Conjugation between bacterial and mammalian cells

Bacterial conjugation, in which DNA is transferred from one bacterium to another, was first reported in 1946 and found to be mediated by the F factor. Although the F and RK2/RP4 prototypic plasmids can mediate the transfer of DNA from bacteria to yeast, there has been no evidence of classical bacterial conjugation to higher eukaryotes. Here, I present evidence of such transfer, using Escherichia coli, the RK2 plasmid system and Chinese hamster ovary CHO K1 cells.

Conjugative transfer in the dissemination of beta-lactam and aminoglycoside resistance

The dissemination of antibiotic resistance among pathogenic bacteria can be attributed largely to conjugative DNA transfer. The general category of conjugative transfer includes both bacterial plasmid conjugation and the transfer of nonreplicative conjugative transposons. Prototypes for these two systems are the plasmid RK2 and the conjugative transposon Tn916. To address the long-term problem of the increasing prevalence and severity of antibiotic resistance, strategies aimed against conjugative transfer are needed, but their development will require a greater understanding of conjugative resistance gene acquisition. Overviews of the two conjugative transfer systems are presented, to summarize and compare current concepts. Observations regarding transfer of conjugative transposons is consistent with the prevailing model for plasmid conjugation, that is, by the transfer of a single-stranded DNA molecule from the donor to the recipient bacterium, and the generation of the single strand by rolling circle DNA replication. The relevance of vegetative plasmid replication and host range to the spread of multiple drug resistance is discussed, and clinical examples of conjugative transfer of multiple antibiotic resistance illustrate the severity of the current situation. Possible directions, traditional and innovative, are offered to address the conjugative transfer problem in drug resistance, and potentially to break the cycle of antibiotic development followed by the bacterial resistance gene acquisition.

Processes at the nick region link conjugation, T-DNA transfer and rolling circle replication

Data from prokaryotic replicative and conjugative systems, which interrelate DNA processing events initiated by a site-specific nick, are reviewed. While the replicative systems have been established in accordance with the rolling circle replication model, the mechanism of conjugative replication has not been elucidated experimentally. We summarize data involving random point mutagenesis of the RK2 transfer origin (oriT), which yielded relaxation-deficient and transfer-deficient derivatives having mutations exclusively in a 10bp region defined as the nick region. Features of the RK2 (IncP) nick region, including the DNA sequence, nick site position, and 5' covalent attachment of the nicking protein, have striking parallels in other systems involving nicking and mobilization of single-stranded DNA from a supercoiled substrate. These other systems include T-DNA transfer occurring in Agrobacterium tumefaciens Ti plasmid-mediated tumorigenesis in plants, and the rolling circle replication of plasmids of Gram-positive bacteria and of phi X174-like bacteriophage. The structural and functional similarities suggest that IncP conjugative replication, originating at the oriT, and T-DNA transfer replication, originating at the T-DNA border, produce continuous strands via a rolling circle-type replication.

Mutational analysis of essential IncP alpha plasmid transfer genes traF and traG and involvement of traF in phage sensitivity

Although the broad-host-range IncP plasmids can vegetatively replicate in diverse gram-negative bacteria, the development of shuttle vector systems has established that the host range for IncP plasmid conjugative transfer is greater than the range of bacteria that sustain IncP replicons. Towards understanding IncP plasmid conjugation and the connection between IncP conjugation and Agrobacterium tumefaciens T-DNA transfer to plants, two sets of mutants were generated in the larger transfer region (Tra1) of the IncP alpha plasmid RK2. Mutagenesis strategies were chosen to minimize transcriptional polar effects. Mutant Tra1 clones were mapped, sequenced, and processed to reconstruct 49.5-kb Tra2-containing plasmid derivatives in order to assay for transfer activity and IncP plasmid-specific phage sensitivity. Focusing on the activities of the gene products of traF and traG in Escherichia coli, we found that mutations in traF abolished transfer activity and rendered the host cells phage resistant and mutations in traG abolished transfer activity but had no effect on phage sensitivity. Complementation of these mutant derivatives with corresponding trans-acting clones carrying traF or traG restored transfer activity and, in the case of the traF mutant, the phage sensitivity of the host cell. We conclude that in E. coli, both TraF and TraG are essential for IncP plasmid transfer and that TraF is necessary (but not sufficient) for donor-specific phage sensitivity, and sequencing data suggest that both TraF and TraG are membrane spanning.

Dissection of IncP conjugative plasmid transfer: definition of the transfer region Tra2 by mobilization of the Tra1 region in trans

We constructed a transfer system consisting of two compatible multicopy plasmids carrying the transfer regions Tra1 and Tra2 of the broad-host-range IncP plasmid RP4. In this system, the plasmid containing the Tra1 region with the origin of transfer (oriT) was transferred, whereas additional functions essential for the conjugative process were provided from the Tra2 plasmid in trans. The Tra2 region, as determined for matings between Escherichia coli cells, maps between coordinates 18.03 and 29.26 kb of the RP4 standard map. The section of Tra2 required for mobilization of the plasmid RSF1010 (IncQ) and the propagation of bacteriophages Pf3 and PRD1 appears to be the same as that needed for RP4 transfer. Tra2 regions of RP4 (IncP alpha) and R751 (IncP beta) are interchangeable, facilitating mobilization of the plasmid carrying the RP4 Tra1 region. The transfer frequencies of both systems are similar. Transcription of Tra2 proceeds clockwise relative to the standard map of RP4 and is probably initiated at a promoter region located upstream of trbB (kilB). From this promoter region the trfA operon and the Tra2 operon are likely to be transcribed divergently. A second potential promoter has been located immediately upstream of trbB (kilB). Plasmids encoding the functional Tra2 region can only be maintained stably in host cells in the presence of the RP4 regulation region carrying the korA-korB operon or part of it. This indicates the involvement of RP4 key regulatory functions that apparently are active not only in the control of replication but also in conjugation.

A natural mutant of plasmid RP4 that confers phage resistance and reduced conjugative transfer

A natural isolate of RP4 (PRC#116) acquired from the Stanford University Plasmid Reference Center differed from the wild-type Incompatibility Group P plasmid in several respects. Cells of Escherichia coli harboring PRC#116 were resistant to the IncP pili-specific bacteriophage PRD1 and GU5, and transferred this plasmid at a lower efficiency than the wild-type RP4. Phage sensitivity was restored, and transfer considerably improved in PRC#116+ bacteria transformed with plasmid constructs containing the origin of transfer (oriT region) of RP4. Mutant RP4 plasmids equivalent to PRC#116 were selected at a high frequency from an RP4+ E. coli population infected with PRD1 indicating that this RP4 variant may be the product of a very common mutation of the wild-type plasmid.

Colicin V virulence plasmids

ColV plasmids are a heterogeneous group of IncFI plasmids which encode virulence-related properties such as the aerobactin iron uptake system, increased serum survival, and resistance to phagocytosis. These plasmids have been found in invasive strains of Escherichia coli which infect vertebrate hosts including humans and livestock. Colicin V was the first colicin to be identified, in 1925, but not until the field experienced a renewed interest has the mechanism of colicin V activity been explored. As encoded by ColV plasmid pColV-K30, the aerobactin iron uptake system has been extensively investigated, but other ColV-encoded phenotypes remain largely uncharacterized. Restriction enzyme mapping of the 144-kb pColV-K30 and of the 80-kb pColV-B188 has facilitated systematic study, so that questions can be addressed by a molecular and comparative approach regarding the contributions of individual factors and plasmids to the virulence of host E. coli in model systems. The family of large ColV plasmids could be analogous to other families of large virulence plasmids, and insights gained from studying these plasmids should contribute to our understanding of cross-genetic interactions and the role of large plasmids in bacterial pathogenesis.

CoIV plasmids pCoIV-B188 and pCoIV-K30: genetic maps according to restriction enzyme sites and landmark phenotypic characteristics

The CoIV plasmids are large virulence plasmids of the incompatibility group IncFI. We have obtained the genetic maps of two of the most studied CoIV plasmids, pCoIV-B188 and pCoIV-K30, according to restriction enzyme sites and landmark phenotypic characteristics such as colicin V, the aerobactin iron uptake system, the transfer region, replication regions, and repeated sequences. Although the two plasmids differ in size (pCoIV-B188 is 80 kb and pCoIV-K30 is 144 kb), the maps reveal many regions which are apparently identical or very similar.

Divergence of the aerobactin iron uptake systems encoded by plasmids pColV-K30 in Escherichia coli K-12 and pSMN1 in Aerobacter aerogenes 62-1

Although the aerobactin-mediated iron uptake system has been characterized genetically in Escherichia coli, the siderophore aerobactin was chemically characterized after purification from culture supernatants of Aerobacter aerogenes 62-1, a member of the Klebsielleae. We have cloned and mapped the genes encoding the aerobactin system genes of A. aerogenes 62-1 and begun characterization of the relevant proteins and enzymatic activities of this plasmid-mediated aerobactin system. Published chemical data indicate that the siderophore aerobactin of E. coli is the same molecule as the aerobactin of Aerobacter aerogenes 62-1, but we have found that both the genes and the complement of proteins making up the biosynthetic enzymes in the two systems have diverged. In contrast, the outer membrane receptors for ferric aerobactin of the two systems showed immunologic cross-reactivity, were of the same molecular size (74 kilodaltons), and were encoded by homologous DNA sequences.

DNA environment of the aerobactin iron uptake system genes in prototypic ColV plasmids

The aerobactin iron uptake system genes in the prototypic plasmid pColV-K30 are flanked by inverted copies of insertion sequence IS1 and by two distinct replication regions. To address the question of how these flanking regions may facilitate the maintenance and spread of the aerobactin system among the plasmids and chromosomes of enteric species, we investigated the DNA environment of 12 ColV plasmids. We found that the aerobactin system-specific genes are conserved in every plasmid phenotypically positive for the aerobactin system. The upstream IS1 and its overlapping replication region (REPI) are also conserved. This replication region was cloned from several ColV plasmids and found to be functional by transforming these cloned derivatives into a polA bacterial host. In contrast, the downstream flanking region is variable. This includes the downstream copy of IS1 and the downstream replication region (REPII). We infer from these results that sequences in addition to the two flanking copies of IS1, in particular the upstream region including REPI, have been instrumental in the preservation and possible spread of aerobactin genes among ColV plasmids and other members of the FI incompatibility group.