Genetic analysis of transfer and incompatibility functions within the IncHI plasmid R27

Characterization of two autoreplicative regions of the IncHI2 plasmid R478: RepHI2A and RepHI1A((R478))

Plasmids of the incompatibility groups HI and HII (IncH plasmids) generally confer multiple antibiotic resistances upon their host pathogenic strain. IncHI group plasmids are distinguished by their property of optimal transfer by conjugation at temperatures below 30 degrees C, allowing for the spread of multiple antibiotic resistance outside their host natural environment, the gut. Plasmids of the IncHI1 subgroup encode multiple replicons. In the present study it is shown that the prototype IncHI2 subgroup plasmid, R478, contains at least two iteron-controlled autoreplicative regions, RepHI2A and RepHI1A((R478)). The DNA sequence and the molecular characteristics of each replicon region are described. RepHI2A is unique to plasmids of the IncHI2 subgroup and contains an unusually large number of iteron sequences downstream of the replication initiator gene. The nucleotide sequence of the replication initiator gene and of the iterons within RepHI1A((R478)) show very close similarity with those of the previously reported RepHI1A replicon of the IncHI1 subgroup plasmid R27. The presence of RepHI1A((R478)) on R478 most likely accounts for the observed incompatibility between R478 and plasmids of the IncHI1 subgroup. These are the first autoreplicative regions from an IncHI2 subgroup plasmid to be described.

Mapping studies and genetic analysis of transfer genes of the multi-resistant IncHI2 plasmid, R478

A bank of transfer-defective Tn7 insertion mutants of the multi-resistant IncHI2 megaplasmid, R478, was generated. Complementation analysis of these mutations identified a large 144-kb transfer-associated region of R478. A 6.8-kb segment from the transfer region was sequenced. The precise locations of Tn7 insertion within four distinct R478::Tn7 transfer-defective mutants were mapped and each insertion was found to disrupt a specific open reading frame. These transfer-associated determinants of R478 were designated htdB (H transfer determinant), htdD, htdT and htdC. Both htdB and htdC encoded amino acid sequences that showed a low homology with pilus biosynthetic proteins encoded by the F plasmid.

Sequencing and characterization of Salmonella typhi plasmid R27 (incompatibility group HI1) trhC, a transfer gene encoding a potential nucleoside triphosphate-binding domain

IncHI1 plasmids are one of the few plasmids known to mediate multiple antibiotic resistance in Salmonella typhi. These plasmids are temperature-sensitive for transfer and R27 is the prototype plasmid. DNA sequencing within the Tra2 region of R27, encoding genes involved in mating pair formation, identified trhC encoding the TrhC protein of 90,000 Da, which was visualized using an in vitro transcription/translation system. Expression of the TrhC protein also identified two smaller protein products of approximately 23 and 25 kDa which are predicted to be protease digestion products. The migration of these smaller products changed when the reactions were run at 28 vs 37 degrees C. The TrhC protein contained a putative nucleotide triphosphate-binding protein and exhibited sequence similarities with several other proteins implicated in bacterial conjugation, including TraC (F), TraB (pKM101), VirB4 (Ti), TrbE (RP4). Phylogenetic analysis showed TrhC was most closely related to TraC. Mini-Tn10 insertions in trhC generated transfer defective mutants, and no pili were specified by the trhC mutants. The trhC gene appeared to be a hot spot for transposon insertion as 37.5% mapped into this open reading frame. One trhC mutant, pDT2990, was able to be complemented by a cloned trhC gene giving a transfer frequency of 1 x 10(-3) transconjugants per recipient in an 18-h mating, whereas the wild-type transfer frequency of R27 was 1 x 10(-2) transconjugants per recipient.

Effect of lipopolysaccharide mutations on recipient ability of Salmonella typhimurium for incompatibility group H plasmids

Previous investigations of the incompatibility group F, P, and I plasmid systems revealed the important role of the outer membrane components in the conjugal transfer of these plasmids. We have observed variability in transfer frequency of three incompatibility group H plasmids (IncHI1 plasmid R27, IncHI2 plasmid R478, and a Tn7 derivative of R27, pDT2454) upon transfer into various Salmonella typhimurium lipopolysaccharide (LPS) mutants derived from a common parental strain, SL1027. Recipients with truncated outer core via the rfaF LPS mutation increased the transfer frequency of the IncH plasmids by up to a factor of 10(3). Mutations which resulted in the truncation of the residues following 3-deoxy-D-manno-octulosonic acid, such as the rfaE and rfaD mutations, decreased the transfer frequency to undetectable levels. Addition of phosphorylethanolamine, a component of wild-type LPS, to the media decreased the frequency of transfer of R27 into wild-type and rfaF LPS mutant recipients tested. Reversing the direction of transfer, by mating LPS mutant donors with wild-type recipients, did not affect the frequency of transfer compared to the standard matings of wild-type donor with LPS mutant recipient. These findings demonstrate that conjugation interactions affected by LPS mutation are not specific for the recipient cell. Our results suggest that LPS mutation does not affect conjugation via altered pilus binding but affects some later steps in the conjugative process, and alteration of transfer frequency by O-phosphorylethanolamine and LPS truncation is due to charge-related interactions between the donor and recipient cell.

Genetic and nucleotide sequence analysis of the gene htdA, which regulates conjugal transfer of IncHI plasmids

IncHI plasmids are naturally repressed for conjugative transfer and do not allow efficient propagation of the IncH pilus-specific phage Hgal. Transposons Tn7, Tn5, and TnlacZ were inserted into IncHI plasmids R478, R477-1, and R27, respectively, leading to the isolation of several plasmid mutants which exhibited increased levels of transfer and also permitted good lysis with phage Hgal. A 4.3-kb HindIII fragment from R478 reversed both phenotypic effects of derepression for the R477-1::Tn5 and the R478::Tn7 derivatives, pKFW99 and pKFW100, respectively. Exonuclease III deletions of this fragment and nucleotide sequence analysis indicated that the gene responsible for transfer repression, named here htdA, encoded a polypeptide of 150 amino acids. Cloning and sequence analysis of pDT2454 (R27::TnlacZ) revealed that the transposon had inserted into an open reading frame (ORF) which had an 83% amino acid identity with the R478 htdA gene. Maxicell analysis showed both the R27 and R478 HtdA products had molecular masses of 19.9 kDa. Conjugation experiments showed that the cloned htdA determinants caused a significant reduction of the transfer frequencies of wild-type R478 and R27 plasmids. Examination of both R478 derepressed mutants, pKFW100 and pKFW101, indicated that both transposon insertions occurred upstream of the htdA ORF. The results suggest that HtdA is a regulatory component of IncH plasmid transfer and also show that the region upstream of the htdA ORF is involved in transfer repression. The locations of the htdA determinants were identified on the plasmid maps of R27 and R478.

Molecular analysis of RepHI1A, a minimal replicon of the IncHI1 plasmid R27

R27, a large conjugative plasmid of the HI incompatibility group, was subjected to a subcloning analysis which revealed the presence of a Poll-independent replicon and determinants contributing to incompatibility within a 2.7 kb SalI/XbaI fragment. The DNA sequence of the minimal replicon revealed the presence of a large open reading frame (ORF) as well two sets of 19 bp repeated oligonucleotides (iterons), in addition to characteristic Escherichia coli origin elements. The protein encoded by the ORF possesses homology with replication initiator proteins encoded by a number of plasmids from different incompatibility groups. Deletion analysis suggested that the iterons are responsible for incompatibility reactions. Dissection of the replicon confirmed this and defined a minimal origin of 230 bp. The putative replication initiator was expressed in an in vitro transcription-translation system, and the 5' end of the mRNA encoding its synthesis was identified. Transcriptional fusion of the repA promoter to lacZ demonstrated an autoregulatory function of RepA. A series of iterons present downstream of the RepA coding sequence are dispensable but are responsible for copy-number control. The minimal replicon appears to be partition-defective.

Isolation and location on the R27 map of two replicons and an incompatibility determinant specific for IncHI1 plasmids

Two replicons were isolated independently from different IncHI1 plasmids. One was isolated from R27, and a second was isolated from pIP522. We demonstrate, by DNA-DNA hybridization experiments, that these maintenance regions are different and that they are specific to, and carried by, all IncHI1 plasmids tested. In view of this specificity we decided to designate the replicon isolated from R27 as RepHI1A and the replicon isolated from pIP522 as RepHI1B. These two autoreplicative regions are not related to a third replicon present in all IncHI1 plasmids that bears homology with RepFIA and that expresses the characteristic incompatibility of IncHI1 subgroup plasmids toward F factor (D. Saul, D. Lane, and P. L. Bergquist, Mol. Microbiol. 2:219-225, 1988; D. E. Taylor, R. W. Hedges, and P. L. Bergquist, J. Gen. Microbiol. 131:1523-1530, 1985). These results demonstrate that all IncHI1 plasmids tested contain at least three replicons. An incompatibility (Inc) region that hybridizes specifically to all the IncHI1 plasmids was previously isolated (M. Couturier, F. Bex, P. L. Bergquist, and W. K. Maas, Microbiol. Rev. 52:375-395, 1988). Although this Inc locus is not located in an autoreplicative region of IncHI1 plasmids, we observed that this locus stabilizes a low-copy-number replicon. This Inc locus is probably a component of an active partition locus involved in the maintenance of IncHI1 plasmids. The nucleotide sequence of the Inc region contains direct repeats of 31 bp. In addition, this incompatibility determinant hybridizes specifically with IncHI1 plasmids but expresses incompatibility toward plasmids of both IncHI subgroups (IncHI1 and IncHI2). In this communication, we present the mapping of these maintenance elements on the R27 genome.

Restriction endonuclease mapping of the HI2 incompatibility group plasmid R478

A restriction map of the 272-kb IncHI2 plasmid R478 was constructed by using the enzymes ApaI, XbaI, SalI, and XhoI. The map was derived from cloned restriction fragments from R478 inserted into cosmid and plasmid vectors as well as from double-digestion analysis of R478 and R478 miniplasmids. All previously known resistance determinants were cloned from R478, and their positions were located on the restriction map. A region involved in incompatibility was cloned and mapped. The location of a previously unreported arsenite resistance gene was also determined. The genes encoding tellurite resistance, colicin B resistance, and phage inhibition were found to be associated with a 6.7-kb SalI fragment of R478.