Spread and Persistence of Virulence and Antibiotic Resistance Genes: A Ride on the F Plasmid Conjugation Module

Abstract

The F plasmid or F-factor is a large, 100-kbp, circular conjugative plasmid of Escherichia coli and was originally described as a vector for horizontal gene transfer and gene recombination in the late 1940s. Since then, F and related F-like plasmids have served as role models for bacterial conjugation. At present, more than 200 different F-like plasmids with highly related DNA transfer genes, including those for the assembly of a type IV secretion apparatus, are completely sequenced. They belong to the phylogenetically related MOB_F12A group. F-like plasmids are present in enterobacterial hosts isolated from clinical as well as environmental samples all over the world. As conjugative plasmids, F-like plasmids carry genetic modules enabling plasmid replication, stable maintenance, and DNA transfer. In this plasmid backbone of approximately 60 kbp, the DNA transfer genes occupy the largest and mostly conserved part. Subgroups of MOB_F12A plasmids can be defined based on the similarity of TraJ, a protein required for DNA transfer gene expression. In addition, F-like plasmids harbor accessory cargo genes, frequently embedded within transposons and/or integrons, which harness their host bacteria with antibiotic resistance and virulence genes, causing increasingly severe problems for the treatment of infectious diseases. Here, I focus on key genetic elements and their encoded proteins present on the F-factor and other typical F-like plasmids belonging to the MOB_F12A group of conjugative plasmids.

Figure 1

A genetic and functional map of the F plasmid is shown. DNA backbone of F and F-like plasmids from the MOB_F12A group is characterized by the presence of the indicated functional modules. Replication (turquoise): RepFIA, RepFIB, and RepFII (disrupted by Tn1000, also termed RepFIC). Partitioning (dark blue): sopABC genes encode a type I ATPase partitioning system. Toxin-antitoxin (TA) modules (pink): Two type I (I) and two type II (II) TA systems are indicated. DNA transfer region: DNA transfer genes represent the largest (approximately 30 kbp) part of the backbone. For a detailed and complete representation of DNA transfer genes, see Fig. 2. oriT: origin of DNA transfer. Leading region (yellow): Genes with known functions for the establishment of the plasmid in a new host are indicated. Cargo genes: Three cargo genes with known or inferred virulence functions are shown (ompP, ychA, ychB). IS sequences and transposons are indicated. Note that finO is disrupted by an IS3 element. In all other MOB_F12A plasmids known so far, finO has remained intact. This map was drawn according to the DNA sequence, accession number AP001918, with SnapGene software.

Figure 2

DNA transfer region of plasmid F. (A) DNA transfer genes and important sequence elements encoded by the F plasmid are shown. tra genes are indicated by capital letters, whereas trb genes are indicated by small initial letters. Genes encoding relaxosomal components and those involved in transfer gene regulation are colored blue or red, respectively. (B) Conserved T4S genes that are present in the prototypical A. tumefaciens P-type T4SS (virB1, virB3-virB10, virD4) are indicated at the positions corresponding to the F-type T4S genes. (C) F-pilus assembly genes that are characteristic for MOB_F12A plasmids are shown. Except for traA (virB2), they are not present in P-type T4SS. (D) Genes encoding mating pair stabilization and surface/entry exclusion are shown. traT as well as vapBC (a TA system) have also been shown to encode virulence factors.

Figure 3

A cargo gene region from the classical MOB_F12A antibiotic resistance plasmids R1 and R100 is shown. In both cases, this resistance gene region is dominated by the composite transposon Tn21 that is flanked by IS1 elements carrying a catA1 gene. Contained within this transposon are mercury resistance genes as well as a class I integron. The basic structure shown is from plasmid R100. R1 does not contain IS1353 in the integron, but additionally has a Tn3 inserted in the merP gene. Thus, this region encodes resistance to chloramphenicol (catA1), sulfonamides (sul1), streptomycin (aadA) in both plasmids, and in R1 additionally to ampicillin (blaTEM-1).

Figure 4

A simplified model of a F-type T4S machine with an attached F-pilus is depicted. The overall structure, shape, and dimensions are drawn according to a published P-type T4S structure (152). Protein components as determined for the P-type T4SS are indicated and labeled according to the A. tumefaciens VirB protein nomenclature. Positions of the indicated F-type T4S proteins are inferred from sequence similarity and experimental data (see text). The attached F-pilus is drawn according to a recently published high-resolution cryo-EM structure (160). The F-pilus has a diameter of 8.7 nm and an inner lumen of 2.8 nm. For each pilin, there is a phosphatidylglycerol (PG) molecule in the polymeric pilus filament. Whereas the pilus could be assembled and disassembled through the periplasm as indicated by two black arrows, TraI and the covalently attached ssDNA are transported via the coupling protein, TraD (pink arrow). OMC: outer membrane complex; IMC: inner membrane complex.