Computer-assisted chromosome mapping by protoplast fusion in Staphylococcus aureus

Cross-genus Boot-up of Synthetic Bacteriophage in Staphylococcus aureus Using a New and Efficient DNA Transformation Method

Staphylococcus aureus is an opportunistic pathogen causing a wide range of infections and food poisoning in humans with antibiotic resistance, specifically to methicillin, compounding the problem. Bacteriophages (phages) provide an alternative treatment strategy, but only infect a limited number of circulating strains and may quickly become ineffective due to bacterial resistance. To overcome these obstacles, engineered phages have been proposed, but methods are needed for efficient transformation of large DNA molecules into S. aureus to boot-up (i.e., rescue) infectious phages. We present a new, efficient and reproducible DNA transformation method, NEST (Non-Electroporation Staphylococcus Transformation), for S. aureus to boot-up of purified phage genomic DNA (at least 150 kb in length tested) and whole yeast-assembled synthetic phage genomes. This method is a powerful new tool for transformation of DNA in S. aureus and will enable the rapid development of engineered therapeutic phages and phage cocktails against Gram-positive pathogens. Importance The continued emergence of antibiotic resistant bacterial pathogens has heightened the urgency for alternative antibacterial strategies. Phages provide an alternative treatment strategy, but are difficult to optimize. Synthetic biology approaches have been successfully used to construct and rescue genomes of model phages, but only in a limited number of highly transformable host species. In this study, we used a new, reproducible, and efficient transformation method to reconstitute a functional non-model Siphophage from a constructed synthetic genome. This method will facilitate not only the engineering of Staphylococcus and Enterococcus phages for therapeutic applications but also the engineering of Staphylococcus strains by enabling transformation of higher molecular weight DNA to introduce more complex modifications.

Construction of a chromosome map for the phage group II Staphylococcus aureus Ps55

The genome size and a partial physical and genetic map have been defined for the phage group II Staphylococcus aureus Ps55. The genome size was estimated to be 2,771 kb by pulsed-field gel electrophoresis (PFGE) using the restriction enzymes SmaI, CspI, and SgrAI. The Ps55 chromosome map was constructed by transduction of auxotrophic and cryptic transposon insertions, with known genetic and physical locations in S. aureus NCTC 8325, into the Ps55 background. PFGE and DNA hybridization analysis were used to detect the location of the transposon in Ps55. Ps55 restriction fragments were then ordered on the basis of genetic conservation between the two strains. Cloned DNA probes containing the lactose operon (lac) and genes encoding staphylococcal protein A (spa), gamma hemolysin (hlg), and coagulase (coa) were also located on the map by PFGE and hybridization analysis. This methodology enabled a direct comparison of chromosomal organization between NCTC 8325 and Ps55 strains. The chromosome size, gene order, and some of the restriction sites are conserved between the two phage group strains.

Adventures with Bacillus megaterium--fusion of bacterial protoplasts

This essay describes the author's studies with bacteria in post-war Hungary; the difficulties encountered, with funding, collaboration and publication; and how the Szeged Institute consolidated itself as the one outstanding scientific institute in Eastern Europe, with the author at the helm.

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.

Genetic approaches to the study of oral microflora: a review

As the study of oral microorganisms intensified almost 2 decades ago, the application of genetic techniques resulted in important contributions to the understanding of this clinically and ecologically important group of bacteria. The isolation and characterization of mutants of cariogenic streptococci helped to focus attention on traits that were important in colonization and virulence. Such classic genetic approaches gave way to molecular genetic techniques, including recombinant DNA methodology in the late 1970s. Gene cloning systems and methods to move DNA into cells have been developed for oral streptococci. Many streptococcal genes thought to be important in colonization and virulence have since been cloned and their nucleotide sequence determined. Mutant strains have been constructed using defective copies of cloned genes in order to create specific genetic lesions on the bacterial chromosome. By testing such mutants in animal models, a picture of the cellular and molecular basis of dental caries is beginning to emerge. These modern genetic methodologies also are being employed to develop novel and efficacious cell-free or whole cell vaccines against this infection. Genetic approaches and analyses are now being used to dissect microorganisms important in periodontal disease as well. Such systems should be able to exploit advances made in genetically manipulating related anaerobes, such as the intestinal Bacteroides. Gene cloning techniques in oral anaerobes, Actinomyces and Actinobacillus, are already beginning to pay dividends in helping understand gene structure and expression. Additional effort is needed to develop facile systems for genetic manipulation of these important groups of microorganisms.

Transformation of a conditionally peptidoglycan-deficient mutant of Staphylococcus aureus with plasmid DNA

A simple and reliable method for polyethylene glycol-induced plasmid transformation of a temperature-sensitive peptidoglycan-deficient mutant of Staphylococcus aureus is described. The procedure uses strains carrying the tofA372 mutation grown under conditions that yield osmotically fragile cells capable of efficient wall regeneration. The peptidoglycan-deficient cells were transformed with plasmids pE194 and pI258 at frequencies comparable with those obtained with protoplasts prepared with lysostaphin treatment. A readily portable tofA372 mutation was constructed by isolating an insertion of the erythromycin resistance transposon Tn551 adjacent to tofA372. tofA372 was shown by protoplast fusion and transformation analyses to be in the gene order hly-421-omega [Chr::Tn551]1059-tofA372-uraB232-omega [Chr::Tn916]1101-thrB106 on the chromosome of S. aureus NCTC 8325.

Genetic analysis of Staphylococcus aureus with Tn4001

Tn4001, a 4.5-kilobase composite transposon with IS256 ends that confers resistance to gentamicin (Gmr), tobramycin, and kanamycin in Staphylococcus aureus, can transpose to diverse chromosomal sites in S. aureus. Chromosomal insertions of Tn4001 were isolated either after UV irradiation of transducing lysates carrying pII147::Tn4001 or by selection for thermoresistant Gmr isolates with strains containing thermosensitive derivatives of plasmids pI258 and pII147 carrying Tn4001. Frequent integration of the entire delivery plasmid occurred under these selective conditions in recombination-proficient hosts. When selection for thermoresistant Gmr isolates was done with these plasmids in recombination-deficient hosts, 99% or more of the Gmr isolates resulted from transposition of Tn4001 in the absence of plasmid integration. Efficient isolation of Tn4001 insertions near markers of interest and the isolation of insertional auxotrophs were achieved. Reversion frequencies of insertional auxotrophs were between 10(-6) and 10(-7) (higher than those observed with Tn551 and Tn917). About 50% of the prototrophic revertants were Gms, and these are attributed to precise excision of Tn4001. The Gmr prototrophic revertants were due to intergenic suppression.

Association of toxic shock toxin-1 determinant with a heterologous insertion at multiple loci in the Staphylococcus aureus chromosome

Most Staphylococcus aureus strains associated with toxic shock syndrome and producing toxic shock syndrome toxin 1 (TSST-1) require tryptophan because of a genetic defect in tryptophan biosynthesis. The association between TSST-1 production and tryptophan auxotrophy was not correlated with the phage type, the colonization site, or the disease status of the patient from whom the isolate came. Protoplast fusion and transformation mapping located the genetic determinant of TSST-1 production (tst) very close to the trp operon in such strains and very close to tyrB in a Trp+ TSST-1+ strain. Southern blot hybridization of ClaI-restricted chromosomal DNA with a tst-specific probe revealed a common homologous segment in all of the Trp+ strains with tst linked to tyrB. These results confirmed that the tst determinant in Trp- strains is located at one site, whereas in Trp+ TSST-1+ strains the determinant is located elsewhere on the S. aureus chromosome. It is suggested that the TSST-1 determinant is associated with the insertion of a transposonlike segment into several sites on the S. aureus chromosome.

Sequential transposition of Tn916 among Staphylococcus aureus protoplasts

Transposition of the Streptococcus faecalis conjugal tetracycline-resistance transposon Tn916 between S. aureus strains occurred when protoplasts of donor and recipient strains were regenerated together without prior fusion. Under these conditions, only Tn916 was transferred; spontaneous fusion of parental protoplasts is therefore unlikely to be responsible for Tn916 transfer. While the exact nature of this transfer remains unclear, it appears to resemble Tn916 conjugal transposition reported in S. faecalis. Evidence for sequential transpositions of Tn916 was obtained by 3-factorial transformation analyses and confirmed by DNA-DNA hybridizations. The ability of Tn916 to transpose within S. aureus and occupy diverse chromosomal sites demonstrates the value of this transposon in genetic studies of S. aureus.

Identification of a Staphylococcus aureus transposon (Tn4291) that carries the methicillin resistance gene(s)

We isolated a transposon (Tn4291) that carries the resistance gene(s) for methicillin in a secondary insertion site on the penicillinase plasmid pI524. Transposition of Tn4291 into pI524 occurred during the transduction of the tetracycline resistance plasmid pSN1 from a methicillin-resistant donor into a recipient that carried the mec allele in the primary site on the chromosome. Insertion of Tn4291 caused extensive rearrangement of pI524 and resulted in the formation of a 27.9-kilobase-pair plasmid (pIT103) which coded for resistance to methicillin and cadmium, but not penicillin. Although resistance to methicillin and cadmium were always linked, Tn4291 was stably maintained only in the presence of a chromosomal mec allele, while in its absence the plasmid was unstable and transposition to the primary site occurred. Subsequently, a 20.1-kilobase-pair plasmid, pIT203, was formed which retained cadmium resistance and regained the ability to express beta-lactamase activity.

Cloning and characterization of the repressor gene of the Staphylococcus aureus lactose operon

The genes responsible for utilization of lactose in Staphylococcus aureus are organized as an inducible operon, with galactose 6-phosphate being the intracellular inducer. To clone the repressor gene of this operon, we constructed an integration vehicle carrying 1.9 kilobases (kb) of DNA sequences from a region upstream of the structural genes of the operon. Through integration and subsequent rescue of this plasmid, we were able to clone approximately 7 kb of staphylococcal chromosomal DNA. We have shown that the plasmid insert complemented lac constitutive mutants. This repressor activity was localized to a 1.8-kb DNA fragment and, through maxicell analysis, was shown to correlate with the presence of a polypeptide with an apparent molecular weight of 32,000. Furthermore, a region between the repressor gene and the other genes of the operon was identified which, when carried on multicopy plasmids, resulted in expression of the operon in the absence of any exogenous induction. This region may represent an operator-type element capable of titrating repressor molecules away from chromosomal operator, allowing transcription of the operon in the absence of induction.

Genetic mapping by means of protoplast fusion in Bacillus subtilis

A new mapping method involving protoplast fusion in Bacillus subtilis is described. Protoplasts from an isogenic standard marker strain containing purA and from a strain containing both purB and the marker, "x", to be mapped were fused with polyethylene glycol, and purA+ purB+ fusants were selected. After isolation of single colonies and determination of unselected markers, marker x was mapped between two standard markers. This method was fully applicable to PBS1-resistant strains (e.g., lyt strains). The results obtained by protoplast fusion, conventional transformation and/or lysed protoplast transformation indicated that a lyt strain, Ni15, contained two new autolysin-minus mutations (lyt-151 and lyt-152). The properties of lyt-15 are also discussed.

Transfer of the conjugal tetracycline resistance transposon Tn916 from Streptococcus faecalis to Staphylococcus aureus and identification of some insertion sites in the staphylococcal chromosome

The Streptococcus faecalis pheromone-dependent conjugative plasmid pAD1::Tn916 and the membrane filter-dependent conjugative plasmid pPD5::Tn916 were used to introduce Tn916 into Staphylococcus aureus by intergeneric protoplast fusions and intergeneric membrane-filter matings. In recombinants obtained by protoplast fusion where no plasmid DNA could be detected, tetracycline resistance resulted from transposition of Tn916 from pAD1 to the S. aureus chromosome. Transformation analyses showed that S. aureus Tn916 chromosomal insertions occurred near pig, ilv, uraA, tyrB, fus, ala, and the trp operon. DNA hybridization analyses of EcoRI- and HindIII-digested chromosomal DNAs confirmed the diversity of chromosomal sites involved and demonstrated that the inserts were Tn916 insertions rather than integrations of all or part of pAD1::Tn916. Both pAD1::Tn916 and pPD5::Tn916 were transferred to S. aureus by membrane-filter matings. These plasmids remained intact and expressed tetracycline resistance in S. aureus. S. aureus strains carrying pAD1::Tn916, but not a chromosomal insert of Tn916, and any one of several conjugal gentamicin-resistance plasmids lost their ability to serve as conjugal donors of the gentamicin-resistance plasmids.

Antimicrobial resistance of Staphylococcus aureus: genetic basis

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Characterization and genetic mapping of a mutation affecting apurinic endonuclease activity in Staphylococcus aureus

Protoplast fusion between the Rec- mutant RN981 (L. Wyman, R. V. Goering, and R. P. Novick, Genetics 76:681-702, 1974) of Staphylococcus aureus NCTC 8325 and a Rec+ NCTC 8325 derivative yielded Rec+ recombinants that exhibited the increased sensitivity to N-methyl-N'-nitro-N-nitrosoguanidine characteristic of RN981. Transformation analyses identified a specific mutation, designated ngr-374, that was responsible not only for N-methyl-N'-nitro-N-nitrosoguanidine sensitivity, but also sensitivity to methyl methanesulfonate, ethyl methanesulfonate, nitrous acid, and UV irradiation. However, ngr-374-carrying recombinants showed no significant increase in their sensitivity to mitomycin C or 4-nitroquinoline 1-oxide and were unaffected in recombination proficiency. In vitro assays showed that ngr-374-carrying strains had lower apurinic/apyrimidinic endonuclease activities than the wild type. The chromosomal locus occupied by ngr-374 was shown to exist in the gene order omega(Chr::Tn551)40-ngr-374-thrB106.

Chromosomal map location of the alpha-hemolysin structural gene in Staphylococcus aureus NCTC 8325

The alpha-hemolysin structural gene, hly+, previously cloned, insertionally inactivated, and introduced into the chromosome by allele replacement (M.O. O'Reilly, J.C.S. De Azavedo, S. Kennedy, and T.J. Foster, Microb. Pathog. 1:125-138, 1986), was shown by protoplast fusion and transformation to be in the gene order purC-hly-uraB-omega[chr::Tn916]1101-thrB on the chromosome of Staphylococcus aureus NCTC 8325. This location is clearly distinct from that of the agr determinant, a regulatory gene affecting several extracellular proteins, including alpha-hemolysin, located between tmn and ilv.

Cloning and expression of the phospho-beta-galactosidase gene of Staphylococcus aureus in Escherichia coli

The phospho-beta-galactosidase gene of Staphylococcus aureus was cloned in Escherichia coli. This was done by first isolating a staphylococcal transposon Tn551-induced mutant which rendered phospho-beta-galactosidase synthesis partially constitutive because of an insertion nearby this lac structural gene. This allowed selection in E. coli of chimeric plasmids which expressed the erythromycin resistance determinant of Tn551. A 26-kilobase (kb) BamHI insert in plasmid pBR322 was isolated which encoded phospho-beta-galactosidase, as determined by phospho-beta-galactosidase activity measurements. Maxicell experiments showed the presence of 56-, 13.5-, and 31-kilodalton proteins encoded by the staphylococcal DNA. The presence of the 56-kilodalton protein correlated with phospho-beta-galactosidase activity and corresponded in molecular weight to the reported value for the purified enzyme. The nature of the other proteins is unknown. Phospho-beta-galactosidase was apparently expressed in E. coli by a promoter contained within a 2.1-kb EcoRI chromosomal DNA fragment. This fragment, when inserted into a chloramphenicol acetyl transferase promoter detection plasmid, was transcriptionally active in both E. coli and Bacillus subtilis but was much more active in the latter host.

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.

Genetics of multiply-resistant Staphylococcus aureus

A substantial portion of recently isolated, multiply-resistant Staphylococcus aureus was shown to carry R-determinants in plasmids. Some of these plasmids were small (3 Mdal) and carried only one R-marker. Others were 18-36 Mdal in size and carried two or more R-determinants. Several of the larger plasmids could be transferred by a conjugation-like process. The location of R-markers on transposable DNA sequences also was observed. Transposition as well as stable integration of R-plasmids into the chromosome can explain the frequent observation of chromosomal location of resistance. Chromosomal resistance might be an advantage for an organism frequently exposed to antibiotics. Molecular evidence suggests that methicillin resistance resides on additional chromosomal DNA. The organization of staphylococcal genomes as well as efficient transfer processes explain the genetic versatility of Staph. aureus, which has resulted in the development of multiply-resistant strains.

Genetic analysis of Bacillus stearothermophilus by protoplast fusion

Efficient and reliable protoplasting, regeneration, and fusion techniques were established for the prototrophic strain Bacillus stearothermophilus NUB36. Auxotrophic mutants were isolated, and protoplast fusion was used to construct isogenic mutant strains and for chromosomal mapping. Markers were mapped using two-, three-, and four-factor crosses. The order of the markers was hom-1-thr-1-his-1-(gly-1 or gly-2)-pur-1-pur-2. These markers may be analogous to hom, thrA, hisA, glyC, and purA markers on the Bacillus subtilis chromosome. No analogous pur-1 marker has been reported in B. subtilis. The relative order of three of the markers (hom-1-thr-1-gly-1) was independently confirmed by transduction.

Differential utilization of Staphylococcus aureus promoter sequences by Escherichia coli and Bacillus subtilis

Promoter-cloning plasmids were constructed and have been used to isolate transcriptionally active DNA fragments from Staphylococcus aureus. The plasmids contain a chloramphenicol acetyltransferase (CAT) gene of Gram-positive (G+) origin which lacks both its promoter and the sequence responsible for CAT inducibility. The ability of S. aureus promoters to direct CAT expression in Escherichia coli and Bacillus subtilis was examined. Two classes of staphylococcal promoter sequences have been obtained. Class I DNA fragments direct CAT expression in S. aureus, B. subtilis, and E. coli, while class II DNA sequences direct CAT expression only in the G+ hosts.

Staphylococcal enterotoxin A is encoded by phage

The gene for staphylococcal enterotoxin A (entA), in two wild-type strains, is carried by related temperate bacteriophages. Hybridization analysis of DNA from entA-converting phage PS42-D and its bacterial host suggests that this phage integrates into the bacterial chromosome by circularization and reciprocal crossover (the Campbell model) and that the entA gene is located near the phage attachment site. DNA from three of eight staphylococcal strains that did not produce enterotoxin A and seven wild-type enterotoxin A-producing (EntA+) strains had extensive homology to the entA-converting phage PS42-D DNA, although there was a high degree of restriction-fragment length polymorphisms. At least one EntA+ strain did not produce detectable viable phage after induction. These data indicate that a polymorphic family of Staphylococcus aureus phages (some of which may be defective) can carry the entA gene.

Transformation and fusion of Streptococcus faecalis protoplasts

Nonconjugative plasmids were transferred by protoplast fusion among Streptococcus faecalis strains and from Streptococcus sanguis to S. faecalis. S. faecalis protoplasts were also transformed with several different plasmids, including the Tn917 delivery vehicle pTV1. Transformation was reproducible, but low in frequency (10(-6) transformants per viable protoplast). A new shuttle vector (pAM610), able to replicate in Escherichia coli and S. faecalis, was constructed and transformed into S. faecalis protoplasts. pAM610 was mobilized by the conjugative plasmid pAM beta 1 in matings among S. faecalis strains and from S. sanguis to S. faecalis. Chimeric derivatives of pAM610 were also transformed into S. faecalis.

Studies on transfection and transformation of protoplasts of Bacillus larvae, Bacillus subtilis, and Bacillus popilliae

Protoplasts of Bacillus larvae NRRL b-3555 and Bacillus subtilis RM125 (restrictionless, modificationless mutant) were transfected with DNA from the B. larvae bacteriophage PBL1c in the presence of polyethylene glycol. B. subtilis 168 and Bacillus popilliae NRRL B-2309M protoplasts could not be transfected with PBL1c DNA. Protoplasts of B larvae NRRL B-3555 were transformed with plasmids pC194 and pHV33 in the presence of polyethylene glycol. The frequency of transformation was much higher when the plasmids were isolated from B. larvae NRRL B-3555 transformants than when they were isolated from B. subtilis 168. These results indicate that the restriction-modification systems found in B. larvae NRRL B-3555 and B. subtilis 168 may be different. Conditions for protoplast formation and cell wall regeneration were developed for B. popilliae NRRL B-2309S. However, no transformation occurred with plasmids pC194 and pHV33 (isolated from B. subtilis 168).

Isolation of transposon Tn551 insertions near chromosomal markers of interest in Staphylococcus aureus

A procedure was developed to isolate insertions of transposon Tn551 near other markers of interest on the chromosome of Staphylococcus aureus NCTC 8325. When an inoculum of strain 8325-4 carrying a thermosensitive mutant of plasmid pI258 (on which Tn551 resides) was inoculated into brain heart infusion agar plus erythromycin and grown to saturation at 43 degrees C, the transforming DNA extracted from this population of cells contained a random collection of different chromosomal insertions of Tn551; this DNA is referred to as pooled Tn551 DNA. When erythromycin-sensitive recipient strains containing chromosomal markers of interest were transformed with pooled Tn551 DNA, and the resulting Emr transformants were screened for coinheritance of the donor allele of the marker of interest, insertions of Tn551 were isolated near several markers, including fus-149, tet-3490, mec-4916, pig-131, ilv-129, pur-140, and uraA141. Many of the insertions were within the linkage groups that contained these markers, and several insertions occupied different positions between the linkage groups in heretofore undefined regions of the circular chromosomal map of S. aureus. These insertions of transposon Tn551 extend the known limits of the existing linkage groups, provide linkage data and map locations for markers not previously mapped, and provide a means to map markers which cannot be directly selected.

Construction of a cloning site near one end of Tn917 into which foreign DNA may be inserted without affecting transposition in Bacillus subtilis or expression of the transposon-borne erm gene

A 1.3-kb restriction fragment carrying a cat gene derived from Staphylococcus aureus was inserted by ligation in both possible orientations into a HpaI restriction site located less than 300 bp from one end of Tn917. The resulting transposon derivatives were unimpaired in their ability to make and resolve transpositions into the chromosome of Bacillus subtilis and they displayed no detectable defect in expression of the inducible erm gene carried by the transposon. This demonstrates that the HpaI site itself, and perhaps the entire 250- to 300-bp region between the HpaI site and the nearest transposon terminal inverted repeat consists of nonessential DNA, and is there fore available to be modified or used as a cloning site with the expectation that the resulting transposon derivatives should be capable of normal transposition activity. To facilitate such manipulations, the HpaI site was "replaced" by a 24-bp DNA segment which contains a BamHI site flanked on either side by SmaI sites; these BamHI and SmaI sites are unique to the transposon. Several of the plasmid constructions undertaken in the course of this work illustrate ways in which homologous recombination may be used in conjunction with ligation in B. subtilis (and other bacteria, such as Streptococcus pneumoniae, which have similar mechanisms for DNA uptake during competence) to facilitate significantly the recovery of certain kinds of recombinant molecules.

Transduction analysis of transposon Tn551 insertions in the trp-thy region of the Staphylococcus aureus chromosome

Previous studies have shown that Tn551, a 5.2-kilobase-pair transposon that determines constitutive resistance to erythromycin, can occupy a variety of chromosomal sites between thy-101 and trp-103 in Staphylococcus aureus 8325. Although many of these insertions were "silent," many others, including lys, thr, met, tyr, and trp, resulted in auxotrophic mutations. The close proximity and erythromycin-resistant phenotypes of the insertions in this region have made their mapping by transformation difficult. Analysis of these sites and similar chemically induced mutations by generalized transduction with phage 80 alpha have defined the order and relationship of these insertion sites and provided a detailed map of this region of the chromosome, including the orientation of the trp operon. The results of this study and a limited phenotypic characterization of the mutants have shown that the divergent pathway from aspartate to lysine, threonine, and methionine, several reactions in tyrosine biosynthesis, and the entire tryptophan operon are determined by this region of the chromosome. The linkage results obtained by transduction have been compared with similar data obtained previously by transformation; this comparison suggests the existence, between thy and lys, of a preferred headful cutting site for transducing phage DNA morphogenesis from the host chromosome.

Mutagenesis of extrachromosomal genetic determinants for exfoliative toxin B and bacteriocin R1 synthesis in Staphylococcus aureus after plasmid transfer by protoplast fusion

In previous studies, we have shown that a 27-megadalton plasmid (pRW002) in Staphylococcus aureus contains genetic determinants for exfoliative toxin B (ET B) and bacteriocin (Bac R1) synthesis and Bac R1 resistance. Attempts to transform or transduce this plasmid to S. aureus or Bacillus subtilis recipients were not successful. However, genetic transfer of the plasmid was possible after polyethylene glycol-induced fusion of S. aureus protoplasts containing pRW002 and S. aureus protoplasts lacking this plasmid. Some of the resulting fusants lost the ability to make ET B, Bac R1, or both products. Fusants that were Bac R1-, Bac R1s, ET B- all lacked the 27-megadalton pRW002 plasmid. The largest class of fusants was Bac R1+, Bac R1r, ET B-. Immunodiffusion analyses of ET B extracts from 28 fusants showed that four ET B+ strains were cross-reacting mutants that produced ET B protein that was serologically related to, but not identical to, the wild-type toxin. Results indicated that genetic transfer of pRW002 after protoplast fusion induced molecular rearrangements that resulted in mutation of the genetic determinants for ET B and Bac R1 synthesis. Recombination of chromosomal genes was enhanced after CaCl2 was added to the protoplast-fusion mixture.

Confirmation of protoplast fusion-derived linkages in Staphylococcus aureus by transformation with protoplast DNA

Transformation provided definitive evidence for linkage between tyrB282::Tn551 ermB321 and omega (Chr::Tn551)34, and thus between the separate large linkage groups containing these markers, in Staphylococcus aureus NCTC 8325. Transformation also defined the chromosomal loci for the purC193::Tn551 and omega (Chr::Tn551)42 markers and the linkage of a tetracycline resistance marker (tet-3490) with a fusidic acid resistance marker (fus-149). The use of DNA isolated from protoplasts under conditions that reduced hydrodynamic shear greatly facilitated the demonstration of most of these linkages. These results provide direct evidence confirming several of the linkages predicted by a microcomputer-assisted protoplast fusion analysis in a previous study (M. L. Stahl and P. A. Pattee, J. Bacteriol. 154:395-405, 1983); those markers whose predicted linkages were not confirmed by transformation are probably separated by chromosomal distances that exceed the limits of detection by transformation, even with protoplast DNA.