A physical and functional analysis of Tn917, a Streptococcus transposon in the Tn3 family that functions in Bacillus

A global genomic perspective on the multidrug-resistant Streptococcus pneumoniae 15A-CC63 sub-lineage following pneumococcal conjugate vaccine introduction

The introduction of pneumococcal conjugate vaccines (PCV7, PCV10, PCV13) around the world has proved successful in preventing invasive pneumococcal disease. However, immunization against Streptococcus pneumoniae has led to serotype replacement by non-vaccine serotypes, including serotype 15A. Clonal complex 63 (CC63) is associated with many serotypes and has been reported in association with 15A after introduction of PCVs. A total of 865 CC63 isolates were included in this study, from the USA (n=391) and a global collection (n=474) from 1998-2019 and 1995-2018, respectively. We analysed the genomic sequences to identify serotypes and penicillin-binding protein (PBP) genes 1A, 2B and 2X, and other resistance determinants, to predict minimum inhibitory concentrations (MICs) against penicillin, erythromycin, clindamycin, co-trimoxazole and tetracycline. We conducted phylogenetic and spatiotemporal analyses to understand the evolutionary history of the 15A-CC63 sub-lineage. Overall, most (89.5 %, n=247) pre-PCV isolates in the CC63 cluster belonged to serotype 14, with 15A representing 6.5 % of isolates. Conversely, serotype 14 isolates represented 28.2 % of post-PCV CC63 isolates (n=618), whilst serotype 15A isolates represented 65.4 %. Dating of the CC63 lineage determined the most recent common ancestor emerged in the 1980s, suggesting the 15A-CC63 sub-lineage emerged from its closest serotype 14 ancestor prior to the development of pneumococcal vaccines. This sub-lineage was predominant in the USA, Israel and China. Multidrug resistance (to three or more drug classes) was widespread among isolates in this sub-lineage. We show that the CC63 lineage is globally distributed and most of the isolates are penicillin non-susceptible, and thus should be monitored.

Exploiting transposons in the study of Staphylococcus aureus pathogenesis and virulence

The opportunistic pathogen Staphylococcus aureus has an extremely complex relationship with humans. While the bacteria can exist as a commensal in many, it can cause a wide range of diseases and infections when turned pathogenic. Its presence is a determinant of chronicity and poor prognosis in numerous diseases, and its genomic plasticity causes S. aureus antimicrobial resistance to be one of the most dire contemporary medical problems to solve. Genetic manipulation of S. aureus has led to numerous findings that are vital in the fight against its pathogenesis. The utilisation of transposon mutant libraries for the systematic inspection of the S. aureus genome led to many landmark discoveries pertaining to the bacteria's pathogenicity, antimicrobial resistance acquisition, and virulence regulation. In this review, we describe mutant libraries, and their significant contributions, from various S. aureus strains created with commonly used transposons. The general workflow for the construction of libraries will be presented, along with a discussion of the challenges of undertaking the task of large-scale library construction. As the accessibility of transposon mutant library construction, screening, and analysis increases, this genetic tool could be further exploited in the study of the S. aureus genome.

Enterococcal Genetics

The study of the genetics of enterococci has focused heavily on mobile genetic elements present in these organisms, the complex regulatory circuits used to control their mobility, and the antibiotic resistance genes they frequently carry. Recently, more focus has been placed on the regulation of genes involved in the virulence of the opportunistic pathogenic species Enterococcus faecalis and Enterococcus faecium. Little information is available concerning fundamental aspects of DNA replication, partition, and division; this article begins with a brief overview of what little is known about these issues, primarily by comparison with better-studied model organisms. A variety of transcriptional and posttranscriptional mechanisms of regulation of gene expression are then discussed, including a section on the genetics and regulation of vancomycin resistance in enterococci. The article then provides extensive coverage of the pheromone-responsive conjugation plasmids, including sections on regulation of the pheromone response, the conjugative apparatus, and replication and stable inheritance. The article then focuses on conjugative transposons, now referred to as integrated, conjugative elements, or ICEs, and concludes with several smaller sections covering emerging areas of interest concerning the enterococcal mobilome, including nonpheromone plasmids of particular interest, toxin-antitoxin systems, pathogenicity islands, bacteriophages, and genome defense.

Insertion of Transposon Tn917 Derivatives into the Lactococcus lactis subsp. lactis Chromosome

Two transposition vectors, pTV32 and pLTV1, containing transposon Tn917 derivatives TV32 and LTV1, respectively, were introduced into Lactococcus lactis subsp. lactis MG1614. It was found that pTV32 and pLTV1 replicate and that TV32 and LTV1 transpose in this strain. A protocol for production of a collection of Tn917 insertions in L. lactis subsp. lactis was developed. The physical locations of TV32 on the chromosomal SmaI fragments of 62 independent transpositions were established by pulsed-field gel electrophoresis. These transpositions could be divided into at least 38 different groups that exhibited no Tn917-dominating hot spots on the L. lactis subsp. lactis chromosome. A total of 10 of the 62 transpositions resulted in strains that express beta-galactosidase. This indicates that there was fusion of the promoterless lacZ of the Tn917 derivatives to a chromosomal promoter. Thus, the Tn917-derived transposons should be powerful genetic tools for studying L. lactis subsp. lactis.

Construction of an Enterococcus faecalis Tn917-mediated-gene-disruption library offers insight into Tn917 insertion patterns

Sequencing the insertion sites of 8,865 Tn917 insertions in Enterococcus faecalis strain OG1RF identified a hot spot in the replication terminus region corresponding to 6% of the genome where 65% of the transposons had inserted. In E. faecalis, Tn917 preferentially inserted at a 29-bp consensus sequence centered on TATAA, a 5-bp sequence that is duplicated during insertion. The regional insertion site preference at the chromosome terminus was not observed in another low-G+C gram-positive bacterium, Listeria monocytogenes, although the consensus insertion sequence was the same. The 8,865 Tn917 insertion sites sequenced in E. faecalis corresponded to only approximately 610 different open reading frames, far fewer than the predicted number of 2,400, assuming random insertion. There was no significant preference in orientation of the Tn917 insertions with either transcription or replication. Even though OG1RF has a smaller genome than strain V583 (2.8 Mb versus 3.2 Mb), the only E. faecalis strain whose sequence is in the public domain, over 10% of the Tn917 insertions appear to be in a OG1RF-specific sequence, suggesting that there are significant genomic differences among E. faecalis strains.

Susceptibilities of oral and nasal isolates of Streptococcus mitis and Streptococcus oralis to macrolides and PCR detection of resistance genes

The susceptibility of viridans group streptococci to macrolides was determined. Thirteen isolates (17%) were resistant to erythromycin. Five strains carried an erm gene that was highly homologous to that in Tn917. Four strains had mefE genes that coded erythromycin efflux ability.

The Staphylococcus aureus transposon Tn551: complete nucleotide sequence and transcriptional analysis of the expression of the erythromycin resistance gene

The complete nucleotide sequence of Staphylococcus aureus transposon Tn551 was determined. The 5,266-bp sequence encoded five putative proteins. Comparison of the nucleotide sequence of Tn551 with that of the enterococcal transposon Tn917 showed that the two transposons were 99.8% identical and differed only at 11 positions along the entire sequence. The genetic organization of Tn551 was also identical to that of Tn917. Northern analysis of RNA prepared from a staphylococcal strain bearing Tn551 displayed three erm-associated transcripts that were constitutively produced. Mapping of the 5' ends of the transcripts by primer extension suggested that the constitutive transcription of erm was initiated from a nucleotide located 5 bp downstream of ORF1. A second set of three erythromycin-inducible transcripts was also detected and these showed a pattern similar to that described for Tn917. A simple and rapid method is described for the use of the Tn551 sequence information in sequencing transposon-inactivated staphylococcal genes.

Molecular characterization and transcriptional analysis of type 8 capsule genes in Staphylococcus aureus

A 20.5-kb contiguous DNA fragment from Staphylococcus aureus Becker affecting type 8 capsule (CP8) biosynthesis was previously cloned. Sequencing analysis indicated that 16 open reading frames (ORFs) encoded within this fragment might be involved in CP8 synthesis. Using various plasmids containing DNA inserts derived from the 20.5-kb region, we showed by complementation of chemical mutants that 8 of the 16 ORFs were required for CP8 synthesis. To determine the involvement of the remaining eight ORFs, nonpolar gene-specific chromosomal mutations located in each of these ORFs were constructed. We found that three additional ORFs were also involved in the CP8 synthesis. Thus, 11 of the 16 ORFs were shown to affect CP8 synthesis. Complementation analyses of these 11 type 8 capsule (cap8) genes affecting CP8 production showed several promoters within the cap8 gene cluster. However, by Northern hybridization using either the entire cap8 gene cluster or the internal fragments of individual ORFs as probes, one 17-kb cap8-specific transcript was detected. Using xylE as the reporter gene, we found that the promoter at the beginning of the cap8 operon was much stronger than any of the internal promoters. These results suggest that the cap8 genes are transcribed mainly as a single large transcript. In addition, Southern hybridization analyses showed that cap8H, cap8I, cap8J, and cap8K, located in the central region of the cap8 gene cluster, were CP8 specific.

Use of Tn917 to generate insertion mutations in the group A streptococcus

The Enterococcus faecalis transposon Tn917 is functional in a broad range of bacteria, including both Gram-positive and Gram-negative species. We cloned Tn917-LTV3, a derivative carrying a promoterless lacZ (beta-galactosidase gene), into the thermosensitive shuttle replicon pG+host4 and assayed for chromosomal insertions in group A streptococcus (GAS). Tn917 transposed into the GAS chromosome at a frequency of (2.8 +/- 3.2) x 10(-5) per colony forming unit (cfu). Transposition products were predominantly simple insertions and no target site preference was detectable. Some transcriptional fusions were identified in which the promoterless lacZ of the transposon appeared to be expressed from an external promoter.

Construction of Tn917ac1, a transposon useful for mutagenesis and cloning of Bacillus subtilis genes

A Tn917 derivative was constructed for the purposes of mutagenesis and cloning of Bacillus subtilis genes. This transposon, Tn917ac1 (4.6 kb), consisted of terminal inverted repeats of Tn917, the res sequence, a ColE1 origin of replication (ori) and two drug-resistance genes. The plasmid carrying this transposon, named pD917, contained the erm-tnpR-tnpA gene cluster of Tn917 and a temperature-sensitive ori of pE194. For the purpose of mutagenesis, transposition of Tn917ac1 was induced by culturing strains harboring pD917 in a medium containing a low concentration of erythromycin. Cells with a Tn917ac1 insertion in the chromosome were selected on agar containing chloramphenicol after heat treatment to eliminate the plasmidic form of pD917. DNA fragments adjacent to Tn917ac1 could be cloned by restriction digestion of the chromosomal DNA and by transforming the self-ligated restriction fragments into Escherichia coli. Sequence analysis revealed that Tn917ac1 was integrated into the chromosome of B. subtilis by transposition in a recE strain and by transposition or integration of pD917 in a wild-type strain. Tn917ac1 has been demonstrated to be useful for mutating and cloning of the genes involved in the biosynthesis of fengycin in B. subtilis F29-3.

Isolation of Tn917 insertional mutants of Bacillus subtilis that are resistant to the protonophore carbonyl cyanide m-chlorophenylhydrazone

Tn917 transposition libraries prepared from Bacillus subtilis were screened for mutants that had insertions in the chromosome resulting in resistance to the protonophore carbonylcyanide m-chlorophenylhydrazone (CCCP). Five such strains were characterized. Three of these were found to have distinct insertion sites that resulted in changes in fatty acid composition of the membrane lipids. The lipid changes were qualitatively similar to changes observed earlier in CCCP-resistant strains of B. subtilis that had been isolated after chemical mutagenesis. However, the extent of the changes was more modest, correlating with a lower level of protonophore-resistance. One of these mutants was disrupted in a gene homologous to the Escherichia coli rho gene, as reported earlier (Quirk et al. (1993) J. Bacteriol. 175, 647-654), one was disrupted in a new member of the two-component signalling systems, and the third was disrupted in a new gene of unknown function that apparently forms an operon with transporter genes. The other two CCCP-resistant mutants were disrupted in genes that are likely to encode membrane transporters; the disruption of these genes may have reduced the transmembrane ion leaks during growth, thus conferring modest protonophore-resistance. In one of these strains, the disrupted gene is part of an apparent operon that is a homologue of iron uptake operons from other prokaryotes.

Genetic analysis of the marine manganese-oxidizing Bacillus sp. strain SG-1: protoplast transformation, Tn917 mutagenesis, and identification of chromosomal loci involved in manganese oxidation

Mature spores of the marine Bacillus sp. strain SG-1 bind and oxidize manganese(II), thereby becoming encrusted with a manganese(IV) oxide. Both the function and mechanism of this oxidation are unknown, although evidence suggests that spore coat proteins are involved. To further study this phenomenon, methods of genetic analysis were developed for SG-1. By a modified protoplast transformation procedure, SG-1 was transformed (approximately 100 transformants per micrograms of DNA) with several different plasmids of gram-positive origin. Transposon Tn917, delivered on the temperature-sensitive plasmid pLTV1, was used to generate mutants of SG-1. Conditions were established that allowed 98% plasmid loss and insertions to be recovered at a frequency of 10(-3). Each mutant was found to be the result of a single insertion event. Restriction analysis of 27 mutants that do not oxidize manganese but still sporulate localized 17 of the insertions within two regions of the chromosome (termed Mnx regions), and a physical map of these regions was generated. Analysis of 18 transposon integrants in which manganese oxidation was unaffected revealed random transposon integration, with none of their insertions mapping within the Mnx regions. The Mnx regions were cloned from wild-type SG-1, and the largest region, carried on the lactococcal plasmid pGK13, was used to complement in trans one of the nonoxidizing mutants. These results demonstrate that the Mnx regions encode factors that are required for the oxidation of manganese, and this represents the first report identifying genes involved in bacterial manganese oxidation.

Identification of a putative Bacillus subtilis rho gene

Transposon Tn917 mutagenesis of Bacillus subtilis BD99 followed by selection for protonophore resistance led to the isolation of strain MS119, which contained a single Tn917 insertion in an open reading frame whose deduced amino acid sequence was 56.6% identical to that of the Escherichia coli rho gene product. The insertional site was near the beginning of the open reading frame, which was located in a region of the B. subtilis chromosome near the spoOF gene; new sequence data for several open reading frames surrounding the putative rho gene are presented. The predicted B. subtilis Rho protein would have 427 amino acids and a molecular weight of 48,628. The growth of the mutant strain was less than that of the wild type on defined medium at 30 degrees C. On yeast extract-supplemented medium, the growth of MS119 was comparable to that of the wild type on defined medium at 30 degrees C. On yeast extract-supplemented medium, the growth of MS119 was comparable to that of the wild type at 30 degrees C but was much slower at lower temperatures; sporulation occurred and competence was developed in cells of the mutant grown at 30 degrees C. To determine whether the protonophore resistance and sensitivity to low growth temperature resulted from the insertion, a chloramphenicol resistance cassette was inserted into the wild-type B. subtilis rho gene of strain BD170; the resulting derivative displayed the same phenotype as MS119.

A truncated Tn916-like element in a clinical isolate of Enterococcus faecium

A 58.7-kb nonconjugative plasmid (pKQ1) previously reported in a clinical isolate of Enterococcus faecium was found to contain both a tetM and an erythromycin resistance (erm) determinant. The plasmid contained a region homologous to the A, F, H, and G HincII fragments of Tn916. However, the 4.8-kb B fragment of Tn916 which contained the tetM determinant was replaced by a 7.3-kb fragment, and the 3.6-kb HincII C fragment of Tn916 was missing. An element homologous to Tn917 was juxtaposed to the truncated Tn916-like element. The Tn917-like element was similar in size to the erm transposon Tn917 as determined by a ClaI restriction digest which spanned approximately 99% of the transposon. When Bacillus subtilis or Streptococcus sanguis were transformed with pKQ1, no zygotically induced transposition of the tetM element was detected. Similarly no transposition of the Tn917-like element was detected.

Functional analysis of the two domains in the terminal inverted repeat sequence required for transposition of Tn3

Bacterial transposon Tn3 has a 38-bp terminal inverted repeat (IR) sequence. The IR sequence has been divided into two domains, A and B, of which domain B is bound by transposase, and domain A is not Here, we defined the two domains more precisely by constructing three IR mutants with a 2-bp substitution at relevant sites within the IR sequence, followed by examination of the binding of transposase to the fragments containing these IR mutants: domain A was located at bp 1-11, whereas domain B was at bp 12-38. To see if the two domains in the IR are functionally distinct, we constructed mini-Tn3 derivatives flanked by two IRs with various 2-bp substitutions within domain A or B, and analyzed their ability to mediate cointegration. The mini-Tn3 derivatives flanked by IR(A+ B+) and IR(A- B+) [or IR(A+ B-)] and those flanked by IR(A-B+) and IR(A+ B-) mediate cointegration more efficiently than the mini-Tn3 derivatives flanked by two IR(A- B+)s or by two IR(A+ B-)s. These results and others presented here indicate that the two domains of IR are functionally distinct in promoting cointegration.

Use of a Plasmid DNA Probe To Monitor Populations of Bacillus pumilus Inoculant Strains in Hay

We are evaluating naturally occurring isolates of Bacillus pumilus for use as microbial hay preservatives. Seven isolates of B. pumilus from hay contained a 42-kb cryptic plasmid (pMGD296). We wished to determine whether pMGD296 could be used as a molecular marker to follow populations of these isolates in hay over time. Southern blots and colony blots of 69 isolates of B. pumilus and other Bacillus spp. were probed with 32 P-labeled pMGD296. Twenty-nine probe-positive isolates were identified; of these, 28 contained a plasmid with a restriction profile identical to that of pMGD296. One isolate from untreated hay contained a 40-kb plasmid (pMGD150) that was homologous to pMGD296 but had a different restriction fragment pattern. Regions of homology between the two plasmids were identified by Southern blotting, and a 1.9-kb Hin dIII- Pst I fragment of pMGD296 lacking strong homology to pMGD150 was cloned in pUC18. The cloned fragment hybridized only with isolates containing pMGD296 and was used to estimate populations of these isolates in treated and untreated hay.

Tn917 transposase. Sequence correction reveals a single open reading frame corresponding to the tnpA determinant of Tn3-family elements

A nucleotide sequence correction on the Enterococcus faecalis transposon Tn917 indicates that what was formerly thought to be two open reading frames (ORF5 and ORF6) is actually one reading frame (ORF5). The latter exhibits homology with the Tn3-family transposase determinants known as tnpA.

Plasmid-associated virulence factors of non-toxigenic (pX01-) Bacillus anthracis

The anthrax toxins and capsule, encoded by plasmids pX01 and pX02, respectively, are the only known virulence factors of Bacillus anthracis and are considered essential for full virulence. Some B. anthracis strains cured of pX01, such as delta Ames-1, remained virulent for mice. The virulence was partially mediated by pX02, as determined by phage transduction. pX02 plasmids from the delta Ames-1 and Pasteur strains were mutagenized with transposon Tn917 to identify loci associated with virulence. The capsule phenotype, virulence and pX02 restriction pattern of the insertion mutants were characterized. Two mutants that produced no detectable capsule were avirulent. One had a deletion of more than 20 kb, which included the structural genes required for capsule synthesis (cap); the second had an insertion outside of cap. Two mutants with reduced encapsulation had insertions at different sites outside cap and were less virulent, whereas one that was normally encapsulated, but had a high rate of pX02 curing, was unaltered in virulence. Mutants that produced greater amounts of capsule than the parental strain were more virulent, and a few that produced wild-type levels of capsule were less virulent.

Dual effect of a Tn917 insertion into the Bacillus subtilis sacX gene

The most common effect of transposon insertion is the inactivation of genes. However, in some cases, transposons can activate in cis the expression of genes in the neighbourhood of their integration site. We previously described an insertion of the transposon Tn917 into the Bacillus subtilis sacXY locus. sacX and sacY encode respectively a negative and a positive regulator involved in induction by sucrose of the exoenzyme levansucrase. Data in this paper show that the Tn917 insertion had two effects: it inactivated sacX and it increased the transcription of sacY. The latter effect involved one or several elements internal to the transposon.

An apparent Bacillus subtilis folic acid biosynthetic operon containing pab, an amphibolic trpG gene, a third gene required for synthesis of para-aminobenzoic acid, and the dihydropteroate synthase gene

McDonald and Burke (J. Bacteriol. 149:391-394, 1982) previously cloned a sulfanilamide-resistance gene, sul, residing on a 4.9-kb segment of Bacillus subtilis chromosomal DNA, into plasmid pUB110. In this study we determined the nucleotide sequence of the entire 4.9-kb fragment. Genes identified on the fragment include pab, trpG, pabC, sul, one complete unidentified open reading frame, and one incomplete unidentified open reading frame. The first three of these genes, pab, trpG, and pabC, are required for synthesis of p-aminobenzoic acid. The trpG gene encodes an amphibolic glutamine amidotransferase required for synthesis of both p-aminobenzoate and anthranilate, the latter an intermediate in the tryptophan biosynthetic pathway. The pabC gene may encode a B. subtilis analog of enzyme X, an enzyme needed for p-aminobenzoate synthesis in Escherichia coli. The sul gene probably encodes dihydropteroate synthase, the enzyme responsible for formation of 7,8-dihydropteroate, the immediate precursor of folic acid. All six of the cloned genes are arranged in a single operon. Since all four of the identified genes are needed for folate biosynthesis, we refer to this operon as a folic acid operon. Expression of the trpG gene is known to be negatively controlled by tryptophan. We propose that this regulation is at the level of translation. This hypothesis is supported by the finding of an apparent Mtr-binding site which overlaps with the trpG ribosome-binding site.

Regulation of histidine and proline degradation enzymes by amino acid availability in Bacillus subtilis

The first enzymes of the histidine (hut) and proline degradative pathways, histidase and proline oxidase, could not be induced in Bacillus subtilis cells growing in glucose minimal medium containing a mixture of 16 amino acids. Addition of the 16-amino-acid mixture to induced wild-type cells growing in citrate minimal medium repressed histidase synthesis 25- to 250-fold and proline oxidase synthesis 16-fold. A strain containing a transcriptional fusion of the hut promoter to the beta-galactosidase gene was isolated from a library of Tn917-lacZ transpositions. Examination of histidase and beta-galactosidase expression in extracts of a hut-lacZ fusion strain grown in various media showed that induction, catabolite repression, and amino acid repression of the hut operon were mediated at the level of transcription. This result was confirmed by measurement of the steady-state level of hut RNA in cells grown in various media. Since amino acid repression was not defective in B. subtilis mutants deficient in nitrogen regulation of glutamine synthetase and catabolite repression, amino acid repression appears to be mediated by a system that functions independently of these regulatory systems.

Insertional inactivation of the mec gene in a transposon mutant of a methicillin-resistant clinical isolate of Staphylococcus aureus

All clinical strains of methicillin-resistant Staphylococcus aureus (MRSA) examined so far contain the mec gene and its product, the penicillin-binding protein (PBP) 2A. Yet the same strains show tremendous variation in the phenotypic expression of antibiotic resistance (MIC), which is under the control of a set of additional, auxiliary genes. Thus, the quantitative contribution of the mec gene to the resistance phenotype of MRSA is not known, and no mutants with the lesion located within the mec gene have been described. We subjected a highly resistant MRSA strain to transposon mutagenesis with the erythromycin resistance transposon Tn551, and a mutant expressing greatly decreased methicillin resistance (RUSA4) was selected to characterize the transposon insertion site. The results indicate that the Tn551 insertion site in mutant RUSA4 is between base pairs 1000 and 1400 of the sequence encoding PBP 2A. Thus, the uniform and greater than 200-fold drop in the methicillin MIC (4 micrograms/ml) for this mutant relative to that for the parent strain (MIC greater than or equal to 800 micrograms/ml) must be related to the inactivation of the PBP 2A gene. The results provide the first unequivocal evidence for the importance of PBP 2A as a quantitative contributor to the MIC for MRSA.

In vivo genetic systems in lactic acid bacteria

A review of in vivo genetic systems covers the key features of transduction and conjugation but emphasises the intramolecular and intermolecular DNA interactions that are often associated with these processes. As well as the transfer of many lactose plasmids, conjugal transfer of nisin genes and the use of conjugation to construct bacteriophage-resistant dairy starter cultures are discussed. The discovery and characterization of insertion sequences in Lactobacillus and Lactococcus and the exploitation of heterologous conjugation and transposition systems in the lactic acid bacteria are described.

Movable genetic elements and antibiotic resistance in enterococci

The enterococci possess genetic elements able to move from one strain to another via conjugation. Certain enterococcal plasmids exhibit a broad host range among gram-positive bacteria, but only when matings are performed on solid surfaces. Other plasmids are more specific to enterococci, transfer efficiently in broth, and encode a response to recipient-produced sex pheromones. Transmissible non-plasmid elements, the conjugative transposons, are widespread among the enterococci and determine their own fertility properties. Drug resistance, hemolysin, and bacteriocin determinants are commonly found on the various transmissible enterococcal elements. Examples of the different systems are discussed in this review.

Excision of transposon Tn917 in Bacillus subtilis

Excision of Tn917 from chromosomal sites in B. subtilis is characterized by reversion studies. We propose different modes of excision depending on the site of transposon insertion. Excision takes place as precise excision in one step which results in reversion of the mutant phenotype, or by a two-step process where nearly precise excision of the transposon moiety is followed by precise excision of the nearly precise excision remnants. For this type of transposons a minor pathway or nonreplicative transposition is proposed to be connected with precise excision.

Listeriolysin O is essential for virulence of Listeria monocytogenes: direct evidence obtained by gene complementation

The role of listeriolysin O in the intracellular multiplication of Listeria monocytogenes and, therefore, its pathogenicity was questioned through a genetic complementation study. A nonhemolytic mutant was generated by inserting a single copy of transposon Tn917 in the bacterial chromosome. This insertion was localized by DNA sequence analysis in hlyA, the gene coding for listeriolysin O. As was another mutant that we previously characterized, this mutant was avirulent in the mouse. It was transformed with a plasmid carrying only hlyA, able to replicate in L. monocytogenes, and stably maintained in vitro and in vivo. The complemented strain displayed a hemolytic phenotype identical to that of the wild-type strain and was fully virulent, therefore attributing a crucial role to listeriolysin O in virulence and excluding the hypothesis of a polar effect of the transposon insertion on genes adjacent to hlyA and possibly involved in virulence.

Sequence and transcription mapping of Bacillus subtilis competence genes comB and comA, one of which is related to a family of bacterial regulatory determinants

The complete nucleotide sequences of the comA and comB loci of Bacillus subtilis were determined. The products of these genes are required for the development of competence in B. subtilis and for the expression of late-expressing competence genes. The major 5' termini of both the comA and comB transcripts were determined. The inferred promoters of both comA and comB contained sequences that were similar to those found at the -10 and -35 regions of promoters that are used by sigma A-RNA polymerase, the primary form of this enzyme in vegetative cells. The comB gene was located approximately 3 kilobase pairs upstream of the comA gene and encoded a 409-amino-acid protein with a predicted molecular weight of 46,693. The comA locus contained two open reading frames (ORFs) and comB contained one ORF. The predicted amino acid sequence of the comA ORF1 gene product consisted of 214 amino acids, with an aggregate molecular weight of 24,132. The ORF1 product was required for competence, while ORF2, which was cotranscribed with ORF1 and encoded a predicted protein of 126 amino acids, was not. The predicted protein sequence of the comA ORF1 gene product was found to be similar to that of several members of the effector class of procaryotic signal transducers. The C-terminal portion of the predicted comA sequence contained a possible helix-turn-helix motif, which is characteristic of DNA-binding proteins. comA ORF1 was cloned on a multicopy plasmid and was shown to complement the competence-deficient phenotype caused by the comA124 insertion of Tn917lac. Also, the presence of comA ORF1 in multiple copies interfered with sporulation. Anti-peptide antibodies raised to the predicted product of comA ORF1 reacted strongly with a single protein band of about 24,000 daltons in immunoblots. The possible roles of multiple signal transduction systems in triggering the development of competence are discussed.

Involvement of Tn4430 in transfer of Bacillus anthracis plasmids mediated by Bacillus thuringiensis plasmid pXO12

The self-transmissible plasmid pXO12 (112.5 kilobases [kb]), originally isolated from strain 4042A of Bacillus thuringiensis subsp. thuringiensis, codes for production of the insecticidal crystal protein (Cry+). The mechanism of pXO12-mediated plasmid transfer was investigated by monitoring the cotransfer of the tetracycline resistance plasmid pBC16 (4.2 kb) and the Bacillus anthracis toxin and capsule plasmids, pXO1 (168 kb) and pXO2 (85.6 kb), respectively. In matings of B. anthracis donors with B. anthracis and Bacillus cereus recipients, the number of Tcr transcipients ranged from 4.8 x 10(4) to 3.9 x 10(6)/ml (frequencies ranged from 1.6 x 10(-4) to 7.1 x 10(-2), and 0.3 to 0.4% of them simultaneously inherited pXO1 or pXO2. Physical analysis of the transferred plasmids suggested that pBC16 was transferred by the process of donation and that the large B. anthracis plasmids were transferred by the process of conduction. The transfer of pXO1 and pXO2 involved the transposition of Tn4430 from pXO12 onto these plasmids. DNA-DNA hybridization experiments demonstrated that Tn4430 was located on a 16.0-kb AvaI fragment of pXO12. Examination of Tra- and Cry- derivatives of pXO12 showed that this fragment also harbored information involved in crystal formation and was adjacent to a restriction fragment containing DNA sequences carrying information required for conjugal transfer.

Construction, transfer and properties of a novel temperature-sensitive integrable plasmid for genomic analysis of Staphylococcus aureus

As an alternative approach to genetic transfer and analysis, a novel integrable plasmid system was developed that should prove useful for mapping and cloning various genes in Staphylococcus aureus and other Gram-positive bacteria. The use of a restriction-deficient recipient strain and an improved protocol for protoplast plasmid transformation facilitated direct cloning of a recombinant plasmid (pPQ126) in S. aureus NCTC 8325-4. Plasmid pPQ126 (13.6 kb) is a novel, temperature-sensitive integrable plasmid containing genes encoding resistance to erythromycin and chloramphenicol (from plasmid pTV1ts), and resistance to gentamicin (from transposon Tn4001). When introduced into an appropriate recipient strain at the permissive temperature (30 degrees C), pPQ126 replicates autonomously. Integration of pPQ126 is directed into homologous chromosomal target sequences (chromosomal insertions of Tn551 or Tn4001) by growing a population of cells containing autonomous pPQ126 in the presence of gentamicin, erythromycin, and chloramphenicol at 39 degrees C (nonpermissive temperature). Elevated temperature both selects for and maintains pPQ126 as an integrated replicon. Integration of pPQ126 occurs at significantly reduced frequency in a recombination-deficient host, and does not occur in the absence of host chromosomal homology. Integrated pPQ126 excises from the chromosome under permissive conditions (30 degrees C), and excision results in derivatives of pPQ126 that harbour DNA of chromosomal origin.

Genetic characterization of a Clostridium difficile erythromycin-clindamycin resistance determinant that is transferable to Staphylococcus aureus

The transferable macrolides-lincosamides-streptogramin B (MLS) resistance determinant of clinical isolates of Clostridium difficile, designated ermZ, has been shown to share homology with ermB, which is associated with Staphylococcus aureus transposon Tn551. Homology within Tn551 was confined to less than or equal to 1.3 kilobases, whereas no homology could be demonstrated between Tn551 sequences external to ermB and MLS-resistant C. difficile. Transfer of ermZ from C. difficile to S. aureus was achieved by means of the filter mating technique, suggesting that (conjugative?) intergeneric exchange between clostridia and staphylococci may also occur in nature. S. aureus transcipients were shown to contain additional DNA from C. difficile besides ermZ. This additional DNA appeared to be present in MLS-susceptible C. difficile and might form part of an as yet undemonstrated insertion sequence element associated with ermZ of resistant strains.

Molecular epidemiology of macrolides-lincosamides-streptogramin B resistance in Staphylococcus aureus and coagulase-negative staphylococci

Macrolides-lincosamides-streptogramin B (MLS) resistance is commonly found in Staphylococcus aureus and coagulase-negative staphylococci (22 and 45%, respectively, among isolates from three New Jersey hospitals). We have examined representative subsets of 107 MLS-resistant isolates for the molecular nature of the resistance determinant, the erm gene, by dot blot and Southern hybridization analysis. All of 35 S. aureus isolates examined and 39 of 42 coagulase-negative isolates examined were found to harbor the ermA or ermC evolutionary variant. Genes of the ermC class occurred exclusively on a small plasmid similar to or indistinguishable from one (pNE131) previously described in S. epidermidis. Genes of the ermA class occurred exclusively in the chromosome, and restriction patterns indicated that they were part of a transposon, Tn554, characteristic of the classical S. aureus ermA strain. Unlike S. aureus ermA strains examined previously, which harbor Tn554 at a single specific (primary) site, four of our S. aureus isolates had second inserts at different chromosomal sites. The majority of our coagulase-negative isolates had two or more inserts, neither of which occurred at the classical primary site and many of which differed from one another in location (as inferred from restriction patterns). Coagulase-negative staphylococci constitute a large reservoir of the ermA and ermC class of determinants, with clear potential for interspecies spread.

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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Transposition of Tn917 in Bacillus megaterium

Transposon Tn917, carried on plasmid pTV1, was introduced into Bacillus megaterium and transposed efficiently and apparently randomly. Insertional mutations included at least eight different auxotrophic loci, two carbon source loci, and sporulation loci. One trp::Tn917 mutation was further verified as an insertion by both reversion and transduction.

Transposition of the gram-positive transposon Tn917 in Escherichia coli

The streptococcal transposon Tn917 was demonstrated to transpose in Escherichia coli from the Bacillus subtilis-E. coli shuttle plasmid pHK1207 into an F' plasmid derivative. Subsequently, a second round of transposition from the F' plasmid into pACYC184 could be readily demonstrated. These results represent the initial demonstration of the transposition of a gram-positive transposon in a gram-negative bacterium at a relatively high frequency.

Identification of Tn4430, a transposon of Bacillus thuringiensis functional in Escherichia coli

The mobile genetic element Tn4430, originating from the gram-positive bacterium, Bacillus thuringiensis, and previously described as the Th-sequence, is the first transposon isolated from the genus Bacillus. In the present work a gene (APH-III) conferring resistance to kanamycin was inserted into this 4.2 kb transposon. Transposition experiments showed that Tn4430 omega APH-III could transpose in the gram-negative host Escherichia coli when its insertion functions were supplied by an intact copy of Tn4430. By transposing Tn4430 omega APH-III directly onto pBR322, it was possible to determine the nucleotide sequence of the terminal inverted repeats of Tn4430 and of the target DNA site. Identical 38 bp in inverted orientation are situated at each end of the transposon and there is a direct duplication of 5 bp at the insertion site. Thus, it is clear that Tn4430 is closely related to the transposons belonging to the Tn3 family (class II elements).

Identification of regions of the Streptococcus faecalis plasmid pCF-10 that encode antibiotic resistance and pheromone response functions

The conjugative plasmid pCF-10 (58 kb) of Streptococcus faecalis has been mapped with restriction enzymes. By restriction mapping and Southern hybridization analysis, a 16-kb segment of the plasmid was shown to resemble closely the conjugative tetracycline resistance transposon, Tn916. Mutagenesis of the plasmid with the erythromycin resistance transposon Tn917 was used to localize a tetracycline resistance determinant and several regions involved in conjugal transfer. Fifty Tn917 insertions (outside the region of the plasmid homologous to Tn916) affecting mating behavior and the ability of donor cells to respond to the sex pheromone cCF-10 were mapped to nine distinct segments, or tra regions. Insertions into tra regions 1-3 and 7-9 led to an enhanced transfer ability of mutant plasmids relative to the transfer frequency obtained for the wild-type plasmid. Cells carrying these mutant plasmids differed in colony morphology or growth in broth culture from cells carrying pCF-10. Insertions into tra regions 4-6 resulted in reduced plasmid transfer, or completely eliminated the mating potential of donor cells. Insertions generating transfer-defective plasmids could be grouped further according to the ability of strains harboring the mutant plasmids to respond to cCF-10. HindIII fragments of pCF-10 coding for transfer functions have been cloned into Escherichia coli.

Broad geographical distribution of homologous erythromycin, kanamycin, and streptomycin resistance determinants among group D streptococci of human and animal origin

The Emr, Kmr, and Smr determinants of the Streptococcus faecalis R plasmid pJH1 were cloned in Streptococcus sanguis with a streptococcal plasmid vector, pVA380-1. Each cloned determinant was used as a probe in hybridization reactions with dot blots containing plasmid-enriched DNA from 91 group D streptococcal isolates resistant to erythromycin, kanamycin, and streptomycin; the isolates were obtained from animal and human sources in a variety of geographical locations. Nearly 70% of the strains contained DNA that hybridized to each of the three resistance determinants from pJH1. Five plasmids mediating resistance to erythromycin, kanamycin, and streptomycin were examined in more detail. These plasmids varied in size between 26 and 105 kilobase pairs (kbp) and exhibited very different EcoRI restriction patterns. However, each plasmid contained the resistance determinants on a single 13- to 20-kbp EcoRI fragment. Southern blot hybridizations and additional restriction endonuclease digests revealed extensive DNA sequence homology and virtually indistinguishable restriction endonuclease maps within a 9- to 11-kbp region of each plasmid which included the resistance determinants.

A 2.6 kb DNA sequence of Streptomyces coelicolor A3(2) which functions as a transposable element

Streptomyces coelicolor A3(2) contains CCC DNA molecules, 2.6 kb in size, with an average copy number of less than one per ten chromosomes. Southern hybridisation revealed, in addition, two linear, integrated copies (A and B) of this "mini-circle" sequence per chromosome. The two integrated copies have similar (if not identical) ends and are present in the same locations in various S. coelicolor A3(2) derivatives. The mini-circle sequence is absent from S. lividans 66 and S. violaceolatus ISP5438 and from several Streptomyces species less closely related to S. coelicolor A3(2). None of a variety of Streptomyces plasmids tested contained homology to the mini-circle sequence. When a 1.8 kb fragment of the mini-circle lacking the ends of the integrated copies was inserted into KC515 (a derivative of the temperate phage phi C31 which is unable to lysogenise host strains by the natural route because the phage attachment site has been deleted) the resulting phage lysogenized S. coelicolor A3(2) (integrating into the genome of this host by homologous recombination with resident minicircle sequences) but not S. lividans or a variety of other phi C31 hosts. In contrast, a KC515 derivative (KC591) carrying the entire 2.6 kb mini-circle sequence linearized at its single BclI site (and therefore containing the integration site of the free mini-circle) lysogenized not only S. coelicolor A3(2) but also S. lividans 66 and most other strains normally lysogenized by phi C31. The KC591 lysogens of the eight Streptomyces species tested contained a linear, integrated prophage with termini apparently identical to those of the linear mini-circle copies of S. coelicolor.(ABSTRACT TRUNCATED AT 250 WORDS)

Construction and properties of Tn917-lac, a transposon derivative that mediates transcriptional gene fusions in Bacillus subtilis

A derivative of Tn917 was constructed, referred to as Tn917-lac, which is capable of generating fusions that connect the transcripts of Bacillus subtilis chromosomal genes to the coding sequence of the lacZ gene of Escherichia coli. Two independent insertions of Tn917-lac into the gltA gene and one insertion into the trpE gene (in the trpEDCFBA operon) of B. subtilis were studied in detail, and the results confirmed that Tn917-lac-mediated transcriptional fusions produce levels of beta-galactosidase that reflect accurately the regulated expression of interrupted genes. To facilitate these studies, a procedure was developed that permits the analysis of Tn917-lac-mediated fusions in partial diploids where insertional mutations are complemented by an intact copy of the interrupted genes. Tn917 is known to function efficiently in bacteria representing three quite different Gram-positive genera (Streptococcus, Bacillus, and Staphylococcus) and is known to display a relatively high degree of randomness in its insertions into bacterial genomes, making it likely that Tn917-lac will be useful for the identification and study of many kinds of regulated genes in a wide range of Gram-positive species.

Complete nucleotide sequence of macrolide-lincosamide-streptogramin B-resistance transposon Tn917 in Streptococcus faecalis

Streptococcus faecalis transposon Tn917 was cloned in Escherichia coli on plasmid vector pBR325. The erythromycin resistance determinant of Tn917 was not expressed in the E. coli background. The nucleotide sequence of Tn917 was determined and found to be 5,257 base pairs in length. Six open reading frames (ORFs) were identified and designated 1 through 6 (5' to 3'); all were on the same DNA strand. A region exhibiting strong homology with known promoters was identified upstream from ORF1. ORFs 1 to 3 were virtually identical to the previously sequenced erythromycin resistance determinant on Streptococcus sanguis plasmid pAM77. At the 3' point, where the homology between Tn917 and pAM77 ends, was a 20-base-pair region about 80% homologous with a component of the res site of Tn3. The amino acid sequence of ORF4 showed homology with other site-specific recombination enzymes, including approximately 30% homology with the resolvase of Tn3. Contained within Tn917 was a directly oriented 73-base-pair duplication of the left terminus. The Tn917 sequence revealed that antibiotic-enhanced transposition might be due to extension of transcription from the resistance-related genes (in ORFs 1 to 3) into transposition genes (in ORFs 4 to 6). Transcription analyses resulted in data consistent with this interpretation.

Ribosomal RNA methylation in Staphylococcus aureus and Escherichia coli: effect of the "MLS" (erythromycin resistance) methylase

Classical acquired resistance to erythromycin in Staphylococcus aureus ("MLS," or macrolide-lincosamide-streptogramin, resistance) was shown by Weisblum and colleagues to be a direct consequence of the conversion of one or more adenosine residues of 23S rRNA, within the subsequence(s) GA3G, to N6-dimethyladenosine (m62A). The methylation reaction is effected by a class of methylase, whose genes are typically plasmid- or transposon-associated, and whose synthesis is inducible by erythromycin. Using a recently obtained clinical MLS isolate of S. aureus, we have further defined the methylation locus as YGG X m62A X AAGAC; and have shown that this subsequence occurs once in the 23S RNA and that it is essentially completely methylated in all copies of 23S RNA that accumulate in induced cultures. Similar findings were obtained with laboratory S. aureus strains containing two well-characterized evolutionary variants (ermB, ermC) of MLS methylase genes. Analyses of a strain of E. coli containing the ermC gene indicated that the specificity of the methylase gene was unchanged, but that its expression was muted. Even after prolonged periods of induction, the strain manifested only partial resistance to erythromycin, and only about one-third of the copies of the MLS subsequence were methylated in such "induced" cultures. Since the E. coli 23S RNA sequence is known in its entirety, localization of the MLS subsequence is in this case unambiguous; as inferred by homology arguments applied earlier to the S. aureus data, the subsequence is in a highly conserved region of 23S RNA considered to contribute to the peptidyl transferase center of the ribosome.

New Ways to Study Developmental Genes in Spore-Forming Bacteria

The regulated activation of numerous sets of genes in multiple chromosomal locations is a hallmark of cellular differentiation in both eukaryotes and prokaryotes. Certain species of bacteria that experience complex developmental cycles are especially attractive as systems in which to study the mechanisms of this kind of gene regulation because they are highly amenable to both biochemical and genetic approaches. Bacillus subtilis, which undergoes extensive cellular differentiation when it sporulates, is one such system. Many new methods are now available in this Gram-positive species for identifying, manipulating, and studying the regulation of genes involved in spore formation, including the use of transposable genetic elements that create gene fusions in vivo as an automatic consequence of insertions into genes.

Evidence for a disseminated erythromycin resistance determinant mediated by Tn917-like sequences among group D streptococci isolated from pigs, chickens, and humans

A total of 199 streptococci isolated from feces of healthy chickens, pigs, and beef cattle and 26 human clinical isolates were tested for resistance to kanamycin, streptomycin, tetracycline, erythromycin, and lincomycin. Of 66 isolates resistant to these antibiotics, 12 transferred one or more resistance traits by conjugation in broth. Erythromycin resistance (Emr) was transferred from 10 of the 12 successful donors. AvaI digests of plasmids isolated from Emr transconjugants derived from two human, two chicken, and one pig isolate contained three fragments similar in size to those produced from Tn3871, an Emr transposon. The three fragments from each of the five digests on Southern blots hybridized to radiolabeled Tn3871. Plasmid DNA from a transconjugant derived from a second pig isolate contained two of the three Tn3871-associated AvaI fragments. One of the AvaI fragments from each of the six plasmids hybridized with a radiolabeled probe containing a cloned AvaI fragment from Tn3871 that contained the Emr determinant. Transposition of the Emr trait was demonstrated for the plasmids derived from one human and one pig isolate. We concluded that extensive DNA homology existed between plasmids from streptococcal strains obtained from two human patients, two chickens, and two pigs and the Emr transposon Tn3871, which is very similar or identical to the well-characterized Emr transposon Tn917. The detection of Tn3871-like sequences in streptococcal isolates from Arkansas, Illinois, North Carolina, and Washington, D.C. indicates wide dissemination of Emr mediated by the same or closely related transposons.

Cloning, sequencing and transcription of an inactivated copy of Bacillus amyloliquefaciens extracellular ribonuclease (barnase)

The gene for Bacillus amyloliquefaciens extracellular RNase (barnase) has been cloned in an inactive form in Escherichia coli following insertional mutagenesis by transposon Tn917. The nucleotide (nt) sequence of the gene was determined and the deduced amino acid (aa) sequence found to correspond almost precisely to the previously determined sequence. An open reading frame (ORF) of 72 codons precedes the mature sequence. The probable translation start site is 46 or 47 codons before the N-terminal alanine of the mature protein, 11 (or 14) bp from a putative ribosome-binding site (RBS). Within this leader sequence is a hydrophobic 15 aa core preceded by three basic residues which is characteristic of a secretory signal sequence. A pro-barnase protein with four extra aa at the N-terminus has been detected extracellularly indicating that the signal peptidase-cutting site lies before the mature protein. An inverted repeat that may act as a transcription terminator was found at the 3' end of the gene. The gene is maintained in E. coli with a short inverted repeat from the termini of Tn917 inserted into the coding sequence. Northern blot analysis of RNA from B. amyloliquefaciens shows an approx. 780-nt transcript produced during exponential and stationary growth phases. The inactive cloned gene produces an approx. 480-nt transcript in E. coli and two transcripts of approx. 480 and 780 nt in Bacillus subtilis.