Involvement of the cell envelope in plasmid maintenance: plasmid curing during the regeneration of protoplasts

Challenges and advances in genetic manipulation of filamentous actinomycetes - the remarkable producers of specialized metabolites

Covering: up to February 2019Actinomycetes are Gram positive bacteria of the phylum Actinobacteria. These organisms are one of the most important sources of structurally diverse, clinically used antibiotics and other valuable bioactive products, as well as biotechnologically relevant enzymes. Most strains were discovered by their ability to produce a given molecule and were often poorly characterized, physiologically and genetically. The development of genetic methods for Streptomyces and related filamentous actinomycetes has led to the successful manipulation of antibiotic biosynthesis to attain structural modification of microbial metabolites that would have been inaccessible by chemical means and improved production yields. Moreover, genome mining reveals that actinomycete genomes contain multiple biosynthetic gene clusters (BGCs), however only a few of them are expressed under standard laboratory conditions, leading to the production of the respective compound(s). Thus, to access and activate the so-called "silent" BGCs, to improve their biosynthetic potential and to discover novel natural products methodologies for genetic manipulation are required. Although different methods have been applied for many actinomycete strains, genetic engineering is still remaining very challenging for some "underexplored" and poorly characterized actinomycetes. This review summarizes the strategies developed to overcome the obstacles to genetic manipulation of actinomycetes and allowing thereby rational genetic engineering of this industrially relevant group of microorganisms. At the end of this review we give some tips to researchers with limited or no previous experience in genetic manipulation of actinomycetes. The article covers the most relevant literature published until February 2019.

An IS257-derived hybrid promoter directs transcription of a tetA(K) tetracycline resistance gene in the Staphylococcus aureus chromosomal mec region

Transcription of the tetA(K) tetracycline resistance determinant encoded by an IS257-flanked cointegrated copy of a pT181-like plasmid, located within the chromosomal mec region of a methicillin-resistant Staphylococcus aureus isolate, has been investigated. The results demonstrated that transcription of tetA(K) in this strain is directed by both an IS257-derived hybrid promoter, which is stronger than the native tetA(K) promoter in the autonomous form of pT181, and a complete outwardly directed promoter identified within one end of IS257. Despite lower gene dosage, the chromosomal configuration was shown to afford a higher level of resistance than that mediated by pT181 in an autonomous multicopy state. Furthermore, competition studies revealed that a strain carrying the chromosomal tetA(K) determinant exhibited a higher level of fitness in the presence of tetracycline but not in its absence. This finding suggests that tetracycline has been a selective factor in the emergence of strains carrying a cointegrated pT181-like plasmid in their chromosomes. The results highlight the potential of IS257 to influence the expression of neighboring genes, a property likely to enhance its capacity to mediate advantageous genetic rearrangements.

A genetic and molecular characterization of the recA gene from Staphylococcus aureus

Previous studies have identified mutant strains of Staphylococcus aureus that have deficiencies in genetic recombination and DNA repair. Although these phenotypes were tentatively attributed to mutations within the S. aureus recA gene, experimental evidence to confirm this has never been reported. To characterize recA from S. aureus, we first isolated transposon insertion mutations that were in close proximity to the recA-like mutation (uvs-568) in strain 112 UVS-1. This allowed for the mobilization of the uvs-568 mutation into strain RN4220, the common laboratory strain of S. aureus. Next, using Bacillus subtilis recA as a probe, we cloned S. aureus recA and determined its nucleotide sequence. The deduced amino acid (aa) sequence of RecA contained 347 aa and was 74% identical to B. subtilis RecA. Using a cloned DNA fragment originating from within S. aureus recA, we then constructed a recA null mutant strain, designated KB103, which exhibited the same phenotypic characteristics imposed by the uvs-568 mutation in the same background. Furthermore, genetic and physical mapping of S. aureus recA placed it in the same region as the uvs-568 mutation. These data strongly suggest that these mutations represent different alleles of the same recA gene.

Protoplast fusion: A tool for intergeneric gene transfer in bacteria

Protoplasts can be isolated from bacterial cells by digestion of the cell wall with the help of lysozyme in presence of osmotic stabilizers. Fusion of protoplasts can be induced by chemical fusogens like polyethylene glycol. The electrofusion technique has been reported in bacteria in which the fusion frequency is much higher than that obtained by PEG induced protoplast fusion. This technology allows recombination to take place not only between related species but also between unrelated genera and is of great potential in the breeding and improvement of industrial strains. This review includes the information and developments on the protoplast fusion in bacteria with special reference to genetic recombination by protoplast fusion between phylogenetically unrelated bacteria.

Regulation of the Staphylococcus aureus plasmid pI258 mercury resistance operon

Experiments involving fusion between the Staphylococcus aureus plasmid pI258-encoded mer operon and the reporter gene beta-lactamase, mutational analysis, and trans-complementation studies have shown that the merR gene of pI258, which shows DNA sequence similarity with known merR genes from other bacteria, regulates the expression of the mer operon in vivo. The merR gene product is a trans-acting protein that activates mer operon transcription in the presence of the inducers Hg2+ and Cd2+. A glutathione-S-transferase-MerR fusion protein specifically bound and protected a 27-nucleotide operator sequence from DNase I digestion. This operator sequence is highly homologous with mer operator sequences of other known systems.

Regulation and expression of the arsenic resistance operon from Staphylococcus aureus plasmid pI258

The arsenic resistance operon from Staphylococcus aureus plasmid pI258 was cloned and sequenced. The DNA sequence contains three genes in the order arsR, arsB, and arsC. The predicted amino acid sequences of the gene products are homologous with those of the products of the ars operons of plasmids pSX267 from Staphylococcus xylosus and R773 from Escherichia coli. The cloned staphylococcal ars operon confers resistances to arsenate, arsenite, and antimonite in S. aureus and Bacillus subtilis. The same operon was also expressed in E. coli and conferred resistance to arsenite but less resistance to arsenate and antimonite. Regulation of the pI258 ars operon was studied by using a translational arsB-blaZ fusion in S. aureus and a transcriptional arsB-luxAB fusion in E. coli. The ars operon was induced by arsenate [As(V)], arsenite [As(III)], and antimonite [Sb(III)], to which the strains were resistant, plus Bi(III) in S. aureus. Only arsenate and arsenite induced the operon in E. coli. Northern (RNA) blot DNA-RNA hybridization analysis showed inducible synthesis of a full-length ars mRNA, about 2.1 kb in size, both in S. aureus and in E. coli. S. aureus ars proteins were expressed in E. coli from the T7 phage promoter under the control of the T7 RNA polymerase. Primer extension (reverse transcriptase) analysis showed that the ars mRNA started at the same position (nucleotides 17 and 18 upstream from the arsR ATG) both in S. aureus and in E. coli. An internal deletion mutation in arsB resulted in decreased resistance to arsenate and total loss of arsenite and antimonite resistances. Partial deletion of 56 bp from the 3' end of the arsC gene resulted in loss of resistance to arsenate; the determinant retained arsenite and antimonite resistances.

Regulation of the cadA cadmium resistance determinant of Staphylococcus aureus plasmid pI258

Regulation of the cadA cadmium and zinc resistance determinant of Staphylococcus aureus plasmid pI258 was demonstrated by using gene fusions and direct measurements of transcription. In growth experiments, cells harboring the intact cadA operon were induced with different cations and challenged by an inhibitory concentration of ZnCl2, a substrate of the CadA resistance system. Uninduced cells did not grow for 8 h after Zn2+ addition, whereas induced cells grew in the presence Zn2+. Cd2+ was a strong inducer, and Bi3+ and Pb2+ also induced well; Co2+ and Zn2+ were weak inducers. A translational beta-lactamase fusion to the cadA gene showed the same induction specificity as that seen with growth experiments with the intact cadA operon. A short beta-lactamase transcriptional fusion to the cadC gene also showed the same pattern of induction, establishing that the cadC gene was not involved in regulation. In Northern (RNA) blot hybridization experiments, a cadmium-inducible, 2.6-kb, operon-length transcript was detected. Primer extension experiments determined that Cd(2+)-inducible transcription of the cadA operon begins at nucleotides 676 and 677 of the published sequence (G. Nucifora, L. Chu, T. K. Misra, and S. Silver, Proc. Natl. Acad. Sci. USA 86: 3544-3548, 1989).

A second gene in the Staphylococcus aureus cadA cadmium resistance determinant of plasmid pI258

Two open reading frames on a 3.7-kb BglII-XbaI fragment which encodes the Staphylococcus aureus cadA cadmium (and zinc) resistance determinant of plasmid pI258 were identified (G. Nucifora, L. Chu, T. K. Misra, and S. Silver, Proc. Natl. Acad. Sci. USA 86:3544-3548, 1989). The [35S]methionine-labelled protein products of the 727-amino-acid CadA ATPase and of the 122-amino-acid CadC polypeptide in Escherichia coli were identified by using the T7 RNA polymerase-promoter expression system. A truncated CadA polypeptide (402 amino acids) did not confer resistance in S. aureus but was expressed in E. coli under control of the T7 RNA polymerase-promoter. Removal of 678 nucleotides from the 5' end of the published sequence (which includes the cadA promoter) abolished resistance to cadmium, whereas a 146-nucleotide-shorter deletion was without effect. The cadC gene is needed in addition to cadA for full resistance to cadmium in S. aureus and Bacillus subtilis. cadC functions both in cis and in trans.

Decreased resistance to antibiotics and plasmid loss in plasmid-carrying strains of Staphylococcus aureus treated with ascorbic acid

The effect of ascorbic acid on plasmid-coded antibiotic resistance in Staphylococcus aureus was investigated. Several strains of S. aureus were cultured in the presence of 1 mM ascorbate for 6 h. This treatment induced an increased loss of resistance markers in 4 of 6 strains tested, and agarose gel electrophoresis showed this disappearance of plasmid DNA in ascorbate-induced susceptible colonies. The presence of ascorbate induced a 50-75% decrease in minimal inhibitory concentrations of different antibiotics for resistant strains. When ascorbate is added, formerly subinhibitory concentrations of penicillin or tetracycline have an increased inhibitory effect on resistant strains and even induced the death of 25-93% of the initial population. These results suggest that ascorbate can induce the loss of several plasmids of S. aureus, and that the levels of antibiotic resistance are also affected by the presence of this compound.

Nucleotide sequence of the beta-lactamase gene from Enterococcus faecalis HH22 and its similarity to staphylococcal beta-lactamase genes

The nucleotide sequence of the constitutively produced beta-lactamase (Bla) gene from Enterococcus faecalis HH22 was shown to be identical to the published sequences of three of four staphylococcal type A beta-lactamase genes; more differences were seen with the genes for staphylococcal type C and D enzymes. One hundred forty nucleotides upstream of the beta-lactamase start codon were determined for an inducible staphylococcal beta-lactamase and were identical to those of the constitutively expressed enterococcal gene, indicating that the changes resulting in constitutive expression are not due to changes in the promoter or operator region. Moreover, complementation studies indicated that production of the enterococcal enzyme could be repressed. The genes for the enterococcal Bla and an inducible staphylococcal Bla were each cloned into a shuttle vector and transformed into enterococcal and staphylococcal recipients. The major difference between the backgrounds of the two hosts was that more enzyme was produced by the staphylococcal host, regardless of the source of the gene. The location of the enzyme was found to be host dependent, since each cloned gene generated extracellular (free) enzyme in the staphylococcus and cell-bound enzyme in the enterococcus. On the basis of the identities of the enterococcal Bla and several staphylococcal Bla sequences, these data suggest the recent spread of beta-lactamase to enterococci and also suggest the loss of a functional repressor.

Isolation, structural and functional characterization of Staphylococcus aureus protoplasts obtained using lysoamidase

The action of the lysoamidase bacteriolytic complex on Staphylococcus aureus VKM B-209P cells has been studied to obtain protoplasts. The cells in the midlogarithmic phase were the most sensitive to lysoamidase action. It led to local destruction of cell wall due to hydrolysis of the peptidoglycan. Protoplast formation occurred in two steps in the presence of 1 M sucrose. First, osmotically fragile spheroplasts were formed. Then, the protoplasts were released from the destructed cell wall. The protoplast yield was about 80%. The protoplasts preserved the intact ultrastructure and were able to synthesize peptidoglycan fibrillae. Mainly the spheroplasts that maintained the cell-wall residues reversed into bacterial forms. The protoplasts had respiratory activity similar to cells. Respiration of cells and protoplasts was stimulated by various substrates. High rates of oxygen consumption were observed with alpha-glycerophosphate and ethanol as substrates.

Conjugative transfer of cadmium resistance plasmids in Rhodococcus fascians strains

The presence of a 138-kilobase plasmid (pD188) correlated with increased resistance to cadmium in Rhodococcus fascians D188. This plasmid could be transferred by a conjugation-like system in matings between R. fascians strains. Transconjugants expressed the cadmium resistance and could be used as donors in subsequent matings. Four other R. fascians strains (NCPPB 1488, NCPPB 1675, NCPPB 2551, and ATCC 12974) could also be used as donors for cadmium resistance in matings. Strain NCPPB 1675 showed a 100% cotransfer of cadmium and chloramphenicol resistance markers.

Genetic transformation in Lactobacillus sp. strain 100-33 of the capacity to colonize the nonsecreting gastric epithelium in mice

Lactobacillus isolates able to colonize the surfaces of the nonsecreting epithelia in the stomachs of monoassociated ex-germfree mice were derived from Lactobacillus acidophilus 100-33. Strain 100-33 was originally isolated from pig feces and is unable to colonize the murine gastric epithelium. In experiments involving attempts genetically to transform the capacity to colonize the epithelium, cells of strain 100-33 were treated with muralytic enzymes and mixed with polyethylene glycol and genomic or plasmid DNA extracted from Lactobacillus fermentum RI. Strain RI was originally isolated from a conventional mouse and has the capacity to colonize the nonsecreting gastric epithelium. The mixtures containing cells, polyethylene glycol, and DNA were plated on a regeneration medium. After overnight incubation, the cells were washed from the plates and introduced by gastric gavage into germfree mice. Only mice that received regenerated 100-33 cells previously mixed with genomic DNA from strain RI had layers of gram-positive bacteria on the keratinized epithelia of their stomachs. Six isolates cultured from the washed gastric tissues of these animals were characterized. When a culture of each or a pool of cultures of the six were orally administered to germfree mice, layers of gram-positive bacterial cells were visible on the keratinized gastric epithelia of the animals within 1 to 3 weeks. Cells of all six, but not of strain 100-33, reacted with antibody made in rabbits to L. fermentum RI cells, as determined by an enzyme-linked immunosorbent assay. Nevertheless, all six had fermentation profiles identical to that of strain 100-33.(ABSTRACT TRUNCATED AT 250 WORDS)

The Streptomyces ghanaensis low copy plasmid pSG2 and its use for vector construction

A plasmid, pSG2, was isolated from Streptomyces ghanaensis and characterized by electron microscopy, buoyant density measurement, and restriction enzyme analysis. The length of 13.8 kb, single restriction sites for HindIII, EcoRV and PvuII and the possibility of deleting non-essential regions of the plasmid made pSG2 a suitable basic replicon for vector development. pSG2 has a copy number of about four. Plasmid pSG2 was fused to a pACYC184 derivative modified to harbour a thiostrepton resistance gene. The resulting plasmid, designated pSW1, is a 16.6 kb shuttle plasmid which replicates in Escherichia coli and in several Streptomyces strains, including S. ghanaensis, S. lividans and S. viridochromogenes. Replacement of a BglII-fragment of plasmid pSG2 by a fragment encoding thiostrepton resistance resulted in a low copy 12.2 kb Streptomyces plasmid. This plasmid, designated pSW2, is a Streptomyces broad host range plasmid.

Antimicrobial resistance of Staphylococcus aureus: genetic basis

Images

Plasmids of Staphylococcus aureus associated with live and processed poultry

A series of 23 strains of Staphylococcus aureus originally isolated from processed poultry was screened for the presence of plasmids. Plasmids were more common in strains of Staph. aureus characteristically associated with live poultry than with strains endemic in poultry plants and strains of human origin. Two plasmids with sizes of 1.65 and 18.2 kilobase pairs (kBp) were present in three strains considered typical of Staph. aureus forma specialis 'altilis' and two plasmids with sizes of 1.65 and 17 kBp were present in three of four strains of Staph. aureus var. gallinae. A 1.65 kBp plasmid was present in all seven strains of these poultry biotypes and in three of 14 'endemic' strains. All the 1.65 kBp plasmids were shown by blot hybridization to share sequence homology. There was also some sequence homology between the 18.2 kBp and 17 kBp plasmids. These results were supported by restriction enzyme digest analyses. A study of cured derivatives of strain PS221 f.sp. 'altilis' suggested that the 18.2 kBp plasmid encoded the genetic determinant(s) responsible for caseolysis. Both the 1.65 and the 18.2 kBp plasmids also exerted an effect on the production of acid from lactose. In no other characteristic did cured strains resemble the plasmid-free 'endemic' strains. This was therefore consistent with the notion that the genetic determinants associated with the cultural characteristics of endemic strains are chromosomally located.

Plasmid instability in regenerating protoplasts of Staphylococcus aureus is caused by aberrant cell division

Elimination of plasmids from regenerating S. aureus protoplasts occurred when the regeneration medium contained sucrose but not when it contained sodium succinate. This difference was caused by the occurrence of cell division prior to regeneration of the cell wall on sucrose but not on succinate. Coexisting compatible plasmids were cured independently; coexisting incompatible plasmids were cured jointly. These results support the hypothesis that plasmid pools exist as physically sequestered units in protoplasts and that curing is a consequence of the segregation of such units during abnormal division of wall-less organisms.

Protoplast transformation in coryneform bacteria and introduction of an alpha-amylase gene from Bacillus amyloliquefaciens into Brevibacterium lactofermentum

The goal of this study was to investigate the likelihood of developing useful transformation systems for coryneform bacteria. Two species of coryneform bacteria, Brevibacterium lactofermentum and Corynebacterium lilium, were transformed with chimeras constructed from pUB110 and a cryptic coryneform plasmid (pGX1901). C. lilium protoplasts were also efficiently transfected with phage CS1 DNA. High transformation and transfection frequencies were obtained after only 2 min of lysozyme treatment of lysozyme-sensitive mutants. A series of experiments was also conducted to determine whether DNA from other species of important industrial microbes from the genus Bacillus could be expressed in coryneform bacteria. Evidence of restriction of Bacillus subtilis DNA by B. lactofermentum was observed but could be overcome. A Bacillus amyloliquefaciens alpha-amylase gene (amyEBamP) was subcloned onto a plasmid able to replicate in B. lactofermentum. B. lactofermentum transformants for this plasmid expressed amylase activity and produced material cross-reactive to amylase antibody.

High frequency conjugal transfer of tylosin genes and amplifiable DNA in Streptomyces fradiae

Genes encoding enzymes for tylosin biosynthesis, genes involved in the expression of resistance to tylosin (Tyl), hygromycin B (Hm), chloramphenicol (Cm), and mitomycin C (MC), and a single copy of an amplifiable unit of DNA (AUD) were jointly transferred at very high frequencies by conjugation from several different Streptomyces fradiae strains to S. fradiae JS85, a mutant defective in many or possibly all tylosin biosynthetic reactions and containing a multiple tandem reiteration of the AUD. No recombination was observed between nar, rif and spc genes in conjugal matings, but recombination was observed between these genes after protoplast fusion. Tylosin biosynthetic genes were transferred at a much lower frequency to S. fradiae JS87, another mutant defective in many or all tylosin biosynthetic reactions, but deleted for the AUD and other DNA sequences. These findings suggest that tylosin structural genes, several genes encoding antibiotic resistance determinants, and amplifiable DNA are present on a self-transmissible element that does not mobilize chromosomal genes, and that JS85 and JS87 contain deletions, and JS85 an amplification, of overlapping portions of this element.

Virulence of Staphylococcus aureus in a mouse mastitis model: studies of alpha hemolysin, coagulase, and protein A as possible virulence determinants with protoplast fusion and gene cloning

Mutants of a genetically well-characterized strain of Staphylococcus aureus [SA113(83A)] were isolated after mutagenization. Alpha-hemolysin- (hla), coagulase- (coa), and protein A- (spa) negative mutants were characterized by more than 90 biochemical tests for production of extracellular proteins and biochemical profile to exclude pleiotropy. Protoplast fusion was then used to isolate double-defective (hla and coa) recombinants and recombinants with regained properties, i.e., production of alpha-hemolysin and coagulase. Studies of such mutants and recombinants in the mouse mastitis model showed that one alpha-hemolysin [SA113(83A) hla-5] and one coagulase-negative [SA113(83A) coa-147] mutant were lower in virulence compared with the wild-type strain SA113(83A). The double-negative mutant SA113(83A) hla-5 coa-147 showed a drastic decline in virulence and only induced very mild changes, as determined by microscopic examinations of infected mammary gland tissue. The recombinant with regained properties, however, was as virulent as the wild-type strain. This suggests that alpha-hemolysin and coagulase are virulence determinants of S. aureus. A high-level protein A-producing mutant (U300) showed the same virulence as the parent strain SA113(83A) in this model. One low virulence protein A-negative mutant (U320) did not markedly increase in virulence when a plasmid containing the cloned gene for protein A (pSPA15) was introduced into this mutant. By these and earlier observations, it seems likely that protein A is not an important virulence determinant in mastitis of mice. The reduced virulence of the protein A-negative mutant U320 compared with the wild-type SA113(83A) may be due to pleiotropic loss of some other unknown virulence determinant(s). Our data confirm earlier findings that pleiotropic changes are common in protein A-negative mutants.

Two plasmid-determined restriction and modification systems in Streptococcus lactis

Two restriction and modification systems were found in Streptococcus lactis strain IL594 which was found to contain 9 plasmids designated pIL1 to pIL9. On the basis of protoplast-induced curing experiments, we showed that a restriction and modification system was related to the presence of pIL6 or pIL7. The pIL6-determined restriction and modification system was confirmed by cotransfer of the plasmid and of the restriction and modification system to a plasmid-free, nonrestricting, and nonmodifying derivative of S. lactis IL594.

Recombination between short direct repeats in Streptomyces lavendulae plasmid DNA

Streptomyces lavendulae S985 carried two plasmids, pSL1 and pSL2. pSL2 contained all of the pSL1 sequences plus a tandem duplication of 900 base pairs from a region of pSL1. Sequence analysis of the duplication junction suggested that the duplication occurred by recombination between short direct repeats of as little as 5 base pairs.

Membrane-bound fractions of R6K plasmid DNA in Escherichia coli

The intracellular location of plasmid DNA has been of interest in an effort to understand the maintenance of these molecules. We have employed a simple procedure which enables us to isolate from exponentially grown cells on sucrose gradients membrane-complexed forms of R6K plasmid DNA. Electron micrographs identified the complexing of membrane fractions to circular forms of R6K DNA. Biochemical studies of the complexed R6K molecules showed the presence of membrane-specific proteins and suggested that complexing of R6K DNA was primarily with inner membrane fractions of Escherichia coli.

Analysis of plasmids in nosocomial strains of multiple-antibiotic-resistant Staphylococcus aureus

Nosocomial infections caused by Staphylococcus aureus strains resistant to methicillin and multiple antibiotics have reached epidemic proportions in Melbourne, Australia, over the past 5 years. Plasmid analysis of representative clinical isolates demonstrated the presence of three classes of plasmid DNA in most strains. Resistance to gentamicin, kanamycin, and tobramycin was usually mediated by an 18-megadalton plasmid but could also be encoded by a related 22-megadalton plasmid. Two distinguishable plasmids of 3 megadaltons each endowed resistance to chloramphenicol, and the third class consisted of small plasmids, each approximately 1 megadalton in size, with no attributable function. An extensive array of resistance determinants, including some which have usually been associated with a plasmid locus, were found to exist on the chromosome. Evidence that resistance to gentamicin, kanamycin, and tobramycin is chromosomally encoded in some clinical isolates suggests that this determinant may have undergone genetic translocation onto the staphylococcal chromosome.

Plasmid complements of Streptococcus lactis NCDO 712 and other lactic streptococci after protoplast-induced curing

The production and regeneration of bacterial protoplasts promoted the loss of three different plasmid-specified traits in Streptococcus lactis subsp. diacetylactis strains. The loss of five different plasmids, including small multicopy molecules, was readily detected in Streptococcus lactis 712 by screening lysates of random protoplast regenerants on agarose gels. In this strain sequential rounds of protoplast regeneration were used to produce a plasmid-free strain and derivatives carrying only single molecules from the plasmid complement. During these experiments a 33-megadalton plasmid, pLP712, was found to encode genes for lactose and protein utilization. Only this plasmid was required for normal growth and acid production in milk; the remaining four plasmids appeared to be cryptic. Lactose-defective derivatives of a strain carrying only pLP712 were readily isolated. Although these derivatives included instances of plasmid loss, deletions of pLP712 were frequently found. Many different deleted derivatives of pLP712, including some in which the lactose or protein utilization determinant or both were lost, were isolated. The molecular instability of pLP712 largely accounted for previous observations of plasmid complements in S. lactis 712 after lactose determinant curing or transfer by conjugation and transduction. Curing of cryptic molecules from multiple plasmid complements by protoplast regeneration may prove to be generally valuable in lactic streptococci and other gram-positive species.

Strategies for cultivation of recombinant organisms

An overview on the strategies for cultivation of recombinant organisms is presented in three sections, that is, the stability of plasmids, expression of cloned genes and an example of a genetically engineered microorganism.

Plasmid ColVBtrp maintenance in Erwinia carotovora

Plasmid ColVBtrp maintenance in Erwinia carotovora cells was followed by measuring kinetics of elimination of plasmid genetic markers and loss of plasmid deoxyribonucleic acid. An E. carotovora mutant stably carrying plasmid ColVBtrp was isolated. Besides stable plasmid maintenance, the mutant showed altered sensitivity to male-specific phage MS2, sensitivity to drugs, and colony morphology.

Plasmid-chromosomal transition of genes important in staphylococcal enterotoxin B expression

Experiments were performed to further elucidate the genetic mechanisms underlying the synthesis of staphylococcal enterotoxin B (SEB). Our laboratory has previously shown that, in strains of Staphylococcus aureus which harbor a 1.15-megadalton plasmid (pentB or pSN2), the plasmid appears to be required for SEB synthesis; in other S. aureus strains, designated chromosomal SEB producers, this 1.15-megadalton plasmid is conspicuously absent. We report here than in both Bacillus subtilis minicells and a coupled translational assay, pSN2 codes for a polypeptide of 18,000 daltons. This product is not immunologically reactive with purified anti-SEB globulin. Nevertheless, pSN2 is necessary but not sufficient for SEB synthesis in strains which harbor the plasmid. Further, the data provide a reasonable link between plasmid-bearing and chromosomal SEB producers: transformational analysis indicates that both require functions specified (in plasmid-bearing strains) by pSN2 for SEB synthesis. The combined genetic and biochemical data suggest that pSN2 is not the reservoir for the SEB structural gene, but that the pSN2-specific functions required for SEB synthesis are regulatory in nature.