F factor conjugation is a true type IV secretion system

The F sex factor of Escheriehia 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 TrbCp, -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 TraGF, 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. (C) 2003 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.