F factor conjugation is a true type IV secretion system

Abstract

The F sex factor of Escherichia coli is a paradigm for bacterial conjugation and its transfer ( tra) region represents a subset of the type IV secretion system (T4SS) family. The F tra region encodes eight of the 10 highly conserved (core) gene products of T4SS including TraA F (pilin), the TraB F, -K F (secretin-like), -V F (lipoprotein) and TraC F (NTPase), -E F, -L F and TraG F (N-terminal region) which correspond to TrbC P, -I P, -G P, -H P, -E P, -J P, D P and TrbL P, respectively, of the P-type T4SS exemplified by the IncP plasmid RP4. F lacks homologs of TrbB P (NTPase) and TrbF P but contains a cluster of genes encoding proteins essential for F conjugation (TraF F, -H F, -U F, -W F, the C-terminal region of TraG F, and TrbC F) that are hallmarks of F-like T4SS. These extra genes have been implicated in phenotypes that are characteristic of F-like systems including pilus retraction and mating pair stabilization. F-like T4SS systems have been found on many conjugative plasmids and in genetic islands on bacterial chromosomes. Although few systems have been studied in detail, F-like T4SS appear to be involved in the transfer of DNA only whereas P- and I-type systems appear to transport protein or nucleoprotein complexes. This review examines the similarities and differences among the T4SS, especially F- and P-like systems, and summarizes the properties of the F transfer region gene products.