METHOD FOR MAKING GENETICALLY MODIFIED MICROORGANISMS

Abstract: A method for making a genetically modified microorgan­ ism wherein DNA material, particularly material comprising a gdh gene, is transferred from a donor microorganism to a recipient microorganism using a plasmid vector. The inven­ tion relates in particular to the use of multicopy plasmids and/or DNA fragments having at their ends target sites for SalI restriction enzymes. The method may involve a plurality of steps, transfer of DNA fragments into intermediate mic­ roorganisms and use of a plurality of plasmids. Genetically modified microorganisms and a process for the production of single cell protein using the genetically modified microorgan­ isms are also claimed.

Description: Method for making genetically modified microorganisms This invention relates to a method for making genetically modified microorganisms, to genetically modified microorganisms and to a process for the production of single cell pro tain (SCP) using such genetically modified microorganisms. In our UE Patent No. 1370892 we describe and claim a process for the production of SOP in which bacteria belonging inter alia to the species Methylophilus methylotroDhus (formerly named Pseudomonas methylotronha) are cultivated in a medium comprising methanol as a carbon source and appropriate inorganic nutrients. In such a process it is desirable that a high carbon conversion (i.e. conversion of substrate (methanol) carbon to cellular carbon) is achieved. Ammonia is a convenient nitrogen source for such a process. In the production of SOP by microbiological assimilation of methanol and ammonia, two pathways for ammonia assimilation by microorganisms have been identified, namely (a) direct (GDH) pathwag Ro2c(c)2co CO2R + NH) + NED(P)H alpha keto glutarate EMI1.1 glutamate This reaction is catalysed by the enzyme glutamate dehydrogenase (GDE) and requires one mole of NAD(P)H, equivalent to 3 moles of ATP per mole of ammonia assimilated. (b) 2-stage (GS-GUS) pathway EMI2.1 glutamate glutamine This reaction is catalysed by the enzyme glutamine synthetase (GS). EMI2.2 This reaction is catalysed by the enzyme glutamine glutamate amino- transferase (GUS) (also known as glutamate synthase and as GOGAT). This combination of reactions forming the two-stage pathway requires one mole of tIP for the first stage and the equivalent of 3 moles for the second stage, i.e. a total of 4 moles of :P, per mole of ammonia assimilated. The exact mechanism by which AUP is consumed is' immaterial. However overall the greater consumption of AvP in the two-stage mechanism results in conversion of more of the methanol to carbon dioxide than in the direct mechanism. We have found that MethYloPhilus methylotrowhus lacks the GDH enzyme and assimilates ammonia by the two-stage pathway and that this pathway predominates amongst methanol-utilizing bacteria proposed to date for SCP production. In our published Us patent Specification No. 2003926A we describe an improved process for the production of single cell protein using microorganisms such as MethyloPhilus methylotrophus which have been genetically modified by incorporation of the gene specifying ammonia assimilation by the GDE enzyme route. This process gives improved conversion of substrate carbon to cellular carbon. We have now developed an improved method for introducing genetic material, such as the gene specifying ammonia assimilation by the GDE enzyme route, into a microorganism, thereby producing improved genetically modified strains of e.g. Methylophilus methylotroohus and an improved process for the production of SOP. In particular our improved genetic modification method provides a means whereby an increased amount of the gene specifying ammonia assimilation by the GDH route can be introduced into e.g. a strain of MethyloPhilus methylotrophus thereby increasing the amount of the GDH enzyme present in the microorganism and improving the efficiency of the ammonia assimilating pathway established in the microorganism. Explanation of terms and symbols In this specification the terms and symbols listed below have the meanings given: NAD(P) ¯ nicotinamide adenine dinucleotide (phosphate), the co-factor of the GDE or GUS enzyme (and many other enzymes) in its oxidised form; NAD(P)H - nicotinamide adenine dinucleotide (phosphate), the co-factor of the GDH or GUS enzyme (and many other enzymes) in its reduced form; ATP - adenosine triphosphate; ADP - adenosine diphosphate; 3sA - deoxyribonucleic acid - genetic material of most organisms; Pi - phosphate ion; ; gdh gene - the gene specifying glutamate dehydrogenase, the key enzyme for ammonia assimilation by the GDH enzyme route; git gene - the gene specifying glutamate synthase, the key enzyme for ammonia assimilation by the GUS enzyme route; git 3 - the gene specifying glutamate synthase, the key enzyme for ammonia assimilation by the GUS enzyme route, in E. coli; fr - rifampicin resistance - other antibiotic re sistances are expressed in similar manner; Tc5 - tetracycline sensitivity - other antibiotic sensitivities are expressed in similar manner; Mu cts - temperature sensitive Nu phage - Mu is a temperate phage; ; transxoson - a segment of DEA which is transportable from one replicon to another and which can attach itself into a plasmid and frequently confer properties such as antibiotic resistances thereon, example transposon th5. According to the present invention we provide a method for making a genetically modified microorganism by incorporating hNA material comprising a gdh gene into a microorganism deficient in this gene, so that expression of the gdh gene product, i.e. the active GDH enzyme, occurs in the previously gdh gene-deficient microorganism which method comprises the steps of (a) introducing into a first microorganism having the gdh gene a plasmid vector for said gene, and (b) transferring said gene on said vector into a second microorganism. The second microorganism may be the gdh gene-deficient microorganism intended as the ultimate ecipient of the IWA material or it may be an intermediate, the DNA material comprising the gdh gene being transferred via one or more intermediate microorganism(s) from the first microorganism to the gdh gene-deficient microorganism. Further according to the invention we provide a method for making a genetically modified microorganism by transferring DNA material comprising one or more genes from a donor microorganism having said gene or genes to a recipient microorganism deficient therein wherein the A material is transferred into the recipient microorganism on a plasmid vector formed from a multicopy plasmid. The method may comprise a plurality of steps involving intermediary microorganisms during transfer of the DWd material from the donor to the recipient microorganism. Preferably the gene transferred is the gush gene. Further according to the invention we provide a method for making a genetically modified microorganism by transferring INA material comprising the gdh gene from a donor microorganism having said gene to a recipient microorganism deficient therein which comprises a step wherein the gdh gene containg 2EA material is transferred on a INA fragment having at its ends target sites for the SalI restriction enzyme. The invention further provides new microorganisms produced by the method thereof, variants and mutants of such microorganisms, processes for producing single cell protein and/or amino acids using such microorganisms, protein derived from such microorganisms, dead cells of such microorganisms and feed - or foodstuffs for animal or human consumption containing such dead cells or protein. The invention further provides as new compositions of matter products produced at intermediate stages of the genetic modification method in particular modified R300E, pRP3Ol and pACYC184 plasmids incorporating the zdh gene. The invention further and particularly provides the novel microorganism MetyloohiIus methylotrophus strain NCIB No. 11585 and variants and mutants thereof, processes for the production of single cell protein and/or amino acids using said strain, variants or mutants, dead cells and amino acids produced by such processes, protein derived from said dead cells and feed or foodstuffs for animal or human consumption containing said dead cells or protein derived therefrom. Preferably the method of the invention is used to introduce the edh gene into gram-negative bacteria lacking this gene. The microorganism Metylophilus methylotrophus strain NCIB No. 11585 was produced by the genetic modification of strain NCIB No. 10515 as described in Example 1 (strain 11585 is Modification 3). A culture of strain NCIB No. 11585 is deposited at the National Collection of Industrial Bacteria, Torrey Research Station, Aberdeen, Scotland, UK. The microbiological characteristics of NCIB No. 10515 and of the species Methylophilus metbylotrohus generally are set out in our UK Patent Specification No. 1370892. The microbiological characteristics of strain NCIB No. 11585 are the same as those of st-rain NCIB No. 10515 except that strain NOtE No. 11585 is resistant to sulphonamide, streptomycin andknnnmycin. Microorganisms which can usefully be modified by the method of the invention to incorporate the gdh gene include the following bacteria: Methylonhilus methylotrophus - cultures of which are deposited as follows : - NCIB 10508 - 10515 and 10592 - 10596, all inclusive; NRRL 3 5352 - B 5364 inclusive; yRI 1215 - 1227 inclusive; Pseudomonas methylonica, the characteristics of which are described in UK Specification No. 1451020 and a culture of which is deposited as follows: : yRI 2247 and 2248 Pseudomonas methanolica - UK Specification No. 1352728 - ATOC 21704; Pseudomonas sp. - UK Specification No. 1326582 - ATCC 21438 and 21439; Metbylomonas methanolica- UK Specification No. 1420264 - NRRL B 5458; Pseudomonas utilis - UK Specification No. 1444072 - iBI 1690 and 1691; Pseudomonas rnaudita - UE Specification No. 1444072 - ERI 1693 and 1694; Metylomonas clara - USP 4166004- ATCC 31226. NB: NCIB is the National Collection of Industrial Bacteris, Aberdeen; NI?RL is the Collection maintained by the-US Department of Agriculture, Peoria, Illinois; ATOC is the American Type Culture Collection; ml is the Fermentation Research Institute, Japan. Such microorganisms may be used in processes for the production of SCP wherein, after modification by the method of the invention, they are aerobically cultured in an aqueous medium containing methanol as a source of assimilable carbon, ammonia and/or ammonium ion at a concentration sufficient to maintain the G3H reaction mechanism, and other inorganic nutrients, followed by recovery of SCP from the culture medium. The conditions of the process for producing SCP are conveniently as described in UK Specifications Nos. 1370892 and 1451020, except that the total concentration of ammonia and ammonium ion is preferably maintained at above 0.5 mM in zones of maximum methanol concentration. As a result of the decreased consumption of AsP there is less overall conversion of methanol to carbon dioxide and consequently the process is less exothermic and requires less cooling to maintain the temperature at the preferred level in the range 34 - 450cm For optimum economy the active microorganism predominantly and preferably exclusively metabolises ammonia by the GDE route. In the method of the invention for introducing a gene into a microorganism deficient in this gene any suitable microorganism may form a source for the gdh gene, gram negative bacteria being preferred. Suitable sources include strains of E. coli, Pseudomonas and Flebsiella, although in general terms there is no a priori reason for selecting any particular gram negative bacterium as the source of the gdh gene. However E. coli is a well-known species whose properties have been widely researched. Hence although a wide variety of microorganisms could serve a sources of the gdh gene it is generally convenient to use E. coli for this purpose. In the particular instance where the gdh gene is to be transferred into a strain of MethyloPhilus methylotrophus, E. coli is further preferred as a source since we have found that the F. coli edh gene can provide a functional GDH pathway in Methylophilus methylotroDhus without any further manipulation of the gene. In the method of the invention for the transfer of the gdh gene a fragment of zW comprising the gdh gene is transferred using one or a succession of plasmid vectors from a microorganism having the ggl gene into a microorganism lacking said gene. In its simplest form the method can consist in introducing into the source microorganism for the gdh gene a plasmid which can act as a vector for the gene, mobilising the gene-containing DNA fragment onto the vector and transferring the gene-carrying vector into the microorganism lacking the gdh gene. However many variations and elaborations of this simple sequence are possible, one being described in detail in Example 1. Por instance the plasmid used to take the gene out of the source microorganism may not be readily transferable into the recipient microorganism and it mag therefore be convenient to introduce an extra step wherein the gene is transferred from the original plasmid vector onto another plasmid more readily transferable into the recipient microorganism. Such transfers may be effected by any suitable means, conveniently by use of appropriate restriction enzymes and ligases. In another instance the original plasmid vector may be large and may carry a considerable quantity of extraneous DEA making it difficult to manipulate. In this case it is convenient to transfer the gene-carrying 2EA fragment onto a smaller plasmid. A suitable sequence of steps for transferring gdh gene from an E. coli strain into a strain of MethyloPhilus methylotroxhus is described in Example 1. The number and exact nature of the steps required will vary in transfers involving different microorganisms depending upon a number of factors including the source and recipient microorganisms concerned and the availability of suitable plasmids and upon general convenience. The plasmids used can be derived from any source. Suitable sources include E. coli, Pseudomonas and Elebsiella. Useful plasmids include those of the "P" incompatibility group, for example RP4, which are freely transmissible amongst a variety of gram negative bacteria. Other suitable plasmids for use during the transfer sequence include R300B, pRP3Ol and pACYC184. Preferably the plasmids are ones which are capable of transfer into the recipient microorganism. To facilitate transfer of the gene, e.g. to improve the utility of a plasmid as a vector or to render it capable of transfer into a recipient microorganism, the properties of a plasmid may be modified by associating with it another INA fragment or fragments. Examples of such other fragments include temperate phages such as Mu and A phages and transposons such as Tn 5. Such phages and transposons can play a useful part in the transfer of the gdh gene. Temperature sensitive Mu phage (Mu cts) is useful in the preparation of the plasmid vector used to take the gdh gene from the source microorganism. When a microorganism containing (Mu cts) is grown at a partially inducing temperature transposition of Mu and adjacent genome INA occurs without further maturation of phage particles. This transposition then facilittransfer of fragments of the microorganism's DEA onto a plasmid such as RP4 present in the microorganism leading to a recombinant RP4 containing segments of genome INA. Temperate phages and transposons can also be used to provide marker antibiotic resistances and target sites for restriction enzymes on otherwise deficient plasmids. For example transposon Tn 5 is very useful as a supplier of target sites for the SalI restriction enzyme to plasmids such as R300E, one such variant of R3O0:Th 5 is p TB70. Groups of plasmids which are readily transferable into strain NCSB No. 1015 and are thus very useful in the invention are the Inc Q, Inc P and Inc W groups of plasmids. Included in the Inc Q group are plasmids R3003 and other plasmids described by PT earth and N J Grinter (J. Bacteriol., (1974), 120, 618 - 30) and plasmid R1162. Included in the Inc P group are plasmids RP4 (described by N Datta et al, J. Bacteriol., (1971), 108, 1244 - 49), pEP 301 (described by P T 3arth at pages 399 - 410 of "Plasmids of Medical, Environmental and Commercial Importance", Editors E N Timmis and A Puhler, Published Elsevier, North Holland, 1979) and R 68.45. Included in the Inc W group are plasmids R388 (described by N Datta et al, J. Gen. Microbiol., 72, 349, (1972)) and R7E. NB. Plasmid pACYC 184 is described by : C Y Chang et al, J. Bacteriol., 134, 1141 - 56, (1978). We have found that the gdh gene of E. coli can be carried on a small easily manipulated DEt fragment having at its ends target sites for the SaIl restriction enzyme. This enables the fragment to be conveniently transferred from one plasmid containing it to another by cutting the DNAs of the fragments with SalI. The cut DNAs are then mixed and treated with a ligase. For such a step the receiving plasmid requires a SalI target either as a normal genotypic feature of the plasmid or imparted to it by an associated transposon or phage. In one embodiment of the invention a gene, e.g. the gdh gene is transferred into a recipient microorganism using a multicopy number plasmid. A multicopy number plasmid is one which, when introduced into a microorganism is capable of replicating itself a sigaificant number of times suitably to produce substantially more than 3 copies of itself per cell, often 8 or 10, preferably at least 20 copies per cell and particularly 20 to 30 copies per cell. By contrast a "single copy1± number plasmid is one which replicates itself in a controlled way such that it resides in the cell as 1 to 3 copies. Using a multicopy plasmid a gene such as the gdh gene may be introduced into a cell more intensively i.e. the replication of the plasmid produces a substantial number of copies of the gene carrying DNA within the cell. Specifically in the instance of the gdh gene introduced into a methanol-utilizing bacterium such as MethYlo7philus methylotrophus it increases the amount of GDR enzyme in the microorganism cells and facilitates establishment therein of an efficient pathway for ammonia assimilation by the GDE pathway. The use of multicopy plasmids has important consequences. A preferred multicopy plasmid for transferring gin gene into Methyloohilus methylotrophus is R30013. Preferably when a microorganism is modified by introduction of the dh gene thereby enabling it to assimilate ammonia by the GDH pathway, it is also modified to prevent it using the altemative GS/GUS pathway thereby enabling it to make the most efficient use of the GR pathway. The GS/GUS pathway can be blocked by ensuring that the microorganism lacks the glt gene. Microorganisms lacking the glt gene can be obtained by forming mutants of the original strain e.g. of Methylovhilus methylotroDhus and selecting mutants which have lost this gene. Two methods of selection by (a) temperature and (b) ammonia growth are described in Example 1. Mutants can be produced by standard techniques, for example by treatment with a physical mutagen such as gamma, X or ultra-violet radiation or a chemical mutagen such as N-methyl-N1-nitro-N-nitroso guanIdine, nitrous acid, a methane- sulphonate ester (e.g. methyl or ethyl), 5-bromo-uridine, 5-bromouracil, 2-amino-purine or nitrogen mustard or a biological mutagen such as Mii-phage. Treatment to produce a deletion mutant is preferred. The mutant normally remains capable of the GS catalysed reaction. The invention is illustrated by the following examples: EXAMPLE 1 Transfer of E. coli gdh gene into Methylorhilus mettylotrophus strain The E. coli gdh gene was transferred into Meth,yloDhilus methNlotrophus strain NCIB No. 10515 by a method comprising the five steps described below. 1. Develonment of vector To generate derivatives of plasmid RP4 carrying the EA-4 coli gEh gene the following E. coli strains were constructed: E401 - donor strain for gssh gene, with the relevant genotypic features: glt 3, gdh+, (Mu cts):RP4 E412 - recipient strain for mating with E401, derived from E. coli strain PA340 (alternativly known as CB100), with the rele vant genotypic features: glt 3, gan, ri, (Mu cts), rec A. The glt 3 and gdh mutations render E412 glutamate depend ent and in a mating with E401 only gdh gene acquistion will give glutamate independence. The rec A mutation renders E 412 recombination deficient. The E401 cells were grown overnight at 370C to induce Mu cts mediated mobilisation of segments of the E 401 chromosome to plasmid RP4. Cells from this overnight culture were then mated with E412 cells grown at 30 C. Matings were performed non-selectively by overnight growth at 300C on L agar plates. Dilutions of cells recovered from the mating plates and control plates (on which donor and recipient cells had been grown separately) were then plated out and selections made for glutamate independence (i.e. the ability to grow using ammonia as nitrogen source), rifam= picin (rif) (a selection for recipient based cells) and kanamycin resistance (Kmr) (a marker borne by the RP4 plasmid). The cells thus selected were E412 cells with the relevant genotypic features: glt 3, gdh+ (Mu cts):RP4; i.e. cells containing an RP4 plasmid with Mu phage and gdh gene attached 3P4 prime gL (RP4tgah). 2. Transfer of gdh gene to small plasmid Plasmid RP4'gdh is a large plasmid carrying much unknown INA and giving problems on transfer and with ENA purification. The gdh gene was therefore transferred as follows to the smaller plasmid pACYCl84. paCYC184 was selected because (a) it contains a gene specifying chloramphenicol resistance (Cm r)- a marker lacking in RP4- and, (b) it has targets for the restriction enzymes EindIII, Bamb@ and SalI located within the INA of the tetracycline resistance gene (Tcr) and therefore insertion of "foreign" DA into these restriction sites on plasmidpffiCYC184 will inactivate the TJ gene. The DNz of plasmid I1P4'Rdh and the gA of plasmid pACYC184 were both cut with SalI restriction enzyme. The two cut DNAS were mixed and a DNA ligase was added. The DNA thus produced was introduced by transformation into E. coli strain Pa340 (relevant geno type features: glt3, B 8 A selection was then made for cells exhibiting glutamate independence, chloramphenicol resistance and tetracycline sensitivity WTc5). Such cells contain paCYC184 plasmids whose Tcr genes have been cut and fragments of RP4'gah DNA inserted thereinto. It should be noted that pAOOOl84 is a multicopy plasmid and that when inserted into a cell it will replicate to produce numerous copies. Whence cultures of selected cells produced cells containing numerous copies of the gdh gene. 3. Transfer of zdh gene to Plasmid transferable to NCIB No. 10515 The plasmidp2CYC184/gdh is not directly transferable into strain NCIB No. 10515. In this plasmid the INA fragment containing gdh is located between targets for restriction enzyme SalI. gdh fragments were transferred to two alternative plasmids which can be transferred to strain NCIB No. 10515 and which will replicate therein. These plasmids were: pRP301 - a single copy P group plasnid which can transfer itself into strain NCT No. 10515, and R3003 - a multicopy plasmid which can be mobilised into strain NOI3 No. 10515 by use of plasmid RP4. Transposon Tn 5 was transposed into plasmid R30013 to give pTB70 as it was required (a) to provide a SalI restriction enzyme target on this plasmid which does not have such a target (Molec. Gen. Genet., 171, 7 - 13, 1979) and (b) to provide a kanamycin resistance ¸r) for the plasmid. The pBCYC18t plasmid ItA was cut with SalI restriction enzyme and, in separate experiments, was mixed with pRP3Ol and pTB370 plasmid DNA similarly cut with SalI restriction enzyme. In both experiments the resulting DNA mixtures were treated with INA ligase. The INAs thus produced in the two experiments were separately introduced by transformation into E. coli strain PA340 (relevant genotypic features: gltB, zdh). Selections were then made in the case of pTB70 for cells exhibiting glutamate independence and kanamycin resistance ( ). These strains were then screened for chloramphenicol sensitivity (Cms) In the case of pEP 301 selections were made for cells exhibiting glutamate independence and ampicillin resistance (amp) and these strains were screened for tetracycline sensitivity. 4. Transfer of gdh gene into strain NOB No. 10515 In step 3 were produced two alternative modified E. coli strains containing respectively plasmid pRP301/gdh and plasmid pEB70:gdh. In separate experiments these two strains were used as alteunative means to transfer the gdh into cells of the strain SC: No. 10515. Transfer using pT870:gdh Plasmid R3003 cannot mobilise itself into cells of strain SC: No. 10515, i.e. R3003 is not transferred from a strain carrying R3003 to another strain. This difficulty was overcome by inserting into the E. coli cells containing pT370:gdh an additional plasmid which can mobilise itself into cells of NCIB No. 10515. The extra plasmid inserted was RP4 whose properties enable it to provide a method for the transfer of plasmids such as plasmid pTB70:gdh from one cell to another. E. coli cells were therefore prepared containing the plasmids pTB70: and RP4. A streptomycin resistance marker (Smffi) is carried by rlasmid R3003. NOIB No. 10515 lacks this resistance. This streptomycin resistance was then used to select for transfer of pIB70:ggh to ECIB 10515. A culture medium suitable for strain NCIB No. 10515 but including streptomycin was prepared and placed upon a petri dish which was divided, one half being used as a control whilst cells of NOI3 No. 10515 were placed on the other half. Both parts were streaked with a culture containing E. coli cells modified as described above. On the control part of the dish E. coli cells did not survive. On the other side, whilst the E. coli cells did not survive. it was possible for 2gA material to be transmitted from them to cells of NOI3 No. 10515. This enabled NOtB No. 10515 cells which had received ggA containing pEB70:gdh with associated (Smr) to survive, i.e. selection was made for The resulting bacterial strain - Modification 2 - was a modification of NCIB No. 10515 with the relevant genotypic features: glut+, gdh+, pew70, Smr Transfer using pRP301/gdh This plasmid was also transferred into strain NIB No. 10515 using suitable methods to give Modification 1 - a modification of NCIB No. 10515 with the relevant genotypic features: glt+, dh+, Rahi, pRP301. Thus both Modifications 1 and 2 possess both of the alternative pathways for ammonia assimilation. 5. Transfer of gdh gene into glt NCIB No. 10515 To obtain an NOI3 No. 10515 strain relying entirely upon the GlH pathway for ammonia assimilation it was necessary first to obtain mutants of NCIB No. 10515 lacking the glt gene. Two methods were employed. (a) Temperature sensitive mutants - NCTi3 No. 10515 cells were treated with N-methyl-N -nitro-N-nitroso guanidime and the resulting variety of mutants was examined at 300C and 37 C. A total of 550 temperature sensitive mutants, i.e. ones capable of growth at 3000 but not at 3700, were collected. These were examined to find mutants in which the git gene product had become temperature sensitive. The mutants were mated as described in 4 above with E. coli cells containing plasmids pT370:gdh and RP4. Four mutants were, after this mating, capable of growth at 37 C. This implied that in these four cases the mutant cells were ones in which the git gene product had become temperature sensitive but that, having acquired pEB70:gdh from the E. coli cells, growth at 37 C became possible. This result was confirmed by incorporating into separate samples of each of the original mutants pEB70:edh and pTB70 (using the method of 4 above) and showing that only transfer of the former plasmid allowed growth at 37 C, and thus that growth was due to a function coded for on the cloned fragment of E. coli DNA carrying the gdh gene. Furthermore, these temperature sensitive mutants carrying pTB70:gdh were unable to form colonies at 370C on plates supplemented with concentrations of ammonia at 0.07 mM or less, on which NCIB No. 10515 can form colonies. This implies that these cells are using the gdh gene product for growth since cells using the glt gene product can grow on much lower concen- trations of ammonia than cells using gdh. Finally it was confirmed by enzyme assays on cells grown at 370C that these temperature sensitive mutants carrying pE370:gdh lacked detectable GUS activity, but had C-DE activity, whereas NCIB No. 10515 had GUS activity but lacked detectable GDH activity. One of the four temperature sensitive mutants modified by inclusion of plasmid pT370:edh was deposited at NOIB as NCI3 No. 11585 (Modification 3) on 5 March 1980. Further modifications were prepared by incorporating into temperature sensitive mutants deficient in the git gene product, the plasmid pRP301:gdh. (b) Ammonia growth Selected mutants - In separate experiments cells of Modifications 1 and 2 were used as "wild-type" strains. These cells contain both the gdh and the glt genes. However mutants may occur lacking the git gene. For efficient assimilation of ammonia by the GIR pathway a high concentration of ammonia is desirable. The alternative GS/GUS pathway functions well at low ammonia concentrations. Hence cells of Modifications 1 and 2, having both pathways should function well at both high and low concentrations of ammonia. In the experiments therefore cells carrying the pTB70:Rdh plasmid were grown on media containing (a) > 30 mM ammonia and (b) < .07 mM and cells were selected which were unable to grow on the medium containing < .07 mM. In this way mutants of strain NCIB No. 10515 were obtained which were gdh+ ana glt (some derived from Modification 1 and some from Modification 2). EXAMPlE 2 Production of SOP using Modification 3 (NOIB No. 11585). The organisms designated NOI3 No. 10515 and NCIB No. 11585, the latter using GDE activity rather than GtrS/GS, were grown in continuous culture in a methanol/mineral salts medium designed to support 30 gms/l dry weight of cells growing at a dilution rate of 0.2 hr Temperature was controlled at 37 C + 10, pH 6.8 - 7.0 by the demand addition of ammonia gas. The cultures were supplied with air as oxygen source. Modification 3 cultures were grown in the presence of not less than 50 mM ammonia in order to saturate GDH. Carbon conversions of methanol carbon to organic cellular carbon were determined by total carbon analysis. Results gave a growth yield improvement of 4 to 7% for NCIB No. 11585 over NCIB No. 10515.

Claims: 1. A method for mnking a genetically modified microorganism by incorporating DNA material comprising a gdh gene into a microorganism deficient in this gene, so that expression of the gdh gene product, i.e. the active GDE enzyme, occurs in the previously gNh gene-deficient microorganism, which method comprises the steps of (a) introducing into a first microorganism having the gdh gene a plasmid vector for said gene, and (b) transferring said gene on said vector into a second microorganism. 2. A method for making a genetically modified microorganism by transferring tNA material comprising one or more genes from a donor microorganism having said gene or genes to a recipient microorganism deficient therein wherein the 2EA material is transferred into the recipient microorganism on a plasmid vector formed from a multicopy plasmid. 3. A method for making a genetically modified microorganism by transferring DWA material comprising the gin gene from a donor microorganism having said gene to a recipient microorganism deficient therein which comprises a step wherein the gdh gene containing DEA material is transferred on a DNA fragment having at its ends target sites for the SalI restriction enzyme. 4. A method according to claim 1 wherein the second microorganism is an intermediate and the gdh gene is transferred via one or more intermediate microorganisms from the first microorganism to the gdh gene-deficient microorganism. 5. A method according to claim 2 which comprises a plurality of steps involving at least one intermediary microorganism during transfer of the DNA material from the donor to the recipient microorganism. 6. A method according to claim 2 or claim 5 wherein the gene transferred is a gdh gene. 7. A method according to claim 3 wherein the INA fragment transferred is transferred on a plasmid vector formed from a multicopy plasmid. 8. A method according to any one of the preceding claims wherein a gdh gene is transferred into a gram-negative bacterium lacking this gene. 9. A method according to claim 8 wherein the gram= negative bacterium is a strain of the species Methylophilus mettylotrophus. 10. A method according to claim 9 wherein the strain is MetkyloPhiulus metkylotrophus NCIB No. 10515. 11. A method according to any one of the preceding claims which comprises a step in which the git gene product is inactivated by mutation in edh gene-deficient microorganism. 12. A method according to any one of the preceding claims wherein the gdh gene containing PIZZA material is obtained from a strain of E. coli. 13. A method according to any one of the preceding claims wherein the Fdh gene containing DNA material is transferred into the gdh gene deficient microorganism on a plasmid vector formed from an Inc Q, Inc P or Inc W group plasmid. 14. A method for making a genetically modified microorganism substantially as described and as shown in Example 1. 15. Genetically modified microorganisms produced by a method according to any one of the preceding claims and variants and mutants of such genetically modified microorganisms. 16. Genetically modified microorganisms according to claim 15 which are genetically modified strains of gram-negative bacteria. 17. Genetically modified microorganisms according to claim 16 which are strains of the species Methylophilus methylotrophus genetically modified by incorporation of DNA material comprising a gdh gene. 18. The microorganism Methyloohilus methyl otrohus strain NCIB No. 11585 and variants and mutants thereof. 19 A process for the production of single cell protein and/ or an amino acid wherein a gram-negative bacterium genetically modified by incorporation of INA material comprising a gdh gene and produced by a method according to any one of claims 1 to 14 is aerobically cultured in an aqueous medium containing methanol as a source of assimilable carbon and ammonia and/or ammonium ion at a concentration sufficient to maintain the GDH reaction mechanism and the single cell protein, a composition contnining it and/or an amino acid is recovered from the culture medium. 20. A process according to claim 19 wherein the genetically modified microorganism is a genetically modified strain of the species Methylophilus methylotronhus. 21. A process according to claim 20 wherein the genetically modified microorganism is Methylophilus metkylotronhus strain NOI3 No. 11585 or a variant or mutant thereof. 22. Dead cells of microorganisms according to any one of claims 15 to 17. 23. Protein derived from microorganisms according to any one of claims 15 to 17. 24. A foodstuff for animal or shaman consumption containing dead cells according to claim 22 or protein according to claim 23. 25. Modified plasmids produced at intermediate stages of a method according to any one of claims 1 to 14. 26. Modified R300S, pry301, pACYC184 plasmids incorporating the dh gene.

Inventors: WINDASS, JOHN DAVID, WORSEY, MICHAEL JOHN
Applicants: IMPERIAL CHEMICAL INDUSTRIES PLC
Application year: 1981
Main classification: C12N15
IPC codes: C12N1, C12N15, A23K1, A23L1, C12N9, C07H21