a regulatory sequence can be one with which the gene coding for the cytotoxic polypeptide is naturally associated or it can be a sequence with which the gene is not naturally associated. In the present context, the term “regulatory DNA sequence” is intended to indicate a DNA sequence which directly or indirectly regulates the expression of the gene coding for the cytotoxic polypeptide at the level of transcription or at the level of translation or at the level of protein function. The regulatory DNA sequence may thus be one, the function of which results in a suppression or inhibition of the activity of the regulatable promoter. 
     Such regulatory DNA sequences are referred to herein as “negatively functioning regulatory DNA sequences”. One interesting example of such a regulatory DNA sequence is a sequence coding for a repressor substance which represses the expression of the gene coding for cytotoxically active polypeptide and which substance may, when a cell containing it is released to a human or an animal body or to the outer environment where the substance is no longer being expressed, undergo a decay whereby the repression of expression of the cytotoxin-encoding gene is gradually reduced and eventually, when the decay of the repressor is completed, the repression is removed. 
     Another example of such a regulatory DNA sequence is a sequence encoding a polypeptide that acts as an antidote or antitoxin for the cytotoxic polypeptide. Such a sequence include the relB gene derived from the relBE operon of  E. coli  K-12 which is capable of binding to the RelE polypeptide and thereby inhibiting its effect. As also shown herein, sequences encoding such antitoxins can be found in Gram-negative and Gram-positive bacteria and in Archae. Such homologues of the relB sequence are encompassed by the present invention. 
     In preferred embodiments of the invention, the regulatory DNA sequence may be present in the cell in one or more recombinant replicons and it may be contained in the same replicon as that containing the cytotoxin-encoding gene or in a different recombinant replicon. 
     One way whereby the expression of the cell function-limiting cytotoxic polypeptide can be regulated is by providing in the cell a gene coding for the polypeptide, which gene is regulated at the level of transcription. 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 factors may either be ones which by their presence ensure expression of the gene coding for polypeptide or may, alternatively, be factors which suppress the expression of the gene so that their absence causes the polypeptide 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 polypeptide may be determined by the environmental conditions, by 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 affecting the expression of genes such as temperature 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 cytotoxic polypeptide is expressed when a certain chemical substance present in a first environment such as the fermentation 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 one which, when it is depleted or 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 cytotoxic polypeptide can 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 promoter. Similarly, the promoter may be 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 fermentation vessel, to a lower temperature prevailing in an outside 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 darkness prevailing in a first environment such as a fermentation vessel. 
     Where microbial cells as defined herein are cells that are to be released to the outer 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 promoter which is activated cyclically, e.g. by changes of the temperature, or 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 expression of the cytotoxic polypeptide 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 or by recombinational excision of a recombinationally excisable negatively functioning regulatory DNA sequence as defined above. 
     It should be noted that in order to ensure a general applicability of the present invention, the promoter used to initiate transcription of the gene coding for the toxic polypeptide 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 polypeptide, the regulatory DNA 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 synthesis of cell surface structures such as fimbriae. Examples of suitable promoters include the  E. coli  trp promoter which becomes activated in the absence of tryptophan, the bacteriophage λ P R  and P L  promoters controlled by temperature sensitive regulatory DNA sequences, the  Bacillus subtilis  sporulation gene promoters which are activated during sporulation, and the  E. coli  and  Salmonella  fimbriae gene promoters which are activated stochastically. 
     In case of chemically regulatable promoters, the chemical substance, the presence or absence of which determines the activation of the promoter, is suitably selected from carbon or nitrogen sources, metabolites, amino acids, nucleosides, purine or pyrimidine bases or metal ions. When the chemical substance is one which, when present, suppresses promoter activity, it is preferably a substance which rarely occurs in the natural environment in such concentrations that the promoter would not be activated when the cell is released to the natural environment. One example of such a promoter 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 environment. A containment system according to the invention 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, which is sufficient to repress the promoter in such an environment, the promoter, however, being derepressed when the cell is released from the first environment to a second environment, e.g. the outer environment which usually contains very low amounts of tryptophan or no tryptophan at all. 
     It is also possible to select a promoter that is regulated by the absence or presence of one or more compounds in exudates of plants colonized with a recombinant organism according to invention. 
     In this context, another useful promoter is an arabinose inducible promoter including that contained in the plasmid pBAD (Guzman et al., 1995). Without arabinose added to the growth medium, the pBAD promoter is completely turned off. However, in the presence of arabinose, strong transcription is induced. This particular promoter is repressible by the addition of glucose to the growth medium. Thus, by the addition of glucose, transcription from pBAD can be rapidly and efficiently turned off. The glucose repression effect is epistatic to the inducer effect by arabinose. Hence, if cells with a pBAD-carrying plasmid are grown in a medium containing both arabinose and glucose, the promoter is not induced. However, if cell growth depletes the medium for glucose, then the promoter will be induced. Therefore, such a plasmid is suitable for the conditional turning on and off the expression of gene, in particular toxin-encoding genes as described herein. 
     Accordingly, in one embodiment of the invention the method is used to contain microbial cells wherein the promoter is suppressible by a first kind of chemical compound and inducible by a second kind of chemical compound whereby, when the first kind of compound is depleted from the medium, the promoter is induced by the second kind of compound. 
     Another example of a regulatable promoter, the activation of which is determined by a chemical substance is the lac promoter which is inducible by e.g. isopropyl-β-D-thiogalactopyranoside (IPTG). 
     As mentioned above, the regulatable promoter may be a promoter, 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 cytotoxin and a regulatable 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 gene coding for a temperature sensitive repressor for the promoter. As one typical example, the λ promoters including those mentioned above may be regulated by a temperature sensitive λ cl repressor. 
     Promoters which are activated stochastically by periodic inversions of the promoter region (in the present context, such promoters are also termed as “invertible promoters” and “inversional switch promoters”) and which are 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  E. 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 frequency of the inversional switch as required by regulating the dosage of expression of the on and off genes. This is e.g. effected by means of suitable promoters transcribing into the on and off genes. The frequency of transcription initiation by these promoters will then determine the relative dosage levels of the on and off gene products being formed. 
     In accordance with the invention, one particular method of stochastically regulating the expression of the gene coding for the toxic polypeptide is the induction of the gene expression as a result of recombinational excision of an excisable negatively functioning regulatory DNA sequence which, while present in the cell, inhibits expression of the gene. In the present context, the term “recombinational excision” refers to the result of a naturally occurring phenomenon of genetic recombination (cross-over) whereby DNA sequences in replicons, 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 recombination in a bacterial cell. 
     When a negatively regulatory DNA sequence is inserted between directly repeated nucleotide sequences of a sufficient length in a recombinationally proficient cell which, in accordance with the invention contains a gene coding for the toxic polypeptide, recombination between the repeats results in the recombinational excision of the negatively regulatory DNA sequence permitting the gene to be expressed. 
     Accordingly, the phenomenon of recombinational excision implies that a DNA subsequence, i.e. the negatively regulatory DNA sequence, is excised from a longer DNA sequence through a recombination event. In essence, 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 DNA sequence, this could recombine to form W-X-Z, with the Y subsequence being excised. 
     As mentioned above, the frequency of the recombination can 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 frequency depending on the degree of mismatch. As an example, it has been found that 10% divergence between nucleotide sequence repeats may reduce the recombination frequency 40-fold. 
     Accordingly, the microbial cell comprising the gene coding for the cytotoxic polypeptide may, in accordance with the invention, be a cell containing a regulatory DNA sequence which is a recombinationally excisable negatively functioning regulatory DNA 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 in other embodiments where a moderate degree of cell function limitation is desirable, it is appropriate to use repeats which are more or less heterologous, but still allowing a desirable lower frequency of recombination to occur. Accordingly, in the present context, the term “sufficiently homologous” is used to indicate a degree of homology between two flanking nucleotide sequence repeats which results in a desired frequency of recombinational events in a cell containing the gene coding for the toxin polypeptide and a negatively regulatory DNA sequence. 
     As it also has been mentioned above, the frequency of recombination 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 100-5,000 base pairs. In certain preferred embodiments, it is advantageous to use flanking sequences, the length of which is in the range of 200-3,000 base pairs. As the flanking sequences can 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. Another example of a useful nucleotide sequence which, when inserted as repeats, results in recombination, is a subsequence 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 3,000 base pairs, such as 598 base pairs. 
     In one interesting embodiment of the invention, the excisable negatively regulatory DNA sequence operably linked to the gene encoding the cytotoxic polypeptide is a gene encoding an antisense RNA which forms an RNA-RNA duplex with said the messenger RNA of the polypeptide-encoding gene and thereby, when it is expressed, inhibits translation of said gene coding for the polypeptide. 
     In another useful embodiment of the present invention, the recombinationally excisable negatively regulatory DNA sequence is a gene encoding a polypeptide repressor of transcription of the polypeptide-encoding gene. Such a repressor may, e.g. be a lac repressor including the repressor encoded by the Laclq gene. It will be appreciated that the negatively regulatory DNA sequence can also be a gene coding for RelB antitoxin or functionally equivalents hereof. 
     In a further useful embodiment of the invention, the excisable negatively regulatory DNA sequence is a transcription termination sequence, preventing the transcription of the cytotoxic polypeptide-encoding gene. In one specific embodiment of the invention, such a suitable terminator sequence is the rpoCt′ transcription terminator isolated from the plasmid pHBA102rpoCt (Squires et al., 1981, Nucleic Acid Res., 9:6827-6839). 
     Negatively regulatory DNA sequences which, in accordance with the invention, are suitable, can be isolated from DNA sequences derived from a virus, or a prokaryotic or eucaryotic cell. Thus, sources of the DNA sequence include bacterial chromosomes, bacterial plasmids, prokaryotic viruses, eucaryotic viruses, eucaryotic plasmids, or eucaryotic chromosomes. 
     In preferred embodiments of the invention, the excisable negatively regulatory DNA sequence 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 DNA sequence comprising at least the above-mentioned sequences and optionally the gene coding for the cytotoxically active polypeptide, 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 allowing 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 toxin polypeptide and operably linked to the negatively regulatory DNA sequence, may be inserted in a replicon which is different from the replicon containing said gene. Optionally, the cassette as defined above is 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 DNA sequence which may be inserted in the microbial cell as defined herein, as a recombinationally excisable DNA sequence in the manner explained in details above. Accordingly, in one embodiment, the present invention provides a microbial cell wherein the toxin-encoding gene is stochastically expressed as a result of recombinational inversion of an invertible promoter sequence. 
     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 resolution 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 DNA 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 been found that such a site-specific recombination system provides a useful mechanism for stochastically regulating the expression of a gene such as the gene coding for the toxic polypeptide as defined herein, since the site-specific recombination may be used to obtain recombinational excision of a negatively regulatory DNA sequence as defined above. 
     Accordingly, in one interesting embodiment, the present invention provides a microbial cell as defined herein in which the negatively regulatory DNA sequence is a sequence flanked by a first site for a site-specific resolution recombinase 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 excisable in the cell. In a specific embodiment, the gene coding for the multimer resolving enzyme is located in trans relative to the sites for site-specific resolution. In the present context, one useful example of a suitable gene is the parA gene isolated from plasmid RP4. 
     In accordance with the invention, the method of controlling the survivability of microbial cells can be based on providing in the cells a gene coding for a cytotoxic polypeptide that is structurally and functionally equivalent to the  E. coli  RelE polypeptide (the relE gene family). Such a genie can be derived from the chromosome or another replicon of a Gram-negative bacterium including  Enterobacteriaceae  spp. such as  E. coli, Hemophilus  spp. such as  H. influenzace, Vibrionaceae  spp. such as  V. cholerae, Pseudomonadaceae  spp.,  Helicobacter  spp. such as  H. pylori  and  Synechosystis  spp, the latter organisms belonging to the group of cyanobacteria. The gene may also be derived from the chromosome and other replicons of Gram-positive bacteria including lactic acid bacteria such as  Streptococcus  spp including  Streptococcus pneumoniae., Bacillaceae  spp such as  B. thuringiensis , and  Mycobacterium  spp. and from species belonging to Arhae such as  Methanococcus jannaschii  and  A. fulgidus . Such genes include those that are defined herein as belonging to the relE gene family. The RelE equivalent polypeptide from  M. jannaschii  was shown to be toxic for  E. coli  when expressed in this organism. 
     However, genes coding for cytotoxins of other proteic killer systems and which are therefore functional equivalents of the  E. coli  K-12 RelE polypeptide can also be used in accordance with the invention for conditionally controlling the survivability of microbial cells. Such genes include the gene coding for the plasmid F CcdB polypeptide, the gene coding for the plasmid R1 PemK polypeptide, the gene coding for plasmid RP4 ParE polypeptide and the gene coding for the prophage P1 Doe polypeptide, as described by Jensen et al., 1995. 
     It will be understood that in this context, the term “functional equivalent” includes variants or derivatives of any of the above first kind of polypeptides the sequences of which have been modified by substitution, deletion or addition of one or more amino acids and the gene product of which has retained at least part of the function of the gene product of the non-modified sequence. 
     In accordance with the invention, the relE family gene or any gene coding for a toxin of a proteic killer system is provided in the microbial cells at a location where it can be expressed effectively. Thus, in useful embodiments the gene is present on the chromosome of the cells whereas in other embodiments it is preferably located on an extrachromosomal element such as a plasmid or a cosmid. In a specific embodiment, the microbial cells according to the invention do not contain a gene coding for a second type of polypeptide that is capable of counteracting the cell toxic effect of the RelE polypeptide or the functional equivalent hereof. 
     However, in other useful embodiments, the microbial cells comprise a gene coding for a second kind of polypeptide that is capable of binding to the relE polypeptide or the functional equivalent hereof, the binding resulting in that the toxic effect of the RelE polypeptide or the functional equivalent is at least partially counteracted. Such a counteracting second kind of polypeptide is, as it is mentioned above, also referred to herein as an antitoxin or an antidote for the cytotoxic polypeptide. 
     Although, in certain uses of the present method, it is preferred that the genes coding for both the toxic polypeptide and the antitoxin herefor is under the control of the same regulatory sequences, it may, in other uses, be advantageous that the gene coding for the second kind of polypeptide is operably linked to a different regulatable regulatory DNA sequence as defined above, permitting that the gene coding for the second kind of polypeptide is suppressed under conditions where the gene coding for the RelE polypeptide or the functional equivalent is expressed. 
     It will be appreciated that the genes coding for the toxin polypeptide and the antitoxin polypeptide, respectively can be present on the same replicon such as a plasmid or on the chromosome, or they can be present on different replicons in the microbial cells. 
     A useful second kind of polypeptide is the RelB polypeptide derived from  E. coli  K-12 which i.a. binds effectively to the  E. coli -derived RelE polypeptide. However, the regulation of the toxic effect of the first kind of polypeptide can also be based on providing in the cells a gene coding for a second kind of antitoxically active polypeptide that is functionally equivalent to the  E. coli  RelB polypeptide. Such a gene can be derived from the chromosome or another replicon of a Gram-negative bacterium including  Enterobacteriaceae  spp. such as  E. coli ,  Hemophilus  spp. such as  H. influenzae ,  Vibrionaceae  spp. such as  V. cholerae ,  Pseudomonadaceae  spp.,  Helicobacter  spp. such as  H. pylori , and  Synechosystis  spp belonging to the group of cyanobacteria. Additionally, genes coding for structural and functional equivalents of the  E. coli  RelB polypeptide can be isolated from Gram-positive bacteria including lactic acid bacterial species such as  Streptococcus  spp.,  Bacillaceae  spp. such as  B. thuringiensis , and  Mycobacterium  spp. and from species belonging to Arhae such as  M. jannaschii  and  A. fulgidus . Sequences for the  E. coli  RelB polypeptide and for equivalents isolated from the above organisms are listed in Table 1.6. 
     Genes coding for functional equivalents of the  E. coli  K-12 RelB polypeptide which in accordance with the invention can be used for containing microbial cells and replicons include the genes coding for the plasmid F CcdA polypeptide, the plasmid R1 Peml polypeptide, the plasmid RP4 ParD polypeptide and the prophage P1 Phd polypeptide. 
     It will be understood that in this context the term “functional equivalent” includes variants or derivatives of any of the above second kind of polypeptides, the sequences of which have been modified by substitution, deletion or addition of one or more amino acids and the gene product of which has retained at least part of the function of the gene product of the non-modified sequence. 
     It is a significant objective of the present invention to provide the means of conditionally controlling the survivability of microbial cells that expresses one or more genes coding for a gene product of interest. In accordance with the invention such an objective is pursued for any type of gene products including enzymes such as proteases, enzymes which are effective in degrading carbohydrates such as starch degrading enzymes, lipid degrading enzymes and nucleases. 
     However, it is of particular interest to provide containment of microbial cells wherein the gene product of interest is selected from an immunologically active gene product, a gene product that is effective in degradation of an environmental pollutant and a pesticidally active product. 
     Accordingly, in such specific embodiments the microbial cells are cells which further comprise a DNA sequence that is 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 determinant) from a pathogenic organism which, when it is administered to the body of a human or an animal, is capable of stimulating the formation of antibodies therein. A microbial cell as defined herein which contains one or more genes encoding such a gene product can be utilized in the preparation of 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 currently used vaccines comprising 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 advantageously contains a multiplicity of genes each of which codes for a specific immunologically active gene product. 
     However, up till now the use of live vaccines has been limited since it is difficult to obtain the right combination of attenuation, viability and adequate immune response. 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 recombinant 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 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 for a cell toxic polypeptide as defined above. In particularly interesting embodiments, the invention provides, as a useful basis for a viable vaccine, the microbial cells as defined above whose expression is stochastically induced. 
     In useful embodiments of the invention, the cell which contains 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 desired 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 toxic polypeptide and the DNA sequence regulating its expression, 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 bacterial species which inherently contains an outer surface structure as mentioned above. Such species include as examples species of Enterobacteriaceae such as  Salmonella  and  E. coli  species,  Vibrionaceae  and  Pseudomonadaceae . It will be understood that strains of such species which are particularly 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 eukaryotic organisms such as fungi, yeast or protozoa. 
     In commercially important embodiments, the microbial cell comprising the gene coding for a cytotoxic polypeptide contains 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 that feed on plants, including insect pests, nematodes and vermins such as rodents or birds, is effective in respect of such pest control. 
     Such pests are currently 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 thuringiensis  that 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 pesticidally active organisms. One approach to solving this problem is to construct genetically engineered organisms having an increased toxic effect or a better survival rate in the environment. In addition to pesticidally active compounds from  B. thuringiensis , such compounds are produced by other microbial organisms including  Bacillus sphaericus , fungal species, algal species and plants. In accordance with the invention, genes coding for such biopesticides can be inserted and expressed in the biologically contained cells of the invention. 
     To the extent such improved organisms are 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 environmental 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 cytotoxic polypeptide can 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 of the toxic polypeptide may, in accordance with the present invention, be adjusted by selecting a first kind of polypeptide that has an appropriate cell function-limiting effect. 
     In another useful embodiment, the invention provides a cell in which the gene coding for a desired gene product is a sequence coding for a pollutant-degrading gene product. It is known that several xenobiotic compounds polluting the outer environment including soil and water can be degraded by microorganisms 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 hydrocarbons. 
     However, the public concern as mentioned above are also relevant in this context and accordingly, the present invention provides useful means of providing improved pollutant-degrading microbial cells, the survival of which can be controlled by regulating the expression of the first kind of polypeptide 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. 
     In addition to the above desired gene products, the microbial cells according to the invention can express any desired gene product including pharmaceutically active products such as e.g. hormones, interleukines and antibiotically active peptides. 
     As mentioned above, the invention provides in a further aspect a method of confining an extrachromosomal replicon to a microbial cell population. Basically, the method comprises the steps of isolating or constructing a microbial cell containing a gene belonging to the relE gene family expressing a first kind of polypeptide that is toxic for the cell and introducing into the cell the extrachromosomal replicon to be confined, which replicon contains a gene coding for a second kind of polypeptide acting as an antitoxin for said first kind of polypeptide, and cultivating the cells under conditions where the genes coding for the first and the second kind of polypeptides are expressed, whereby a daughter cell that does not receive a copy of the extrachromosomal replicon is killed by the first kind of polypeptide being expressed in the absence of expression of the second kind of polypeptide. 
     In preferred embodiments of such a method the cell population consists of cells that comprise a gene coding for a gene product of interest as defined above. 
     The above method of confining an extrachromosomal replicon is particularly useful when the replicon is a plasmid that naturally occurs in a host cell in a low copy number. Accordingly, the method is useful for confining a plasmid occurring in the microbial cells at a copy number which is in the range of 1-30 including the range of 1-10 such as the range of 1-5. 
     Microbial cells to which a replicon can be confined in accordance with the invention include Gram-negative bacterial species such as species belonging to  Enterobacteriaceae, Hemophilus, Vibrionaceae  and  Pseudomonadaceae  and Gram-positive bacterial species, fungal cells including yeast cells, animal cells including human cells and insect cells, and plant cells. 
     In a still further aspect, the invention provides a method of post-segregationally stabilizing a plasmid in a microbial host cell population as described above. As it is mentioned above, the method comprises the steps of (i) inserting into the plasmid a gene coding for a first kind of polypeptide as defined herein and a gene coding for a second kind of polypeptide as also defined herein that is capable of being degraded in the host cell at a higher rate than that at which the first kind of polypeptide is degraded, (ii) cultivating the cell population under conditions where the genes coding for the first kind and second kind of polypeptides are expressed, whereby a daughter cell that does not receive at least one copy of the plasmid is killed as a result of the faster degradation of the second kind of polypeptide. 
     The invention also provides a recombinant microbial cell as defined above, comprising a gene coding for a first kind of polypeptide. Such a cell can be a bacterium of a Gram-negative bacterial species including  Enterobacteriaceae  spp.,  Hemophilus  spp.,  Vibrionaceae  spp. and  Pseudomonadaceae  spp or it can be of a Gram-positive bacterial species such as a  Bacillus  species or lactic acid bacterial species, a fungal cell including a yeast cell, an animal cell including a human cell and an insect cell, and a plant cell. 
     As also mentioned above, the invention pertains in another aspect to a method of limiting the survival of a cell population in a first or a second environment, which method comprises as the first step that the cells are transformed with a gene coding for a cytotoxic polypeptide, which gene is selected from the group consisting of the gene coding for the  E. coli  K-12 RelE polypeptide, the gene coding for the plasmid F CcdB polypeptide, the gene coding for the plasmid R1 PemK polypeptide, the gene coding for plasmid RP4 ParE polypeptide, the gene coding for the prophage P1 Doc polypeptide and a gene coding for a functionally equivalent polypeptide for anyone of said polypeptides. 
     In a specific embodiment of such a method, the survival of the cell population is limited in a first environment in which the gene is expressed whereby the cell population is contained in said first environment. In another embodiment, the survival of the cell population is not limited when present in a first 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 cytotoxically active polypeptide 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. 
     In a still further embodiment of the above method, the survival of a cell population is being limited by providing in the cells a gone coding for a cytotoxic polypeptide which is operably linked to a DNA sequence encoding an antitoxin repressor substance which can undergo a decay when said cells are released to the outer environment to an extent whereby the repressor substance is converted to a non-functional form, whereby as a result of said decay, the function of the cells of the population will be gradually limited. 
     In yet another aspect of the invention, there is provided 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 as the first step providing on the recombinant extrachromosomal replicon a gene whose expression results in the formation of a cytotoxic polypeptide selected from the group consisting of the  E. coli  K-12 RelE polypeptide, the plasmid F CcdB polypeptide, the plasmid R1 PemK polypeptide, the plasmid RP4 ParE polypeptide, the prophage P1 Doc polypeptide and a functionally equivalent polypeptide for anyone of said polypeptides. 
     In one specific embodiment of such a method the gene product which inhibits the expression of the expression of the gene coding for the polypeptide or the cell function-limiting effect of the polypeptide is selected from the  E. coli  relB polypeptide, the plasmid F CcdA polypeptide, the plasmid R1 Peml polypeptide, the plasmid RP4 ParD polypeptide, the prophage P1 Phd polypeptide and a functionally equivalent polypeptide of anyone of such polypeptides. 
     The invention also provides a method as defined above of stochastically limiting in an environment the survival of a cell population. Such a method is particularly useful in the containment of recombinant cells which are to released to the outer environment or the animal or human body. 
    
    
     
       The invention will now be described in further details in the following examples and the drawings wherein 
         FIG. 1  illustrates relB K-12 ::lacZ and relE K-12 ::lacZ translational fusions. Shown are relevant parts of the lacZ reporter plasmids pKG4001 (carrying a relB K-12 ::lacZ fusion) and pKG4002 (carrying a relE K-12 ::lacZ fusion). Numbers to the right in the Figure indicates lacZ expression levels in Miller units. The low expression level of relE::lacZ in pKG4002 is, in part, due to the presence of an intact relB gene located on the plasmid. The relB gene product represses the relBE promoter c. 130-fold; 
         FIG. 2  illustrates in vitro translation of relBE P307 -carrying plasmids. Lane 1: pBR322; lane 2: pHA402 (pBR322-relB + ); lane 3: pHA403 (pBR322-relBE+); lane 4: pBR322; lane 5: pHA100 (pBR322-E11 contains the P307 relBE genes in their natural context); lane 6: pKG325; lane 7: pHA110 (pBR325-relB + ); 
         FIG. 3  shows the structure of expression plasmid pNDM220. The plasmid is a mini-R1 vector whose copy number is amplifiable at 42° C. due to the insertion of the temperature inducible λ P R  promoter upstream of the replication control region. The plasmid also carries the Cl 857  temperature-sensitive allele of the Cl repressor. Genes shown are copB (copy number control), repA (initiation of replication), parM and parR (plasmid stability loci), bla (β-lactamase) and lacl q . The plasmid contains the LaCl regulated pA1/O4/O3 promoter upstream of a multiple cloning site that contains unique BamHI and EcoRI restriction sites. Thus genes inserted downstream of the promoter are inducible with IPTG; 
         FIG. 4  illustrates cell killing by relE K-12  and anti-killing by relB K-12 . Shown are optical density at 450 nm and viable counts as function of time for strains MC1000/pMG223 (relE + ) (A, B), MC1000/pMG223/pMG2201 (relB −  control plasmid) (C, D) and MC1000/pMG223/pMG2202 (relB +  plasmid) (E, F). At time zero, transcription of relE on plasmid pMG223 was induced by the addition of IPTG (1 mM). Filled symbols indicate that IPTG was added. As seen from (E) and (F), the presence of relB on a second plasmid counteracted relE mediated cell killing; 
         FIG. 5  shows the structure of expression plasmid pBAD33. The plasmid is a medium copy number pACYC-derived vector. The plasmid carries the arabinose inducible pBAD-promoter and the araC gene of  E. coli . Thus upon addition of arabinose to pBAD33 containing cells, genes inserted downstream of pBAD are transcriptionally induced. Genes shown in the Figure are: pACYC-ori: origin of replication; CM(R): gene encoding chloramphenicol acetyl transferase; bla′: truncated (nonfunctional) gene encoding β-lactamase; mRNA1 encodes AraC activator protein; pBAD: arabinose-inducible promoter; 
         FIG. 6  A/B illustrates cell killing by RelE P307  and anti-killing by RelB P307 . Shown are optical density at 450 nm (A,C) and viable counts (B, D) as a function of time for strains MC1000/pHA810/pBR322 (A, B) or MC1000/pHA810/pHA110 (carrying relB P307 ). At time zero, transcription of relE P307  on plasmid pHA810 was induced by the addition of arabinose (0.02%). Filled symbols indicate that arabinose was added. As seen from (C) and (D), the presence of relB P307  on a second plasmid counteracted relE P307  mediated cell killing; 
         FIG. 7  shows maps of pHA705 and pHA715; 
         FIG. 8  illustrates OD 450  of MC1000/pHA-Sp2, MC1000/pHA705 and MC1000/pHA715 (+/−IPTG); 
         FIG. 9  shows viable counts of MC1000/pHA-Sp2, MC1000/pHA705 and MC1000/pHA715 (+/−IPTG); 
         FIG. 10  is the DNA sequence of the relBE Sp2  locus of  S. pneumoniae;    
         FIG. 11  is a map of pHA-Sp2; 
         FIG. 12  is a map of pHAG33; 
         FIG. 13  is a map of pHAG33-2; 
         FIG. 14  is a map of pHAG33-3; 
         FIG. 15  is a map of pHAG33-4; 
         FIG. 16  illustrates OD 450  of KT2440/pHAG33-2, KT2440/pHAG33-3 and KT2440/pHAG33-4 (+/−IPTG); 
         FIG. 17  shows viable counts of KT2440/pHAG33-2, KT2440/pHAG33-3 and KT2440/pHAG33-4 (+/−IPTG); 
         FIG. 18  is a map of pHA810; 
         FIG. 19  illustrates Glucose run-out, OD 450  of MC1000/pHA810; 
         FIG. 20  illustrates Glucose run-out, viable counts of MC1000/pHA810; and 
         FIG. 21  illustrates that RelE K12 , RelE P307  and RelE Mj  inhibit translation in vitro 
     
    
    
     EXAMPLES 
     Materials and Methods 
     (i) Bacterial Strains 
     The  E. coli  K-12 strain MC1000 (Casadaban and Cohen, 1980) which contains a chromosomal copy of the relBE genes was used as the standard cloning strain and when a chromosomal copy of the relB operon was required. The  E. coli  K-12 strain JS115 (leu, thy, thi, supE, ΔrelB), which contains a deletion covering the entire relB operon was provided by Olle Karlström. The latter strain was used for the regulatory studies of relBE. 
     (ii) Plasmids Used 
     Plasmid pOU253 is a mini-R1 based translational fusion vector carrying the lacZ gene of pNM482 (Minton, 1984). The fusion vector is segregationally stable due to the presence of the parA system of plasmid R1 (Dam and Gerdes, 1994). 
     Plasmid pNDM220 is a low copy-number mini-R1 expression vector carrying a multiple cloning site (mcs) placed between the Lacl regulated pA1/O4/O3 promoter (Lanzer and Bujard, 1988) and two transcriptional terminators. 
     pNDM220 was deposited on 30 Apr. 1998 under the Budapest Treaty with the DSMZ-Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH under the accession No. DSM 12157. 
     Plasmid pBD2430 (+388-+1899) is a pUC18 derivative carrying the complete relBE operon and gene IV located downstream of relF (Olle Karlström, unpublished). The relevant  E. coli  DNA present in pBD2430 is shown in Table 1.1 below. 
     pBD2430 was deposited on 30 Apr. 1998 under the Budapest Treaty with the DSMZ-Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH under the accession No. DSM 12161. 
     (iii) Plasmids Constructed 
     pKG4001: pBD2430 was digested with EcoRI and XhoI and the fragment carrying the relB promoter (Table 1.1) was inserted into pOU253 producing an in-frame translational fusion between relB K-12  and lacZ. Thus, pKG4001 carries a relB K-12 ::lacZ translational fusion. 
     pOU253 was deposited on 30 Apr. 1998 under the Budapest Treaty with the DSMZ-Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH under the accession No. DSM 12158. 
     pKG4002: pBD2430 was digested with EcoRI and Bst11071 and the resulting fragment was inserted into pOU253 producing an in frame translational fusion between relE K-12  and lacZ. Thus pKG4002 carries an intact relB K-12  gene and a relE K-12 ::lacZ translational fusion. 
     pMG223: relE K-12  was amplified by PCR on pBD2430 with primers relE1B (5′-CCCCGGATCCATAAGGAGTTTTATAAATGGCGTATTTTCTGGATTTTGACG, SEQ ID NO:1) containing the parA Shine &amp; Dalgarno (Gerdes and Molin, 1986) and relE2 (5′-CCCCCCTCGAGGTCGACTCAGAGAATGCGTTTGACCGC-3′, SEQ ID NO:2). The resulting relE K-12  carrying fragment was inserted into pNDM220 using the BamHI and SalI restriction sites. Plasmid pMG223 expresses RelE K-12  upon addition of IPTG. 
     pMG2201: this plasmid contains the EcoRI-Eco47III fragment from pBD2430 inserted between the EcoRI and ScaI sites of pBR322. Plasmid pMG2201 carries the relB K-12  promoter and the 5′ part of the relB K-12  gene. 
     pMG2202: pBD2430 was digested with EcoRI and Bst11071 and the relB K-12 -carrying fragment was inserted into pBR322 EcoRI-ScaI. The resulting plasmid carries the relB promoter and relB K-12 . 
     pHA100: Plasmid pNZ945 is a pBS(+) derivative that carries a 4.3 kb EcoRI fragment from plasmid P307. This fragment encodes the RepFIB replicon and the relBE genes of P307 (Saul et al., 1989). The 4.3 kb EcoRI fragment (designated E11) of pNZ945 was purified and restricted with PstI. The resulting 2.2 kb EcoRI-PstI fragment was inserted into pBR322 restricted also with EcoRI and PstI. The pBR322-derived plasmid carrying the 2.2 kb EcoRI-PstI fragment was designated pHA100. Plasmid pHA100 codes for the entire relBE system from P307. 
     pNZ945 was deposited on 30 Apr. 1998 under the Budapest Treaty with the DSMZ-Deutsche Sammlung von Mikroorganismen und Zellkulturen GmhH under the accession No. DSM 12160. 
     pHA110: The 2.2 kb EcoRI-PstI fragment of pHA100 was purified and digested with ApoI (EcoRI isoschizomer). The resulting EcoRI-ApoI DNA fragment (+1 to +1122) was inserted into the EcoRI site of pKG325 which was constructed as follows: Plasmid pBR325 was restricted with PstI, which has a unique recognition site in the plasmid. The resulting vector DNA fragment was made blunt ended with T4 DNA polymerase according to the manufacturer&#39;s instructions, and religated. Transformants that were resistant to chloramphenicol and tetracycline, but sensitive to ampicillin were selected. Thus, pKG325 is a TcR, CmIR and ApS derivative of pBR325. 
     pKG325 was deposited on 30 Apr. 1998 under the Budapest Treaty with the DSMZ-Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH under the accession No. DSM 12159. 
     Plasmid pHA110 contains the relB promoter (prelB P307 ) and gene relB P307 . 
     pHA205: Plasmid pHA205 is a derivative of the low copy-number mini-R1 expression vector pNDM220 that contains the relB gene from P307. The PCR fragment generated from pNZ945 using primers RelB-P307/1: 5′-CCCCCGGATCCCAGTCTTGAAAGGTGGC-3′ (SEQ ID NO: 3) and RelB-P307/2: 5′-CCCCCGAATTCTCATAGGTATTTATCCAG-3′ (SEQ ID NO:4) was restricted with BamHI and EcoRI and inserted downstream of the pA1/04/03 promoter of pNDM220. pHA210: Gene relE P307  was PCR-amplified from pNZ945 with the primers: relE-p307/3 (5′-CCCCGGATCCAGATCTGGATAAATACC, SEQ ID NO:5) and relE-P307/2 (5′-CCCCCGAATTCGTAACTTTCTGTGTTTATTGC, SEQ ID NO:6). The resulting PCR DNA fragment was restricted with BamHI and EcoRI and inserted into pNDM220 also restricted with BamHI and EcoRI. Plasmid pHA210 (+1089 to +1417) is thus a mini-R1 derivative carrying a pA1/O4/O3::relE P307  gene fusion which renders relE P307  inducible with IPTG. 
     pHA215: Genes relBE P307  were PCR-amplified from pNZ945 with the primers RelB-P307/1 (5′-CCCCCGGATCCAGTCTTGAAAGGTGGC, SEQ ID NO:3) and relE-P307/2 (5′-CCCCCGAATTCGTAACTTTCTGTGTTTATTGC, SEQ ID NO:6). The resulting PCR-generated DNA fragment was restricted with BamHI and EcoRI and inserted into pNDM220 also restricted with BamHI and EcoRI. Plasmid pHA215 (+840 to +1417) is thus a mini-R1 derivative carrying a pA1/O4/O3::relBE P307  gene fusion rendering the relBE P307  genes inducible with IPTG. 
     pHA402: A PstI-AatII fragment from plasmid pHA205, which carries laCl q  and the pA1/O4/O3::relB P307  gene fusion was inserted into pBR322 also restricted with PstI and AatII. Thus, the high copy-number plasmid pHA402 contains a relB P307  gene which is inducible with IPTG. 
     pHA403: A PstI-AatII fragment from plasmid pHA215, which carries laCl q  and the pA1/O4/O3::relBE P307  gene fusion was inserted into pBR322 also restricted with PstI and AatII. Thus, the high copy-number plasmid pHA403 contains the relBE P307  genes which can be conditionally induced by the addition of IPTG. 
     pHA810: A DNA fragment encoding relE P307  was generated by PCR using primers relE-P307/4 (5′-CCCCCGAGCTCAGATCTGGATAAATACC, SEQ ID NO:7) and relE-P307/5 (5′-CCCCCGCATGCGTAACTTTCTGTGTTTATTGC, SEQ ID NO:8). The fragment was digested with SaCl+SphI and inserted into the expression plasmid pBAD33 also digested with SaCl+SphI. The resulting plasmid, pHA810 (+1089-+1417), contains the pBAD::relE P307  gene fusion that renders relE P307  inducible with arabinose. 
     An overview of the bacterial strains and plasmids used herein is shown in Table 0.1 below. 
                     TABLE 0.1               Bacterial strains and plasmids                                                    Strains   genotypes   Reference/Source                       MC1000   Δlac leu ara   Casadaban &amp; Cohen, 1980           JS115   ΔrelB leu thy thi supE   J. P. Bouche, unpublished                                             Plasmids   Replicon   Resistance a)     relBE co-ordinates b)     Reference/Source               pOU253   mini-R1   Ap R     none   lab. collection       pBAD33   pACYC   Cml R     none   Guzman et al., 1995       pNDM220   mini-R1   Ap R     none   Gotfredsen &amp; Gerdes, 1998       pBR322   ColE1   Ap R , Tc R     none   Bolivar et al., 1978       pKG325   pBR325   Tc R     none   lab. collection       pBD2430   pUC   Ap R     +388-+1899   Olle Karlström collection       pNZ945   pUC   Ap R      +1-+4298   Saul et al., 1989       pKG4001   mini-R1   Ap R     +388-+596    Gotfredsen &amp; Gerdes, 1998       KG4002   mini-R1   Ap R     +388-+921    Gotfredsen &amp; Gerdes, 1998       pHA100   pBR322   Tc R      +1-+2198   Grønlund &amp; Gerdes, 1998       pHA110   pBR325   Tc R      +1-+1122   Grønlund &amp; Gerdes, 1998       pHA205   mini-R1   Ap R     +840-+1111   Grønlund &amp; Gerdes, 1998       pHA210   mini-R1   Ap R     +1089-+1417    Grønlund &amp; Gerdes, 1998       pHA215   mini-R1   Ap R     +840-+1417   Grønlund &amp; Gerdes, 1998       pHA402   pBR322   Tc R     +840-+1111   Grønlund &amp; Gerdes, 1998       pHA403   pBR322   Tc R     +840-+1417   Grønlund &amp; Gerdes, 1998       pHA810   pACYC   Cml R     +1089-+1417    Grønlund &amp; Gerdes, 1998       pMG223   mini-R1   Ap R     +733-+1020   Gotfredsen &amp; Gerdes, 1998       pMG2201   pBR322   Tc R     +388-+597    Gotfredsen &amp; Gerdes, 1998       pMG2202   pBR322   Tc R     +388-+921    Gotfredsen &amp; Gerdes, 1998                 a) Tc R , tetracycline resistance; Ap R , ampicillin resistance; Cml R , chloramphenicol resistance.         b) Co-ordinates refer to Table 1.1 (relBE K-12 , pMG-plasmids) or Table 1.2 (relBE P307 , pHA-plasmids)            
(iv) Growth Media and Antibiotics
 
     The growth medium was LB medium (Bertani, 1951) or A+B minimal medium (Clark and Maaløe, 1967) supplemented with 0.2% glucose and 1% casamino acids. For growth on solid media, LA-plates were used. LA is LB containing 15 g agar per liter. All media were supplemented with 50 μg/ml thymine for growth of the strain JS11507-05-99ΔrelBEF K-12 . Antibiotics were added at the following concentrations: ampicillin, 30 μg/ml, and tetracycline, 10 μg/ml. When indicator plates were used X-gal (5-Bromo-4-chloro-3-indolyl-β-D-galactoside) was added to a final concentration of 40 μg/ml. 
     (v) Conditions of Cell Growth. 
     Cells were diluted in LB+antibiotics from an overnight culture to an OD 450  of 0.005. The cultures were then grown at 37° C. until an OD 450  of 0.4 and then diluted to an OD 450  of 0.01 in 37° C. LB containing 1 mM IPTG and antibiotics. Samples for OD 450  measurements and viable counts were taken at the time points indicated. Viable counts were made by plating dilutions of the cultures onto LA plates containing the proper antibiotics. 
     (vi) Coupled in vitro Transcription and Translation. 
     The reactions were performed using the  E. coli  S30 Extract System For Circular DNA as described by the supplier (Promega Corp.). 4 μg of DNA was used in all reactions. The reactions were run on a 16% Tricine-SDS-PAGE gel essentially as described by Schäger and von Jagow (1987). 
     (vii) β-galactosidase Assays. 
     β-galactosidase assays were performed essentially as described by Miller (1972). 
     (viii) Homology Search. 
     BLAST searches were performed at the GENESTREAM BLAST network server CRBM Montpellier, France. Standard conditions were used except that the blosum 80 matrix was used. 
     Example 1 
     The Occurrence of relBE Operons in Bacteria and Archae 
     1.1. Nucleotide Sequence of the relBE Operon of  E. coli  K-12 
     The DNA sequence of the relBE operon from  E. coli  K-12 is shown in Table 1.1. In this Table the transcriptional start site of the relBE mRNA is indicated with two asterisks (heterogeneity). IR indicates inverted repeats in the promoter and terminator regions. Start codons and stop codons are shown in bold. The transcriptional termination point (ttp) of the relBE mRNA is also indicated with a vertical arrow. The DNA sequence is from Bech et al., 1985. 
     By visual inspection of the relB K-12  and relE K-12  genes there was found striking similarity with the so-called “proteic plasmid stabilization systems” as described by Jensen and Gerdes (1995). First, relE K-12  codes for a very basic protein (RelE K-12 ; pl=9.7) of 95 amino acids (aa), and relB K-12  codes for a very acidic protein (RelB K-12 ; pl=4.8) of 79 aa. 
     The sequences of proteins RelB K-12  and RelE K-12  are shown in Tables 1.5 and 1.6, respectively. These Tables show multiple sequence alignments of the RelB and RelE gene families. Conserved amino acids at a given position are shown with shading as follows: two amino acids are considered conserved if they both belong to one of the following groups: group 1: D and N; group 2: E and Q; group 3: S and T; group 4: K and R; group 5: F, Y and W; group 6: L, I, V and M. Light grey shading indicates 60-80% conservation, dark grey indicates 80-99% conservation and black indicates 100% conservation. Note in Table 1.6 the fully conserved glycine at position 69 (G in consensus line) and the fully conserved arginine at position 79 (R in consensus line). The entrez database accession numbers of the protein sequences are given in Tables 1.3 and 1.4. 
     The relB K-12  and relE K-12  genes are co-transcribed with a third gene, relF (also denoted orf-3 or hokC), which is homologous to the hok gene from plasmid R1 (Gerdes et al., 1986). The start site (i.e. the 5′-end) of the relBE mRNA was determined to be 31 nucleotides upstream of the relB K-12  AUG start-codon (Bech et al., 1985) and was confirmed (M. Gotfredsen and K. Gerdes, 1998). Inverted arrows in the relBE promoter region (Table 1.1) indicate putative binding sites for regulators of transcription (i.e. the RelB K-12  and RelE K-12  proteins themselves). 
     The properties described above suggested that RelE could be a cytotoxin and that RelB could be an antitoxin which counteracts the toxicity elicited by RelE. 
     
       
         
           
               
             
               
                 TABLE 1.1 
               
               
                   
               
               
                 DNA sequence of the relBE operon from  E. coli  K-12 (SEQ ID NO:9) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 1 
                 CTTAATTTCA GGCCCCATCG GATCACACAT GGAGAGTTTT TATGAATAAC 
                   
               
               
                   
               
               
                 51 
                 CCCGTCTGTC TTGATGACTG GTTGATTGGC TTTAAAAGCT TGTTGACAGG 
               
               
                   
               
               
                 101 
                 GGTAAACGTT CGGCAATAAT TTTCTGCCGC ATGCGGGTGT TGCATAAAAC 
               
               
                   
               
               
                 151 
                 GTGTTACGTT CCTTTATCGA CAGGTCAGGT CACCGCTCAC CCGCCGACGA 
               
               
                   
               
               
                 201 
                 GAAAGCAACA CTGACATGCT AAAGCAAAAA ATAGATGAAT AAGTTGAGTT 
               
               
                   
               
               
                 251 
                 GTGCATATGT AGCCTGACCG TCACAAAGTA TATGGTGTCT GTACCAGTAA 
               
               
                   
               
               
                 301 
                 GATGATGGCC GGACTCTTTA AAAACGAGCT GACCTGCACA ATACAGGATG 
               
               
                   
               
               
                 351 
                 GACTTAGCAA TGGCTGCTCC TGGCACAAAG CGGACAGTGA TCACCGTTCT 
               
               
                   
               
               
                 401 
                 TACGACTACT TTCTGACTTC CTTCGTGACT TGCCCTAAGC ATGTTGTAGT 
               
               
                   
               
               
                   
                             **→relBEF mRNA                   relB start 
               
               
                 451 
                 GCGATACTTG TAATGACATT TGTAATTACA AGAGGTGTAA GAC ATG GGTA 
               
               
                   
                       ---- ---- --→IR←-- ------- -- 
               
               
                   
               
               
                 501 
                 GCATTAACCT GCGTATTGAC GATGAACTTA AAGCGCGTTC TTACGCCGCG 
               
               
                   
               
               
                 551 
                 CTTGAAAAAA TGGGTGTAAC TCCTTCTGAA GCGCTTCGTC TCATGCTCGA 
               
               
                   
               
               
                 601 
                 GTATATCGCT GACAATGAAC GCTTGCCGTT CAAACAGACA CTCCTGAGTG 
               
               
                   
               
               
                 651 
                 ATGAAGATGC TGAACTTGTG GAGATAGTGA AAGAACGGCT TCGTAATCCT 
               
               
                   
               
               
                   
                                     End relB      Start relE 
               
               
                 701 
                 AAGCCAGTAC GTGTGACGCT GGA TGA ACTC TG ATG GCGTA TTTTCTGGAT 
               
               
                   
               
               
                 751 
                 TTTGACGAGC GGGCACTAAA GGAATGGCGA AAGCTGGGCT CGACGGTACG 
               
               
                   
               
               
                 801 
                 TGAACAGTTG AAAAAGAAGC TGGTTGAAGT ACTTGAGTCA CCCCGGATTG 
               
               
                   
               
               
                 851 
                 AAGCAAACAA GCTCCGTGGT ATGCCTGATT GTTACAAGAT TAAGCTCCGG 
               
               
                   
               
               
                 901 
                 TCTTCAGGCT ATCGCCTTGT ATACCAGGTT ATAGACGAGA AAGTTGTCGT 
               
               
                   
               
               
                 951 
                 TTTCGTGATT TCTGTTGGGA AAAGAGAACG CTCGGAAGTA TATAGCGAGG 
               
               
                   
               
               
                   
                              End relE 
               
               
                 1001 
                 CGGTCAAACG CATTCTC TGA  ACCAAAGCAT GACATCTCTG TTTCGCACCG 
               
               
                   
               
               
                   
                                                      Start hokC (relE) 
               
               
                 1051 
                 AAGGTGACAC TTCTGCTTTG CGTTGACAGG AGAAGCAGGC T ATG AAGCAG 
               
               
                   
               
               
                 1101 
                 CAAAAGGCGA TGTTAATCGC CCTGATCGTC ATCTGTTTAA CCGTCATAGT 
               
               
                   
               
               
                 1151 
                 GACGGCACTG GTAACGAGGA AAGACCTCTG CGAGGTACGA ATCCGAACCG 
               
               
                   
               
               
                   
                                                               End hokC 
               
               
                 1201 
                 ACCAGACGGA GGTCGCTGTC TTCACAGCTT ACGAACCTGA GGAG TAA GAG 
               
               
                   
               
               
                 1251 
                 ACCCGGCGGG GGAGAAATCC CTCGCCACCT CTGATGTGGC AGGCATCCTC 
               
               
                   
               
               
                 1301 
                 AACGCACCCG CACTTAACCC GCTTCGGCGG GTTTTTGTTT TTATTTTCAA 
               
               
                   
                                  ---- -- IR ---- - ttp 
               
               
                   
               
               
                 1351 
                 CGCGTTTGAA GTTCTGGACG GTGCCGGAAT AGAATCAAAA ATACTTAAGT 
               
               
                   
                   (data base accesion number X02405) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 1.3 
               
             
            
               
                   
               
               
                 relE homologues from Gram-positive and Gram-negative bacteria and 
               
               
                 Archae 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 entrez 
                   
                 Number 
                   
                   
               
               
                 Bacterial species 
                 accession 
                 gene a)   
                 of aa 
                 MW (kD) 
                 pl 
               
               
                   
               
            
           
           
               
            
               
                 Gram-negative bacteria: 
               
            
           
           
               
               
               
               
               
               
            
               
                   E. coli  K-12 
                 132284 
                 relE K-12   
                 95 
                 11.2 
                 9.7 
               
               
                   E. coli  K-12 
                 984581 
                 relE SOS   b)   
                 92 
                 10.8 
                 9.5 
               
               
                   E. coli  plasmid 
                 516611 
                 relE P307   
                 95 
                 11.2 
                 9.9 
               
               
                 P307 
               
               
                 
                   H. influenzae 
                 
                 1175293 
                 relE Hi   
                 102 
                 11.9 
                 6.7 
               
               
                 
                   V. cholera 
                 
                 396846 
                 relE Vc   
                 96 
                 11.2 
                 9.9 
               
               
                 
                   H. pylori 
                 
                 2314031 
                 relE Hp   
                 88 
                 10.4 
                 7.9 
               
               
                 
                   Synechosystis 
                 
                 1653777 
                 relE Sy   
                 120 
                 13.7 
                 7.9 
               
            
           
           
               
            
               
                 Gram-positive bacteria: 
               
            
           
           
               
               
               
               
               
               
            
               
                 
                   B. thuringiensis 
                 
                 520407 
                 relE Bt   
                 74 
                 8.6 
                 9.7 
               
               
                   M. tuberculosis #1 
                 2612811 
                 relE Mt1   
                 87 
                 10.2 
                 11.0 
               
               
                   M. tuberculosis #2 
                 2695832 
                 relE Mt2   
                 97 
                 11.1 
                 9.5 
               
            
           
           
               
            
               
                 Archae: 
               
            
           
           
               
               
               
               
               
               
            
               
                   M. jannaschii #1 
                 1498833 
                 relE Mj1   
                 90 
                 11.0 
                 10.2 
               
               
                   M. jannaschii #2 
                 1499953 
                 relE Mj2 (*) 
                 88 
                 10.6 
                 10.0 
               
               
                   M. jannaschii #3 
                 1591583 
                 relE Mj3 (*) 
                 91 
                 11.1 
                 10.1 
               
               
                   A. fulgidus #1 
                 2648176 
                 relE Af1   
                 87 
                 10.6 
                 10.3 
               
               
                   A. fulgidus #2 
                 2649499 
                 relE Af2   
                 92 
                 11.0 
                 9.9 
               
               
                   A. fulgidus #3 
                 2649496 
                 relE Af3   
                 85 
                 10.0 
                 10.0 
               
               
                   A. fulgidus #4 
                 2649514 
                 relE Af4   
                 86 
                 10.2 
                 9.9 
               
               
                   
               
               
                   a) relE homologues marked with (*) are not located adjacent to a relB partner 
               
               
                   b) The relBE SOS  system of  E. coli  K-12 contains a LexA binding-site in the promoter region (Lewis et al., 1994) 
               
            
           
         
       
     
                     TABLE 1.4                  relB homologues from Gram-positive and Gram-negative bacteria and       Archae                                         entrez       Number               Bacterial species   accession   gene a)     of aa   MW (kD)   pl                         Gram-negative bacteria:                                       E. coli  K-12   132283   relB K-12     79   9.1   4.8         E. coli  K-12   984582   relB SOS   b)     86   9.4   5.2         E. coli  K-12   984588   relB K-12,2 (*)   97   11.2   5.5         E. coli  plasmid   516610   relB P307     83   9.2   4.4       P307         S. typhimurium     731639   relB St (*)   68   7.6   5.3         H. influenzae     1573712   relB Hi     98   11.0   4.7         V. cholera     396847   relB Vc     82   8.9   4.4         H. pylori     2314037   relB Hp     95   11.4   9.8         Synechosystis     1653776   relB Sy     86   9.9   4.7                 Gram-positive bacteria:                                       B. thuringiensis     520406   relB Bt     85   10.1   4.5         M. tuberculosis #1   2612810   relB Mt1     93   10.2   4.6         M. tuberculosis #2   2695833   relB Mt2     89   9.8   5.1                 Archae:                                       M. jannaschii #1   1498832   relB Mj1     82   9.6   4.5         A. fulgidus #1   2648190   relB Af1     65   7.8   4.8         A. fulgidus #2   2649516   relB Af2     62   7.4   4.3         A. fulgidus #3   2649510   relB Af3     72   8.5   4.5         A. fulgidus #4   269513   relB Af4     57   6.7   4.1                 a) relB homologues marked with (*) are not located adjacent to a relE partner.         b) The relBE SOS  system of  E. coli  K-12 contains a LexA binding-site in the promoter region (Lewis et al., 1994).              
1.2. Nucleotide Sequence of the relBE Operon of Plasmid P307
 
     By database searching it was found that the  E. coli  plasmid P307 codes for a gene system which exhibits both structural and sequence similarity with the  E. coli  relBE genes described above. 
     The DNA sequence of the relBE P307  genes is shown in Table 1.2. The transcriptional start site of the relBE mRNA is indicated with an asterisk, and the −10 and −35 sequence elements of the relBE promoter are underlined. The Shine &amp; Dalgarno sequence of the relB and relE genes are doubly underlined. The DNA sequence is from Saul et al., 1989. 
     Again, relE P307  codes for a very basic protein of 95 aa (pl=9.9), and relB P307  codes for a very acidic protein of 83 aa (pl=4.4), see Tables 1.3 and 1.4. The protein sequences of RelE P307  and RelB P307  are also shown in Tables 1.5 and 1.6, respectively. The start site (i.e. the 5′-end) of the relBE P307  mRNA was determined to be located 27 nucleotides upstream of the relB P307  AUG start codon. Inverted arrows in the relBE P307  promoter region (Table 1.2) indicate putative binding sites for regulators of transcription (i.e. the RelB P307  and RelE P307  proteins). 
                         TABLE 1.2               DNA sequence of the relBE operon from the             E. coli  plasmid P307 (SEQ ID NO:44)                                            301   GAGTATCATA TTAGGATACG GGTGGGTGAC GCCCACCTCT GGCATAGAAC                   351   GGACATTCAT TGATGCCATG CCAGAATGGA CGTTCAGGTT ATTCCGTCCA               401   GTTCTGCTGG CAACGCGAGA TCTCCCCTGG TATAGTGATG CCACAGCAAA               451   GCGCTCAAAC AGGGATAATA TGATGGAAAT CAAGGCTCAA CAGTTTTGTC               501   ACATCAACGG GGCGGCAAGT CCTTACTGAC AACGGACAAC AAGGTATGGG               551   CGGCGTGGCG GGTATCGGTT CCACGACTGA AAAGCATCAG GGGCGCGTGG               601   CGGAAGCGAT TTTTGCGAAC TGCGCGGAAC TGGATAACGA CCAGCTTAAC               651   GAGATCATCG AGTGGGTTCG GCTCTATCAG CGCTGAATGC CACTATCAGG               701   CTGCGCAAGC GGCCTTTTTT ACGCCCCTTG TTTAATTCCC GCACTACCTG                                                                      −35       751   GACGTTCAGG TGATTCTGTC CATCTGTACA AAAAACAATA AAAGAC TTGT                                           −10         *→ relBE P307  mRNA       801     TA ACAGGTCA TGTAAGGAG T ATCTT TGAGA CTGGTTAAAC AGTCTT GAAA                     SD      start relB       851     GGTG GCCT AT G CCTAACATT ATTCTCAGTG ATACAAGCGC CAGTGTCAGC               901   GAGCTGAAGA AAAACCCGAT GGCGACAGTC AGCGCCGGTG ATGGTTTCCC               951   GGTCGCTATC CTGAACCGTA ATCAGCCTGC TTTCTACTGT GTACCCGCAG               1001   AGCTGTACGA AAAGATGCTT GATGCCCTAG ACGATCAGGA GTTGGTTAAA                                                                    SD       1051   CTGGTAGCCG AACGCAGCAA CCAACCGCTG CATGATGTAG ATCT GGA TAA                   end relB/start relE       1101   ATACCT ATGA  GGTATCAGGT AAAATTCAGG GAAGATGCGC TGAAAGAGTG               1151   GCAAAAACTG GACAAGGCTA TTCAGCAACA GTTTGCGAAA AAGCTAAAAA               1201   AGTGCTGTGA CAATCCGCAT ATTCCTTCCG CAAAACTGCG TGGGATAAAG               1251   GACTGCTACA AAATAAAATT ACGTGCGTCA GGTTTTCGCC TGGTCTATCA               1301   GGTGATTGAC GAACAATTAA TTATCGCTGT TGTAGCTGTG GGTAAACGTG                                                                end relE       1351   AGCGCAGTGA CGTTTATAAT CTTGCCAGCG AAAGAATGAG A TAA AAGCAA               1401   TAAACACAGA AAGTTACTCT GGCGTTATGG GGTAATGCAA AGTATGAGTC               1451   GTAGAGGGAA TTGCCTGGAT AATTCGCCGA TGGAAAGAGT CTTTCGCAGC               1501   CTTAAAAGTG AATGGCTTCC GAAAGGTGGT TATGGTGATT TTAGCCATGC           (database accession number D426308)                    
1.3. Nucleotide Sequence and Proteins of a relBE Homologous Operon from  Bacillus thuringiensis    
     Using BLAST database searching (Altschul et al., 1990) it was found that transposon Tn5401 from the Gram-positive organism  B. thuringiensis  contains, in one end or asymmetrically located, a two-component system which exhibits both structural and sequence similarity with the above described relBE systems from  E. coli . This homology is surprising given that it has not previously been described that relBE-like genes are found in organisms other than  E. coli.    
     The nucleotide sequence of the relBE operon from Tn5401 is shown in Table 1.7. In this Table the transcriptional start-site of the relBE mRNA is indicated with an asterisk (Baum, 1994). IR indicates inverted repeats in the relBE Bt  promoter region. Start codons and stop codons are shown in bold. The Shine &amp; Dalgarno sequence of the relB Bt  gene is doubly underlined. The DNA sequence is from Baum et al., 1989. 
     The relE Bt  gene codes for a very basic protein of 74 aa (pl=10.6) and the relB Bt  gene codes for an acidic protein of 87 aa (pl=4.4). The protein sequences of RelE Bt  and RelB Bt  are aligned with the other RelE and RelB homologues in Tables 1.5 and 1.6, respectively. The modular, structural and physico-chemical similarities between the  B. thuringiensis  system and the  E. coli  systems suggested that the genes may exert similar functions in very different bacteria. 
                         TABLE 1.7               DNA sequence of the relBE operon from the           Gram-positive organism  B. thuringiensis  (SEQ ID NO:45)                                            3701   CTCGTTTTTT CTGTTGGTAC AAACTTAATT GATTTTGAAT AATTTGTTTG                   3751   TACCAGTCCT TTTTGCTTAG CCCAGTCAAA ATAACGTTTG ATTGAATTAA               3801   TGCGCCGGTT AATCGTAGAA GGTTTTAGTA ATCTTGTAAC TTGCATATGC               3851   CCTCGATATC GAGCAATAGT GCGAGCGGTA ACTTCTATTG GATGAAAAAG               3901   AGTATCCTCA GCATGTTTTC CCCACACATT TTCAAACCAA AATACAAAAT               3951   CTTTTAAATC ACTCGTATAT TCTTTTAGTG TTTTTGTATG CAAATCTCCT               4001   TCTTGAGATA AGCTAGAAAT AAAATCGGAA ATCAAAGATG TTGCTTGTAT                                                                  −35       4051   AGAAATTGTT TTAGTGGAAT GCATAAATAC CTCCTCTTTT A TTGACT TAC                                    −10        *→ relBEBt mRNA       4101   ATTAGCGGAC ATGA TATTTT  AATCTTATCA ATTATGTTAG CGGACATCAA               4151   ACATTTATTT TCCCACACTT CATGTCCACT AATATTAATT AGTGGACATT                                  --------- --→ IR ←-- ---------                                     SD           Start relB       4201   TAAAACTATC TC GAAAGTAG GTGT AACAC A TG GCTATTCG TAAAGATGAA               4251   TTGTATCGGT TAATTGATCA CCTGGATCAA CAAGATGAAA AAGCAGCATT               4301   TGACTTTTTA GAATTTCTTG TTCAACGGTC AAGAAGAAAA CCTAAAGAAT               4351   GGGAAAAAAT TGATATGGCA GATCCTGATC ATGAACCGCT GTCTACACAA               4401   GAGTTAGAAC AGTTAAACAG TGAAGAAGGA TATGTATCAG GGGAGGACGC                                                   End relB    Start relE       4511   AAAACGTGAA TTCGGACTAC AAATTGATTT ACCA TAA GTC CGCGGTGAAA               4501   TTTATTGCAA AGCAAGAAAA AGGGATTCAA AAAAGAATTG CAGAAGGATT               4551   GAAGGGACTT CTTAAGATTC CTCCTGAAGG AGATATTAAA AGTATGAAAG               4601   GTTACACAGA ACTATATCGA TTACGGATTG GAACCTTTCG AATTTTATTT               4651   GAAATAAATC ATGATGAGAA AGTCATATAC ATACAAGCAA TTGGAAATCG                               End relE       4701   TGGTGACATC TATAAA TAA G GCAAACATGC ATTTTTAAAA GAAAGGTCTT               4751   CTGAATCGAA GAACCTTCCT TTTTTGTGTG CGAATAATGT CCGCTAATGC               4801   TTGTTGCGTG ATTCTGTTCC ATTGCTACAC ATACCCC           (database accession number U03554)                    
1.4. The Archaeon  Methanococcus jannaschii  Encodes a relBE Homologous System
 
     Again using database searching it was found that the completely sequenced genome of the methanogenic archaeon  Methanococcus jannaschii  codes for three relE homologous genes, one of which are located just downstream of relB homologous genes. This finding was surprising since, in many respects, archaeal organisms are more similar to eukaryotes than to bacteria (e.g. in their macromolecular synthesis apparatuses). 
     The DNA sequence of the relBE Mj1  system is shown in Table 1.8. In this Table start codons and stop codons are shown in bold. The DNA sequence is from Bult et al., 1996. 
     Gene relE Mj1  codes for a very basic polypeptide of 90 aa (pl=11.0) and gene relB Mj1  codes for an acidic polypeptide of 82 aa (pl=4.4). The aa sequences of the RelE Mj1  and RelB Mj1  proteins are aligned with the other RelBE homologues in Tables 1.5 and 1.6, respectively. Thus, these basic similarities suggested that the relBE Mj1  system may carry out similar or related functions in bacteria and archae. The properties of the second and third relE homologues of  M. jannaschii  are also given in Table 1.3. These comparisons show that  M. jannaschii  codes for one complete relBE homologous gene system and for two relE homologues without an adjacent relB partner. 
                         TABLE 1.8               DNA sequence of a relBE homologous gene system           from the archaeon  Methanococcus jannaschii         (SEQ ID NO:46)                                            751   CCGATACCGT TGCTGGAGAC ATAGCTGGAG CTTTGAAGGC GGAGAAGCTT                   801   ATTTTAATAA CAGATGTTGA TGGAATAATG GATGATATAA ATAATCCAGA               851   GACGTTGCAT AGAAAATTAA CAGCTTCAGA ACTAAAAGAA ATGATAGAAG               901   ATGGAAGAAT AAAGGGAGGG ATGATTCCAA AGGCTGAAAG TGCCTTATAT               951   GCCTTAGAGC ATGGAGTTAA GAGCGTTCAT ATAATAAATG GAAAGATTCC               1001   TCATGCTTTG TTGTTGGAGA TATTTACAGA GGAGGGTATT GGGACGATGA               1031   TAACAAGAGA TTAAAGTTTT TATATTATAA ACTACTTAAG AATTAAAATA                                         Start relB Mj1         1101   AGACAAATAA GGGGATAACT  ATG CTCAATA TAAACAAAGA GATAGCACAA               1151   ATAGAAACTG AATTGAATGA ATTGAAAAAA TTGAGAGATG AAATCTCTGA               1201   AAGGATTGAA AAATTAGAAA TAAAGTTATT AAAATTGAAA GCATTAGCTA               1251   TTCCAGAGGA GGAATTTGAA GAGGATTATG AAGAAATTAT AGAAGATCTT               1301   AAAAAATCTC TGGATAAAAA AGAGACTGTG CCAGCAGAAG AGGCTTTGAA                          End relB Mj1 /start relE Mj1         1351   AGAATTGGGA TTATT ATGA A GTTTAACGTT GAGATACATA AAAGAGTCTT               1401   AAAAGATTTA AAGGATTTGC CTCCCTCAAA CTTAAAGAAG TTTAAAGAAC               1451   TAATAGAAAC ATTAAAAACC AATCCCATTC CAAAAGAAAA ATTTGATATT               1501   AAAAGATTAA AAGGCAGTGA TGAGGTTTAT AGAGTTAGAA TTGGAAAATT               1551   TAGAGTTCAA TATGTTGTTT TATGGGATGA TAGAATAATA ATAATTAGAA                                                     End relE Mj1         1601   AGATAAGTAG AAGAGAAGGA GCTTATAAAA ATCCC TAA GC TATTAAAAAT               1651   TCTAATGGCT ACATTTTTAT ATCTCTTTTC TTAATTCAAA TAGAAAAAAC               1701   AGATTCGGCT GATACCATGA TTATTCTTTT AGATTTAAAT GGAACAATAG           (database accession number U67464)                    
1.5. relBE Homologous Genes are Ubiquitous in Prokaryotes
 
     Further relBE homologous two-component systems were discovered. The corresponding RelB and RelE homologous proteins are aligned in Tables 1.5 and 1.6, respectively. It appears that relE homologous genes are present in a wide variety of Gram-negative bacteria ( E. coli, H. influenzae, V. cholera, H. pylori  and  Synechosystis ), in Gram-positive bacteria ( B. thuringiensis  and  M. tuberculosis ) and in Archae ( M. jannaschii  and  A. fulgidus ). Most strikingly, the archaeon  A. fulgidus  contains four complete relBE homologous gene systems. 
     A number of features become evident from the alignments of the proteins (Tables 1.5 and 1.6) and from the properties listed in Tables 1.3 and 1.4. First, all RelE homologues are basic with pH&#39;s around 8-10 whereas the RelB homologues are acidic with pl&#39;s about 4-5. Secondly, the RelE proteins are in general slightly larger (90-120 aa) than the RelB homologues (70-80 aa). Thirdly, the start codons of the relE genes are juxtaposed or even overlap with the stop codons of the linked relB partner, thus indicating translational coupling of relE to relB. These properties suggest that the proteins could exert similar functions in very different organisms. 
     Example 2 
     Demonstration of Translation of the relB K-12  and relE K-12  Genes 
     Using the low copy-number lacZ fusion vector pOU253 (Table 0.1) in frame gene fusions between relB K-12  and relE K-12  and the lacZ gene were constructed (see Materials and methods). Thus plasmid pKG4001 (+388 to +596) carries a fusion between relB K-12  and lacZ, and pKG4002 (+388 to +921) carries a fusion between relE K-12  and lacZ. The structure of the relevant parts of these reporter plasmids are shown in  FIG. 1 . When present in strain MC1000, both plasmids expressed significant amounts of galactosidase fusion proteins, indicating that genes relB K-12  and relE K-12  are translated ( FIG. 1 ). The relE K-12 -lacZ fusion (pKG4002) expressed significantly lower amounts of β-galactosidase than the relB K-12 -lacZ fusion, mainly because pKG4002 encodes an intact relB K-12  gene which produces the RelB K-12  autorepressor which inhibits transcription from the relB promoter. 
     Example 3 
     Demonstration of Translation of the relB P307  and relE P307  Genes 
     To detect authentic RelB P307  and RelE P307  proteins, in vitro translation reactions were carried out using high copy number pUC-plasmids carrying genes relE P307  (pHA403), relB P307  (pHA402) or both genes (pHA100) (for construction of these plasmids, see Materials and Methods and Table 0.1). Proteins produced in the in vitro translation reactions were labelled with  35 S-Methionine and separated by SDS-page.  FIG. 2  shows the direct visualization of RelB P307  and RelE P307 , thus providing evidence that the corresponding genes are translated. 
     Example 4 
     Demonstrating that relE K-12  is a Cytotoxin 
     The low copy-number cloning vector pNDM220 contains laClq and the LaCl regulated pA1/O4/O3 promoter (Lanzer and Bujard, 1988) upstream of a multiple cloning site (mcs). The genetic structure of pNDM220 is shown in  FIG. 3 . Without IPTG added to the growth medium, the pA1/O4/O3 promoter is almost completely turned off. However, with IPTG, strong transcription is induced towards the cloning site. Therefore plasmid pNDM220 is suitable for the conditional expression of genes, in particular toxin-encoding genes. 
     The relE K-12  gene of  E. coli  K-12 (Bech et al., 1985) was PCR amplified and inserted into the mcs of pNDM220, resulting in pMG223 (for the construction of pMG223, see Materials and Methods). Plasmid pMG223 (+733 to +1020) was established in MC1000, which contains a chromosomal copy of the relBE operon. However, it was not possible to transform pMG223 into the JS115 strain, which carries a deletion of the relBE operon (ΔrelB). Therefore, the induction experiments shown in  FIG. 4  were accomplished using strain MC1000, which contains the chromosomal copy of relBE K-12 . 
     Strain MC1000/pMG223 was grown in LB at 37° C. At time zero, IPTG was added to the growth-medium. After two hours of induction with IPTG, the viable counts decreased c. 600-fold ( FIG. 4B ). The decline started immediately and continued exponentially for about 2 hours. On plates containing IPTG, viable counts decreased even further (data not shown). The optical density (OD 450 ) increased during the first 20 minutes after addition of IPTG and then the culture became stationary ( FIG. 4A ). Addition of IPTG to growing cells containing the vector-plasmid had no effect (not shown). These results indicate that the relE gene encodes a cell toxin. 
     Example 5 
     Demonstrating that RelB K-12  is an Antitoxin 
     Plasmid pMG2202 (+388 to +921) is a pBR322 derivative that contains the relB gene expressed from its own promoter (see Table 1.1). Plasmid pMG2201 (+388 to +597) is a pBR322 derivative that contains the relB promoter and the first part of relB K-12 . Thus, pMG2201 does not contain an intact relB gene and was included in the analyses as a control plasmid. The strains MC1000/pMG223 (pA1/O4/O3::relE+)−/pMG2202 (relB + ) and MC1000/pMG223/pMG2201 (relB − ) were subjected to a physiological growth experiment similar to the one described in Example 4. As seen from  FIGS. 4E and 4F , the presence of the high copy-number relB-carrying plasmid suppressed relE-dependent cell killing. The antitoxin effect was dependent on an intact relB reading frame, since the control-plasmid (pMG2201) carrying the promoter region and the first part of the relB reading frame did not prevent the relE mediated cell killing ( FIG. 4C ,  4 D). 
     Example 6 
     Demonstrating that relE P307  Encodes a Very Efficient Cytotoxin 
     The medium copy number expression vector pBAD33 contains an arabinose inducible promoter (pBAD) with a multiple cloning site (mcs) and the araC gene (Guzman et al., 1995). The genetic structure of pBAD33 is shown in  FIG. 5 . Without arabinose added to the growth medium, the pBAD promoter is completely turned off. However, with arabinose, strong transcription is induced towards the cloning site. On top of this property, the pBAD promoter is repressible by the addition of glucose to the growth medium. Thus, by the addition of glucose, transcription from pBAD can be rapidly and efficiently turned off. 
     The glucose repression effect is epistatic to the inducer effect by arabinose. Hence, if cells with a pBAD-carrying plasmid are grown in a medium containing both arabinose and glucose then the promoter is not induced. However, if cell-growth depletes the medium for glucose, then the promoter will be induced. Therefore, plasmid pBAD33 is suitable for the conditional turning on and off of the expression of genes, in particular toxin-encoding genes as described herein. 
     The relE gene of the  E. coli  plasmid P307 (Saul et al., 1989) was PCR amplified and inserted into the mcs of pBAD33, resulting in pHA810 (for the construction of plasmid pHA810, see Materials and methods). Thus plasmid pHA810 contains the relE P307  gene inserted downstream of the pBAD promoter. Strain MC1000/pHA810 was grown in LB-medium without glucose at 37° C. At time zero, the culture was diluted into medium containing either 0 or 0.2% arabinose. In the arabinose-containing culture, an immediate decline in viable counts was observed ( FIG. 6B , closed symbols). The decline continued exponentially throughout the experiment. After 240 min of induction with arabinose, viable counts had decreased more than five orders of magnitude. Without arabinose, cells containing pHA810 continued to grow exponentially ( FIGS. 6A and 6B , open symbols). On plates containing arabinose, none or very few viable cells were detected. These results show that relE P307  gene encodes an extremely efficient cell toxin. 
     Example 7 
     Demonstrating that RelB P307  is an Antitoxin 
     Plasmid pHA110 (+1 to +1122) is a pBR322 derivative that contains the relB P307  gene expressed from its own promoter. The strain MC1000/pHA810/pHA110 (relB P307 ) was subjected to a physiological growth experiment as described in Example 6. It appeared that the presence of the relB P307 -carrying plasmid pHA110 prevented relE P307  dependent inhibition of cell growth ( FIG. 6C ) and cell killing ( FIG. 6D ). This observation shows that relB P307  codes for an antitoxin that counteracts the cell killing caused by RelE P307 . 
     Example 8 
     Determination of the Frequency of Spontaneous Mutants that are Resistant to the Killing Effect of RelE 
     Strain MC1000/pHA810 was grown exponentially to an OD of 0.5 and serial dilutions of the cell suspension were plated on LA plates containing chloramphenicol (selecting for plasmid pHA810) and with or without 0.02% arabinose (which induces expression of relE present in pAH810). On such plates without arabinose the plating efficiency of strain MC1000/pAH810 was normal, i.e. more than 99% of the viable cells produced a colony. This indicated that the presence of pAH810 in itself had no effect on the viability of the cells. However, with arabinose the plating efficiency was reduced by about 109 fold, thus indicating that expression of RelE is extremely toxic to the cells. The few surviving colonies that appeared eventually were retransformed with the RelE expression plasmid pHA210 which can co-exist with pAH810. However, none of the surviving cells from the first round of selection (i.e. using pHA810) survived induction of RelE (by addition of IPTG) from the second plasmid pAH210. 
     These results show that resistance against RelE toxicity is a very rare event, as based on this experiment it is less than about 10 −9 . 
     Example 9 
     Demonstrating that RelE of the Archeon  Methanococcus jannaschii  is Toxic to  E. coli    
     The relE gene of  M. jannaschii  was amplified from genomic DNA using primers MJ-relE/2CWW (5′-CCCCCGAATTCGCATGCGCCATTAGAAT, SEQ ID NO:47) and MJ-relE/1 CW (5′-CCCCCGGATCCGAGCTCGAGGCTTTGAAAGAATTGGG, SEQ ID NO:48). The resulting DNA fragment was cleaved with BamHI and EcoRI and cloned into plasmid pNDM220 ( FIG. 3 ) thus yielding pHA705 ( FIG. 7 ). Similarly, relB and relE from  M. jannaschii  were PCR amplified using primers relB-M.jannCW (5′-CCCCGGATCCGTCGACGACAAATAAGGGGATAACTATG, SEQ ID NO:49) and MJ-relE/2CWW. The resulting DNA fragment was cleaved with BamHI and EcoRI and cloned into pNDM220, thus yielding pHA715 ( FIG. 7 ). 
     Plasmids pHA705 (carrying relE) and pHA715 (carrying relBE) were transformed into  E. coli  K-12 strain MC1000. Cells were grown exponentially and followed after the addition of IPTG.  FIG. 8  shows that the addition af IPTG inhibited the growth of MC1000/pHA705 but not that of MC1000/pHA715, and  FIG. 9  shows that viable count was significantly reduced in the case of MC1000/pHA705 but not in that of MC1000/pHA715, thus demonstrating that RelE of  M. jannaschii  is toxic to  E. coli.    
     Example 10 
     Demonstrating that RelE of the Gram-positive Bacterium  Streptococcus pneumoniae  is Toxic to  E. coli    
     Using BLAST database searching, we identified two homologues of the relBE genes of  S. pneumoniae . The DNA sequence of the homologue designated relE Sp2  is shown in  FIG. 10 . Gene relE Sp2  was PCR amplified from genomic DNA of  S. pneumoniae  strain RP46 using primers relE-Sp2/cw (5′-CCCCGGATCCGATGCATGATTTAGGCTTGAAG, SEQ ID NO:50) and relE-Sp2/ccw (5′-CCCCGAATTCGAATGAAAATTTACTTGAAAAAAG, SEQ ID NO:51). The resulting DNA fragment was cleaved with BamHI and EcoRI and cloned into pNDM220 thus yielding plasmid pHA-Sp2 ( FIG. 11 ). 
     Plasmid pHA-Sp2 (carrying relE Sp2 ) was transformed into  E. coli  strain MC1000. Cells were grown exponentially and followed after the addition of IPTG.  FIG. 8  shows that the addition af IPTG inhibited the growth of MC1000/pHA-Sp2, and  FIG. 9  shows that viable counts were dramatically reduced, thus demonstrating that expression of relE Sp2  is highly toxic to  E. coli.    
     Example 11 
     Cloning of the relE Genes of Plasmid P307 , M. jannaschii  and  E. coli  K-12 into the Broad-host-range Vector pHAG33 
     The broad-host-range vector pVLT33 is an RSF1010 derivative that can be mobilized by an appropriate conjugation system (de Lorenzo, Eltis, L., Kessler, B. and Timmis, K. N. 1993. Analysis of  Pseudomonas  gene products using laCl q /Ptrp-lac plasmids and transposons that confer conditional phenotypes. Gene 123, 17-24). It also contains the tac-promoter (ptac) and laCl q . Since ptac is leaky and therefore unsuitable for the regulated expression of toxins, the promoter was replaced by the pA1/O4-O3 promoter of pNDM220. The resulting plasmid, pHAG33, is shown in  FIG. 12 . The relE genes of pHA210 (relE P307 ), pHA705 (relE Mj ) and pMG223 (relE K12 ) were cloned into pHA33, resulting in plasmids pHA33-2 ( FIG. 13 ), pHA33-3 ( FIG. 14 ), and pHA33-4 ( FIG. 15 ), respectively. 
     Example 12 
     Demonstrating that RelEs of  E. coli  K-12, P307 and  M. jannaschii  are Toxic to  Pseudomonas putida    
     Plasmids pHA33-2 (relE P307 ), pHA33-3 (relE Mj ) and pHA33-4 (relE K12 ) were transformed into the  E. coli  K-12 strain S17-1. This strain contains the conjugation system of RP4 and is thus able to mobilize pHA33-derived plasmids as described above (Simon et al., 1986). After conjugation on solid medium to  P. putida  strain KT2440 according to standard procedure, strains KT2440/pHA33-2, KT2440/pHA33-3 and KT2440/pHA33-4 were established. 
     The strains were grown exponentially in LB containing 30 μg/ml ampicillin and 50 μg/ml kanamycin and followed after the addition of 2 mM IPTG. As seen from  FIG. 16 , the increment in cell-growth as measured by OD 450  was reduced by IPTG in all three cases. Furthermore, measurements of viable counts ( FIG. 17 ) showed cell-killing in all three cases, most severe in the case of relE K12  (pHA33-4(+) in  FIG. 17 ). Thus, RelE proteins of P307 and  M. jannaschii  are toxic to  P. putida  and RelE of  E. coli  K-12 is extremely toxic to  P. putida.    
     Example 13 
     Demonstrating Biological Containment by the Depletion of a Carbon Source 
     Plasmid pHA810 ( FIG. 18 ) was constructed by inserting the relE gene of P307 into the expression vector pBAD33 ( FIG. 5 ). The promoter (designated p BAD ) upstream of relE P307 ) in pHA810 is repressed by glucose and induced by arabinose. The repression by glucose overrides induction by arabinose such that the simultaneous presence of glucose and arabinose in the growth medium results in repression of the promoter. 
     To simulate a realistic scenario in which the carbon source was depleted, we grew MC1000/pHA810 in ABT minimal salts medium at 35° C. in the presence of a limiting amount of glucose (0.025% w/v) (represses p BAD ) and varying the concentration of arabinose (induces p BAD ). Optical density ( FIG. 19 ) and viable counts ( FIG. 20 ) typical for such an experiment were obtained. As seen in  FIG. 19 , the rate of increase in OD 450  is severely reduced by the highest amounts of arabinose (0.050% and 0.075%). This was expected, since arabinose induces pBAD and the limited amount of glucose (0.025%) cannot fully suppress pBAD at high concentrations of arabinose. The glucose added (0.025%) was depleted by cell growth at an OD 450 =approx. 0.1. At this OD 450 , a dramatic cell killing was seen in the case of 0.005%, 0.010%, and 0.025% of arabinose ( FIG. 20 ). This result shows, that depletion of the carbon source (glucose) leads to massive cell killing, and thus to biological containment of the plasmid that carries relE P307 . 
     Example 14 
     Demonstrating that RelE of  E. coli  K-12 and  M. jannaschii  are Toxic to Human Cells 
     The cell line 293 is a permanent line of primary human embryonal kidney cells transformed by human adenovirus type 5 (Ad 5) DNA (ATCC CRL-1573). The cells are particularly sensitive to human adenovirus, are highly permissive for adenovirus DNA, and contain and express the transforming genes af AD 5 (Graham, F. L., Smiley, J., Russell, W. C., and Nairn, R. 1977. Characteristics of a human cell line transformed by DNA from human adenovirus type 5. J. Gen. Virol. 36, 59-74). 
     Plasmid pcDNA3.1 (+) (Invitrogen) carries the constitutive promoter P CMV  from cytomegalovirus upstream of a multiple cloning site (mcs). Genes relE of  E. coli  K-12 and  M. jannaschii  were inserted in the mcs, resulting in plasmids p5.4 and p5.3, respectively. 
     Plasmids pcDNA3.1(+)(control), p5.4 and p5.3 were transfected into cell line 293 by selection in medium containing G418 (geneticin), which selects for cells expressing the neomycin gene present on the plasmids. After 12 days, the cell density was measured by inspection. In the case of p5.4 (relE K-12 ), between 0 and 5% of the cells had survived (as compared to the control). In the case of p5.3 (relE Mj ), between 5 and 10% of the cells had survived. These results indicate that the bacterial RelE K-12  and the archival RelE Mj  toxins both are lethal to human cells. 
     Example 15 
     Demonstrating that RelE K-12 , RelE P307 , and RelE Mj  Inhibit Translation in vitro 
     DNA fragments comprising genes relE K-12 , relE P307  and relE Mj  were PCR amplified such that a T7 RNA polymerase promoter was placed upstream of the corresponding genes (according to Thisted et al., 1994. 
     The following primers were used: relE K-12  (P1: 5′-( TGTAATACGACTCACTATA GATAAGGAGTTTTATAAATGGCGTATTTTCTGGATTTTG, SEQ ID NO:52) and P2 (CACCTTCGGTGCGAAACAG, SEQ ID NO:53); relE P 307 (P3; 5′- TGTAATACGACTCACTATA GATAAGGAGTTTTATAAATGAGGTATCAGGTAAAATTCA (SEQ ID NO:54) and P4: 5′-CTTTCCATCGGCGAATTATC, SEQ ID NO:55); relE Mj  (P5: 5′- TGTAATACGACTCACTATA GATAAGGAGTTTTATAAATGAAGTTTAACGTTGAGATAC SEQ ID NO:56) and P6: (5′-ATCATGGTATCAGCCGAATC, SEQ ID NO:57). T7 RNA polymerase sequences are underlined, and the strong Shine-Dalgarno (SD) sequence from the parA system of plasmid R1 is shown in italics. 
     Using in vitro transcription with T7 RNA polymerase according to standard procedures, mRNAs encoding relE K-12 , relE P307 , and relE Mj  were produced and subsequently purified from a denaturing polyacrylamide gel. To facilitate the quantification of the mRNAs they were labelled with tritium (alfa- 3 H-CTP) during their synthesis. The relE-encoding mRNAs (1.5 pmol) were used as templates in in vitro translation reactions employing an S30 extract (obtained from Promega) containing 150 μM of each amino acid except Methionine which was 1 μM. 
       FIG. 21  shows SDS-PAGE (tricine-gel) analysis of such an experiment. The in vitro translation reactions were initiated with unlabelled Methionine in order to produce RelE toxin in the reaction. Ten minutes after the addition of the unlabelled Methionine, radioactive 35-S-Methionine (5 pmol in a 15 μl volume) was added and the reaction continued for an additional 20 minutes. C in  FIG. 21  denotes a control lane without exogenous mRNA added. The protein bands seen in this lane originate from translation of mRNAs present in the S30 extract. In lane 1, the in vitro translation reaction contained an mRNA encoding the relE gene of  E. coli  K-12. As seen, the translation reaction was severely inhibited. In lane 2, a mRNA encoding a mutated relE (denoted relEmE and described in Gotfredsen et al, 1998) gene was added. As seen, the presence of this mRNA did not inhibit the reaction. This result shows that the RelE protein produced during the initial incubation-period without 35-S-Met added inhibits the in vitro translation reaction (i.e. compare lanes 1 &amp; 2). Furthermore, this lack of inhibition is correlated with loss of cell killing activity in vivo (since the mutated relE gene, relEmE, used in lane 2 is not toxic to  E. coli  cells), thus indicating that inhibition of translation is the actual cause of cell death in vivo. In lanes 3 and 4, mRNAs encoding relE of plasmid P307 and the archaeon  M. jannaschii  were added. As seen, the presence of these mRNAs inhibited the in vitro translation reactions as well. These results indicate that the RelE toxins from  E. coli  K-12 , M. jannaschii  and plasmid P307 all act by inhibition of translation. 
     Example 16 
     Demonstrating that RelE K-12  is Toxic to Yeast Cells 
     1. Yeast Strain 
     In these experiments the yeast strain  Saccharomyces cerevisiae  281288DIV-36 (MATa his 4-5; LEU2 THR4 ura3-52 trp1 CYH2 KAR1) was used. 
     2. PCR Amplification of RelE Coding Region 
     The RelE coding region was PCR amplified from the plasmid pMG223 using two oligonucleotide primers. The primer S-RelE was 24 nucleotides long (5′-TAGGTACCATGGCGTATTTTCTGG-3′, SEQ ID NO:58). It contains KpnI and NcoI endonuclease restriction sites at the 5′ end with an 8 nucleotide overhang. Primer AS-RelE was 23 nucleotides long (5′-GAGACCCCACACTACCATCGGCG-3′, SEQ ID NO:59) and hybridises 400 nucleotides downstream the RelE termination codon and 392 nucleotides downstream the EcoRI site in plasmid vector pMG223. PCR amplifications were performed using Vent© Polymerase (New England Biolabs), 200 μM of each dNTP. PCR reaction buffer (10 mM KCl; 10 mM (NH 4 ) 2 SO 4 ; 20 mM Tris-HCl (pH 8.8); 2 mM MgSO 4 ; 0.1% Triton X-100) with 0.2 μM of each of the primers. After 5 min denaturation (95° C.) PCR was performed with 20 cycles, each cycle consisting of 1 min denaturation (92° C.), 1 min primer annealing (50° C.) and 1 min primer extension (72° C.). Successful PCR products of 706 bp DNA fragments were identified and purified from a 1% agarose gel after a run of 1 hour at 40 mA. The PCR product was digested with the two restriction enzymes KpnI and EcoRI and the fragment of 304 bp containing the RelE open reading frame was isolated after electrophoresis on a 1.2% agarose gel, and purified using a gel extraction kit (Pharmacia). 
     3. Cloning of the Amplified relE Gene 
     The isolated DNA fragment of the relE gene flanked with KpnI and EcoRI sites was ligated into the pYES2 expression vector (Invitrogen) previously digested with KpnI and EcoRI using standard procedures (Sambrook). After ligation,  E. coli  Top10 (Invitrogen) was transformed with the ligation mixture using electroporation using the  E. coli  gene pulser (BioRad). After phenotypic expression for 2 hours in SOC medium the culture was spread onto selective LB medium (Sambrook) containing 50 μg of ampicillin per ml. Transformed colonies were identified using PCR amplification and a positive clone designated pPK727 was further tested by restriction enzyme analysis. The functionality of the PCR amplified RelE gene was tested in  E. coli  by cloning the NcoI-EcoRI fragment from pPK727 into the  E. coli  expression vector pUHE24. Induction with IPTG led to cell killing in  E. coli.    
     4. Yeast Transformation 
       S. cerevisiae  was grown overnight (ON) in YDP medium (1% yeast extract; 2% Bacto peptone; 2% glucose). For a single transformation, cells from 1 ml ON culture were spun down (5,000 rpm for 30 sec using an Eppendorf minicentrifuge) and washed twice in sterile water. Cells were resuspended in 200 μl lithium acetate buffer (10 mM Tris-HCl pH 7.6 with 100 mM LiOAc, 1 mM EDTA). After incubation for 15 min. at 25° C. with agitation two transformations were made adding 20 μl carrier DNA (10 mg/ml salmon sperm DNA, sonicated and heat denatured) and 100 ng of the plasmids pPK727 and pYES2 (vector control), respectively. A volume of 1.2 ml 40% PEG 4,000 in 0.1 M lithium acetate buffer was added to each transformation mixture. The transformation mixtures were incubated in a 25° C. incubator for 30 min before transferring to a 42° C. water bath for 15 min. Cells were spun down (5,000 rpm for 30 sec.) and washed once with sterile water before plating on Uracil drop-out medium (1% Bernstein acid; 0.1% NaOH; 2% glucose; 0.67% Bacto yeast nitrogen base; 0.1% amino acids (without uracil); 2% agar). 
     After three days growth at 30° C. single colonies were picked and streaked onto plates with Uracil drop-out medium. After two days at 30° C. cells were transferred to induction medium (Uracil drop-out medium with 2% galactose as the sole carbon source) by replica-plating. 
     Single colonies of  S. cerevisiae  containing pPK727 and pYES2 were transferred to liquid Uracil drop-out medium (1% Bernstein acid; 0.1% NaOH; 2% glucose; 0.67% Bacto-yeast nitrogen base; 0.1% amino acids (without uracil)) and incubated ON. To compensate for difference in cell density, a volume of 50 μl upper OD 540  (optical density at 540 nm) was used to inoculate 50 ml of liquid Uracil drop-out medium with either glucose or galactose as sole carbon source, respectively. The four flasks with  S. cerevisiae  (pPK727) and  S. cerevisiae  (pYES2) in Uracil drop-out medium with or without galactose were incubated at 30° C. with moderate shaking (200 rpm). To monitor growth, samples were taken at different time point and OD540 was determined. Samples were taken in duplicates and the average OD 540  calculated and plotted against time of sampling 
     5. Results. 
     All colonies containing either the plasmid pPK727 or the pYES2 control plasmid were able to grow on plates with glucose as carbon source. When transferred to plates with galactose as the sole carbon source leading to gene expression from the P-gall promoter only cells with the pYES2 control plasmid showed normal growth, whereas cells containing the pPK727 were strongly inhibited in growth. 
     In liquid media, the yeast cells in which the relE gene was induced showed a remarked growth inhibition when compared to the uninduced control and to the controls with only the plasmid pYES2 without insert. 
     These results that are summarised in the below Table 16.1 clearly suggest that inhibition of cell growth in yeast cells is due to expression of the relE gene. 
     
       
         
           
               
             
               
                 TABLE 16.1 
               
             
            
               
                   
               
               
                 Growth of  Saccharomyces cerevisiae  transformed with 
               
               
                 pYES2 +/− relE gene 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Plasmid + relE 
                 no 
                 plasmid (control) 
                 no 
               
               
                 time (hours) 
                 Galactose 
                 galactose 
                 galactose 
                 galactose 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 5.5 
                 0.018 
                 0.024 
                 0.023 
                 0.028 
               
               
                 15.5 
                 0.013 
                 0.186 
                 0.055 
                 0.176 
               
               
                 23 
                 0.190 
                 1.195 
                 0.286 
                 0.963 
               
               
                 28 
                 0.010 
                 1.480 
                 0.816 
                 1.649 
               
               
                 29 
                 0.035 
                 3.930 
                 1.660 
                 2.650 
               
               
                 65 
                 0.990 
                 3.950 
                 6.180 
                 3.820 
               
               
                   
               
            
           
         
       
     
     REFERENCES 
     
         
         Altschul, S. F., Gish, W., Miller, W., Myers, E. W., and Lipman, D. J. (1990). Basic local alignment search tool. J Mol Biol 215:403-410. 
         Baum J. A. 1994. Tn5401, a new class II transposable element from  Bacillus thuringiensis . J Bacteriol 176:2835-2845. 
         Bech F. W, Jorgensen S. T, Diderichsen B, Karlstrom O. H. 1985. Sequence of the relB transcription unit from  Escherichia coli  and identification of the relB gene. EMBO J 4:1059-1066. 
         Bertani, G. (1951). The mode of phage liberation by lysogenic  Escherichia coli . J Bacteriol 62:293-300. 
         Bolivar, P. (1978). Construction and characterization of new cloning vehicles. III. Derivatives of plasmid pBR322 carrying unique EcoRI sites for selection of EcoRI generated recombinant DNA molecules. Gene 4:121-136. 
         Bult C. J, White O, Olsen G. J, Zhou L, Fleischmann R. D, Sutton G. G, Blake J. A, FitzGerald L. M, Clayton R. A, Gocayne J. D, Kerlavage A. R, Dougherty B. A, Tomb J. F, Adams M. D, Reich C. I, Overbeek R, Kirkness E. F, Weinstock K. G, Merrick J. M, Glodek A, Scott J. L, Geoghagen N. S. M, Weidman J. F, Fuhrmann J. L, Venter J. C et al. 1996. Complete genome sequence of the methanogenic archaeon,  Methanococcus jannaschii . Science 273:1058-1073. 
         Casadaban M. J, Cohen S. N. 1980. Analysis of gene control signals by DNA fusion and cloning in  Escherichia coli . J Mol Biol 138:179-207. 
         Clark, D. and Maaløe, 0.1967. DNA replication and the division cycle in  Escherichia coli . J Mol Biol 23:99-112. 
         Dam, M. and Gerdes, K. 1994. Partitioning of plasmid R1: Ten direct repeats flanking the parA promoter constitute a centromere-like partition site parC, that expresses incompatibility. J. Mol. Biol. 236:1289-1298. 
         Fleischmann R. D, Adams M. D, White O, Clayton R. A, Kirkness E. F, Kerlavage A. R, Bull C. J, Tomb J. F, Dougherty B. A, Merrick J. M et al. 1995. Whole-genome random sequencing and assembly of  Haemophilus influenzae  Rd. Science 269:496-512. 
         Gerdes, K., Bech, F. W., Jørgensen, S. T., Løbner-Olesen, A., Atlung, T., Boe, L., Karlstrøm, O., Molin, S, and von Meyenburg, K. 1986. Mechanism of postsegregational killing by the hok gene product of the parB system of plasmid R1 and its homology with the relF gene product of the  E. coli  relB operon. EMBO J. 5:2023-2029. 
         Gotfredsen, M. and Gerdes, K. 1998. The  Escherichia coli  relBE genes belong to a new toxin-antitoxin gene family.  Molec. Microbiol,  29:1065-1076. 
         Guzman L. M, Belin D, Carson M. J, Beckwith J. 1995. Tight regulation, modulation, and high-level expression by vectors containing the arabinose PBAD promoter. J Bacteriol 177:4121-4130. 
         Jensen, R. B. and Gerdes, K. 1995. Microreview. Programmed cell death in bacteria: proteic plasmid stabilization systems. Mol Microbiol 17:205-210. 
         Klenk H. P, Clayton R. A, Tomb J. F, White O, Nelson K. E, Ketchum K. A, Dodson R. J, Gwinn M, Hickey E. K, Peterson J. D, Richardson D. L, Kerlavage A. R, Graham D. E, Kyrpides N. C, 
         Fleischmann R. D, Quackenbush J, Lee N. H, Sutton G. G, Gill S, Kirkness E. F, Dougherty B. A, McKenney K, Adams M. D, Loftus B, Venter J. C et al. 1997. The complete genome sequence of the hyperthermophilic, sulphate-reducing archaeon  Archaeoglobus fulgidus . Nature 390:364-370. 
         Lanzer M, Bujard H. 1988. Promoters largely determine the efficiency of repressor action. Proc Natl Acad Sci U.S.A. 85:8973-8977. 
         Miller, J. F. (1972). Experiments in molecular genetics. Cold Spring Harbor Laboratory Press, New York. 
         Minton N. P. 1984. Improved plasmid vectors for the isolation of translational lac gene fusions. Gene 31:269-273. 
         Saul D, Spiers A. J, McAnulty J, Gibbs M. G, Bergquist P. L, Hill D. F. 1989. Nucleotide sequence and replication characteristics of RepFIB, a basic replicon of lncF plasmids. J Bacteriol 171:2697-2707. 
         Shägger, H. and von Jagow, G. (1987). Tricine-Sodium Dodecyl Sulfate-polyacrylamide gel electrophoresis for the separation of proteins in the range from 1 to 100 kD. Anal Biochem 166:368-379. 
         Simon, R., O&#39;Connell, M., Labes, M. and Puhler, A. 1986. Plasmid vectors for the genetic manipulation of  rhizobia  and other gram-negative bacteria. Methods. Enzymol. 118:640-659. 
         Thisted, T., Nielsen, A. and Gerdes, K. 1994. Mechanism of post-segregational killing: translation of Hok, SrnB, and PndA mRNAs of plasmids R1, F and R483 is activated by 3′-end processing. EMBO J. 13:1950-1959.