Plasmids pick a bacterial partner before committing to conjugation

Abstract

Bacterial conjugation was first described by Lederberg and Tatum in the 1940s following the discovery of the F plasmid. During conjugation a plasmid is transferred unidirectionally from one bacterium (the donor) to another (the recipient), in a contact-dependent manner. Conjugation has been regarded as a promiscuous mechanism of DNA transfer, with host range determined by the recipient downstream of plasmid transfer. However, recent data have shown that F-like plasmids, akin to tailed Caudovirales bacteriophages, can pick their host bacteria prior to transfer by expressing one of at least four structurally distinct isoforms of the outer membrane protein TraN, which has evolved to function as a highly sensitive sensor on the donor cell surface. The TraN sensor appears to pick bacterial hosts by binding compatible outer membrane proteins in the recipient. The TraN variants can be divided into specialist and generalist sensors, conferring narrow and broad plasmid host range, respectively. In this review we discuss recent advances in our understanding of the function of the TraN sensor at the donor-recipient interface, used by F-like plasmids to select bacterial hosts within polymicrobial communities prior to DNA transfer.

Graphical Abstract

Mating pair stabilization and conjugation species specificity is mediated by specific pairing of a TraN sensor in the donor with an outer membrane protein in the recipient.

Figure 1.

Genetic arrangement, function and subcellular localization of each gene products encoded by the F plasmid tra operon.

Figure 2.

Conservation analysis of TraN. Sequence conservation of TraNs mapped onto the TraN encoded by the pKpQIL, as calculated by Consurf (61,62) (A) and the AlphaFold model (B). The conservation increases from green to purple. TraN is divided into three functional regions: the base, which anchors the protein to the outer membrane, the scaffold tip, and a distal sensor. The base shows the highest degree of sequence conservation whereas the tip and sensor the least.

Figure 3.

The TraN sensors. (A) The predicted structures of the evolved F-like plasmid-encoded TraNα, TraNβ, TraNγ and TraNδ tip sensors. The tip scaffold consists of conserved β-sheets, shown by gray ribbons. The colored structural motifs represent the surface exposed TraN sensor, each binding a specific receptor on the surface of the recipient. (B) The different TraN sensors in the donor (D), which recognize distinct OMPs in the recipient (R), mediate plasmid spread and conjugation species specificity. (C) The crystal structures of the K. pneumoniae OmpK36 (PDB ID: 6RD3) (59) and the E. coli OmpC (PDB ID: 2J1N) (63) are highly similar, yet the TraNβ sensor specifically recognizes recipients expressing OmpK36 but not OmpC.

Figure 4.

A phylogenetic tree of TraN. The tree (made with IQ-Tree) (64) consists of 639 TraN protein sequences (clustered following sequence alignment with Clustal Omega). These include 632 from Uniprot, filtered with 500–800 amino acids, ≤27 Cys residues and ≥ 30% amino acid similarity to TraN from the F plasmid. An additional seven TraN sequences from the R100-1 (TraNα1), pSLT (TraNα2), pKpQIL (TraNβ1), MV2 (TraNβ2), F (TraNγ), MV1 (TraNδ1) and MV3 (TraNδ2) reference plasmids were included and highlighted. Metadata blocks show the host genus of the plasmid and the number of Cys residues in TraN. The scale bar represents the number of substitutions per site. Each entry is associated with a UniProt accession code, and the structure prediction for each variant is available at: https://alphafold.ebi.ac.uk. An interactive version of the tree is available at: https://microreact.org/project/tran.