The nucleotide sequence of the tetracycline resistance gene of plasmid pNS1981 from Bacillus subtilis differs from pTHT15 from a Thermophilic bacillus by two base pairs

Plasmid-Mediated Antimicrobial Resistance in Staphylococci and Other Firmicutes

In staphylococci and other Firmicutes, resistance to numerous classes of antimicrobial agents, which are commonly used in human and veterinary medicine, is mediated by genes that are associated with mobile genetic elements. The gene products of some of these antimicrobial resistance genes confer resistance to only specific members of a certain class of antimicrobial agents, whereas others confer resistance to the entire class or even to members of different classes of antimicrobial agents. The resistance mechanisms specified by the resistance genes fall into any of three major categories: active efflux, enzymatic inactivation, and modification/replacement/protection of the target sites of the antimicrobial agents. Among the mobile genetic elements that carry such resistance genes, plasmids play an important role as carriers of primarily plasmid-borne resistance genes, but also as vectors for nonconjugative and conjugative transposons that harbor resistance genes. Plasmids can be exchanged by horizontal gene transfer between members of the same species but also between bacteria belonging to different species and genera. Plasmids are highly flexible elements, and various mechanisms exist by which plasmids can recombine, form cointegrates, or become integrated in part or in toto into the chromosomal DNA or into other plasmids. As such, plasmids play a key role in the dissemination of antimicrobial resistance genes within the gene pool to which staphylococci and other Firmicutes have access. This chapter is intended to provide an overview of the current knowledge of plasmid-mediated antimicrobial resistance in staphylococci and other Firmicutes.

Characterization and heterologous expression of the tetL gene and identification of iso-ISS1 elements from Enterococcus faecalis plasmid pJH1

The tetracycline-resistance (TcR) determinant of the Enterococcus faecalis plasmid pJH1 has been identified and located on a 2.2-kb RsaI-EcoRI fragment. The fragment was cloned in Escherichia coli, and specified TcR in this host. The nucleotide (nt) sequence of the cloned fragment showed the presence of an open reading frame (ORF) of 1374 bp, designated tetL. The nt sequence of tetL from pJH1 was identical to that of the tetL present on pLS1 from Streptococcus agalactiae. Upstream of the pJH1 tetL, part of another ORF was found that, except for two single-nt substitutions, was identical to an iso-ISS1 element from Lactococcus lactis. Hybridization studies indicated the presence of several ISS1-like elements in plasmid pJH1, but not on the En. faecalis chromosome. To study its usefulness as a marker in Gram+ organisms, the pJH1 tetL was cloned on the broad-host-range plasmid pNZ124, resulting in pNZ280, that was found to give resistance to 40 micrograms Tc/ml in Lc. lactis and Bacillus subtilis.

Nucleotide sequence and phylogeny of the tet(L) tetracycline resistance determinant encoded by plasmid pSTE1 from Staphylococcus hyicus

The nucleotide sequence of the tetracycline resistance (tet) gene and its regulatory region, encoded by the plasmid pSTE1 from Staphylococcus hyicus, was determined. The tet gene was inducible by tetracycline and encoded a hydrophobic protein of 458 amino acids. Comparisons between the predicted amino acid sequences of the pSTE1-encoded Tet from S. hyicus and the previously sequenced Tet K variants from Staphylococcus aureus, Tet L variants from Bacillus cereus, Bacillus stearothermophilus, and Bacillus subtilis, Tet M variants from Streptococcus faecalis and Staphylococcus aureus as well as Tet O from Streptococcus mutans were performed. An alignment of Tet amino acid sequences revealed the presence of 30 conserved amino acids among these Tet variants. On the basis of the alignment, a phylogenetic tree was constructed. It demonstrated large evolutionary distances between the Tet M and Tet O variants on one hand and the Tet K and Tet L variants on the other hand. The pSTE1-encoded Tet proved to be closely related to the Tet L proteins originally found on small Bacillus plasmids. The observed extensive similarities in the nucleotide sequences of the tet genes and in the deduced Tet amino acid sequences allowed the assignment of the pSTE1-encoded Tet to the Tet proteins of class L.

The large Bacillus plasmid pTB19 contains two integrated rolling-circle plasmids carrying mobilization functions

Plasmid pTB19 is a 27-kb plasmid originating from a thermophilic Bacillus species. It was shown previously that pTB19 contains an integrated copy of the rolling-circle type plasmid pTB913. Here we describe the analysis of a 4324-bp region of pTB19 conferring resistance to tetracycline. The nucleotide sequence of this region revealed all the characteristics of a second plasmid replicating via the rolling-circle mechanism. This sequence contained (i) the tetracycline resistance marker of pTB19, which is highly similar to other tetL-genes of gram-positive bacteria; (ii) a hybrid mob gene, which bears relatedness to both the mob-genes of pUB110 and pTB913; (iii) a palU type minus origin identical to those of pUB110 and pTB913; and (iv) a plus origin of replication similar to that of pTB913. A repB-type replication initiation gene sequence identical to that of pTB913 was present, which lacked the middle part (492 bp), thus preventing autonomous replication of this region. The hybrid mob gene was functional in conjugative mobilization of plasmids between strains of Bacillus subtilis.

Origin of tetracycline efflux proteins: conclusions from nucleotide sequence analysis

The sequences of six tetracycline efflux proteins and three transport proteins which have some resemblance to them were compared. The tetracycline efflux proteins fall into three families: (i) those encoded by pBR322, RP1, and Tn10 (Escherichia coli); (ii) pT181 (Staphylococcus aureus) and pTHT15 (Bacillus subtilis); and (iii) tet347 (Streptomyces rimosus). There is global sequence homology within each of the first two families, but there is none between the families. The pT181/pTHT15 family shares close homology with the N-terminal half of the methylenomycin A efflux protein (Streptomyces coelicor), while tet347 resembles the C-terminal half. Portions of the N-terminal half of the Tn10-encoded protein show significant resemblance to portions in the N-terminal half of the pT181/pTHT15 family, but this sometimes occurs among transport proteins which do not have a common substrate. Tetracycline efflux proteins, therefore, appear to have arisen on at least two, or possibly three, separate occasions, probably from other transport proteins.

A limited number of Bacillus subtilis strains carry a tetracycline-resistance determinant at a site close to the origin of replication

Several strains of Bacillus subtilis, e.g., 168 derivatives and R, were found to carry a single copy of a tetracycline-resistance (TcR) determinant (named tetBS908) at a site close to the origin of replication on the chromosome. This gene is highly homologous (80% identical) to the TcR determinant of plasmids widely dispersed among aerobic spore-forming bacilli. B. subtilis RM125 (168 strain) transformants which carry a varying number of tetBS908 sequences in a tandem array on the chromosome were constructed and examined for their TcR level. A nearly proportional relationship between the TcR level and copy number of tetBS908 existed.

Analysis of the tet gene of plasmid pCIS7 isolated from Bacillus subtilis

We have previously shown that plasmid pCIS7, which contains 11.5 kb of Bacillus subtilis DNA isolated from a tetracycline-sensitive (TcS) strain, confers Tc resistance when integrated and amplified in the chromosome of TcS B. subtilis 168trpC2 [Ives and Bott, J. Bacteriol. 171 (1989) 1801-1810]. Here, we report that the number of integrated plasmid sequences required to confer Tc resistance is greater than the 20 copies seen with increasing chloramphenicol selection and, by dot-blot analysis, exceeds 100 copies per cell. The amplification is accompanied by a corresponding increase in mRNA encoding the tet gene. The tet gene sequence of pCIS7 has been compared to B. subtilis tetGSY908 [Sakaguchi et al., Biochim. Biophys. Acta. 94 (1988) 49-57] and other Gram-positive tet genes. The tet gene of pCIS7 is a member of the class L TcR determinants, and probably confers Tc resistance by increasing the efflux of Tc from the bacterial cell.

Characterization of the tetracycline resistance gene of plasmid pT181 of Staphylococcus aureus

Some genetic and biochemical properties of the tetracycline resistance element of the Staphylococcus aureus plasmid pT181 have been studied. Resequencing of a portion of the tetracycline resistance gene (tet) showed the presence of a single open reading frame of 1,299 nucleotides capable of encoding a polypeptide of 433 amino acids. Analysis of BAL 31 nuclease-generated deletion mutants of the tet gene showed the presence of two complementation groups within this region. Northern blot hybridizations demonstrated that the tet gene encodes a single mRNA, and its initiation site has been mapped by S1 nuclease protection experiments. We also identified an approximately 52,000-dalton tetracycline-inducible polypeptide in Bacillus subtilis minicells carrying pT181. Induction of the tet gene by tetracycline resulted in a 4-fold increase in the levels of TET mRNA and at least a 15-fold increase in the amount of TET protein in B. subtilis minicells.

Nucleotide sequence homology of the tetracycline-resistance determinant naturally maintained in Bacillus subtilis Marburg 168 chromosome and the tetracycline-resistance gene of B. subtilis plasmid pNS1981

The nucleotide sequence (1579 bp) of tetracycline-resistance determinant and flanking regions of the cloned 5.1 kb DNA fragment from Bacillus subtilis GSY908 chromosome (Sakaguchi, R. and Shishido, K. (1988) Biochim. Biophys. Acta 949, 49-57) were determined and compared with those of the B. subtilis tetracycline-resistance plasmid pNS1981. The tetracycline-resistance structural (tet) genes of the B. subtilis GSY908 chromosome (tetBS908) and pNS1981 (tetpNS1981) were found to be highly homologous (80% identical). Both tet genes were composed of 1374 bp and 458 amino-acid residues initiating from a GTG codon preceded by a ribosome-binding site (RBS-2). Upstream from tetBS908 there exists a short open reading frame (20 amino acids) initiating from a ATG codon preceded by its own RBS (RBS-1). This leader sequence was also highly homologous to that of tetpNS1981 except for a deletion of one bp between the RBS-1 and the ATG codon.

Molecular cloning of a tetracycline-resistance determinant from Bacillus subtilis chromosomal DNA and its expression in Escherichia coli and B. subtilis

Bacillus subtilis GSY908 DNA fragments (5.1 and 4.4 kilobase pairs (kb)) containing a tetracycline-resistance determinant were cloned in Escherichia coli using a shuttle plasmid vector pLS353. Restriction endonucelase analysis showed that the 4.4 kb fragment is a spontaneous deletion derivative of the 5.1 kb fragment. E. coli tetracycline-resistance transformants carrying pLS353 with the 5.1 kb fragment (named pTBS1) and that with 4.4 kb fragment (pTBS1.1) could grow at tetracycline concentrations up to 80 and 50 micrograms per ml, respectively. B. subtilis MI112 and RM125 were transformed by pTBS1, resulting in isolation of transformants of MI112 maintaining pTBS1 and RM125 maintaining either pTBS1 or pTBS1.1. Maximum tetracycline concentrations permitting growth of plasmidless MI112 and MI112 with pTBS1 were 4 and 10 micrograms per ml, respectively, while those of plasmidless RM125, RM125 with pTBS1 and RM125 with pTBS1.1 were 7, 50 and 80 micrograms per ml, respectively. It was interesting to note that the tetracycline-resistance level in E. coli conferred by the 5.1 kb fragment is higher than that conferred by the 4.4 kb fragment, but in B. subtilis the 4.4 kb fragment, in contrast, confers a higher level of tetracycline resistance. The level of tetracycline resistance in B. subtilis conferred by the cloned determinant clearly depends on the host strain. The tetracycline resistance conferred by the cloned determinant was associated with decreased accumulation of the drug into the cells. However, it was constitutive in E. coli, but inducible in B. subtilis. The cloned tetracycline-resistance determinant was detected specifically on the chromosome of B. subtilis Marburg 168 derivatives.

Nucleotide sequence analysis reveals similarities between proteins determining methylenomycin A resistance in Streptomyces and tetracycline resistance in eubacteria

Previous studies had localised the gene (mmr) for resistance to methylenomycin A (Mm) to a 2.5-kb PstI fragment in the middle of a cluster of Mm biosynthetic genes from the Streptomyces coelicolor plasmid SCP1. In this paper, the gene has been more precisely located by sub-cloning, and the nucleotide sequence of the whole fragment has been determined. The predicted mmr-specified protein (Mr 49238) would be hydrophobic, with some homology at the amino acid level to tetracycline-resistance proteins from both Gram-positive and Gram-negative bacteria. Comparisons of hydropathy plots of the amino acid sequences reinforces the idea that the proteins are similar. It is suggested that Mm resistance may be conferred by a membrane protein, perhaps controlling efflux of the antibiotic. No significant homology was detected by hybridisation analysis between mmr and a cloned oxytetracycline (OTc)-resistance gene (tetB) of the OTc producer Streptomyces rimosus, and no cross-resistance was conferred by these genes. Sequences on both sides of mmr appear to encode proteins. The direction of translation in each case would be opposite to that of mmr translation. This suggests that mmr is transcribed as a monocistronic mRNA from a bidirectional promoter. An extensive inverted repeat sequence between the stop codons of mmr and the converging gene may function as a bidirectional transcription terminator.