An updated view of plasmid conjugation and mobilization in Staphylococcus

Abstract

The horizontal gene transfer facilitated by mobile genetic elements impacts almost all areas of bacterial evolution, including the accretion and dissemination of antimicrobial-resistance genes in the human and animal pathogen Staphylococcus aureus. Genome surveys of staphylococcal plasmids have revealed an unexpected paucity of conjugation and mobilization loci, perhaps suggesting that conjugation plays only a minor role in the evolution of this genus. In this letter we present the DNA sequences of historically documented staphylococcal conjugative plasmids and highlight that at least 3 distinct and widely distributed families of conjugative plasmids currently contribute to the dissemination of antimicrobial resistance in Staphylococcus. We also review the recently documented "relaxase-in trans" mechanism of conjugative mobilization facilitated by conjugative plasmids pWBG749 and pSK41, and discuss how this may facilitate the horizontal transmission of around 90% of plasmids that were previously considered non-mobilizable. Finally, we enumerate unique sequenced S. aureus plasmids with a potential mechanism of mobilization and predict that at least 80% of all non-conjugative S. aureus plasmids are mobilizable by at least one mechanism. We suggest that a greater research focus on the molecular biology of conjugation is essential if we are to recognize gene-transfer mechanisms from our increasingly in silico analyses.

Keywords: MRSA; Mob; antibiotic resistance; horizontal gene transfer; mating pore; mobilization; pGO1; pSK41; plasmid; relaxase; type IV secretion.

Figure 1.

Mechanisms of conjugative mobilization in Staphylococci. The conjugative plasmid encodes all genes required for formation of the mating pore, as well as the coupling protein, DNA relaxase and an oriT. Mobilizable plasmids can exploit the conjugative-plasmid mating pore by either: (A) encoding a mimic sequence of the conjugative-plasmid oriT; (B) encoding a distinct relaxase (Mob) compatible with the conjugative plasmids coupling protein and its own cognate oriT or; (C) by carrying a replicative relaxase (Rep) compatible with the conjugative-plasmid coupling protein.

Figure 2.

pWBG4, a third family of staphylococcal conjugative plasmids. The internal circle represents the gene map of pWBG4, showing the positions and predicted products of the putative pWBG4 conjugation cluster detA-detV (white arrows with black outlines) and other open-reading frames. The outer circles represent ungapped circular BLASTN alignments of pWBG4-family plasmids, created using BRIG software.

Figure 3.

The diversity of oriT mimics on large and small resistance plasmids in staphylococci. (A) The plasmid map of the rolling-circle plasmid pNewbould305, illustrates the presence of 3 potential mobilization mechanisms. The repB gene of pNewbould305 shares 56% amino-acid identity over 96% of its length with the pBS42/pUB110 RepB protein, which enables mobilization by ICEBs1-family elements. Downstream of the repB gene is an oriT mimic sequence of the pWBG749-family, subfamily OT45 and a pSK41-like oriT mimic sequence. (B) The atypically large staphylococcal plasmid pWBG762, carries 4 oriT mimics. Three are of the pWBG749 family and one is of the pSK41 family. (C) Alignment of the pWBG749-family oriT mimic sequences carried by pNewbould305 and pWBG762 below the pWBG749 oriT region, illustrating IR2 sequence divergence. Conserved nucleotides are shaded. The AR1-AR3 repeats of the full oriT required for mobilization by pWBG749 have been truncated in this figure for clarity (D) Alignment of the pSK41-like oriT mimic sequences from pNewbould305 and pWBG762, below the pSK41 oriT region, showing divergence of the IR sequence; the Nes relaxase nick site is denoted by a vertical arrowhead.