title: F-like type IV secretion systems encode proteins with thioredoxin folds that are putative DsbC homologues.
abstract: F and R27 are conjugative plasmids of enteric bacteria belonging to the IncF and IncHI1 plasmid incompatibility groups, respectively. Based on sequence analysis, two genes of the F transfer region, traF and trbB, and three genes of the R27 transfer region, trhF, dsbC, and htdT, are predicted to encode periplasmic proteins containing a C-terminal thioredoxin fold. The C-X-X-C active-site motif of thioredoxins is present in all of these proteins except TraF(F). Escherichia coli carrying a dsbA mutation, which is deficient in disulfide bond formation, cannot synthesize pili and exhibits hypersensitivity to dithiothreitol (DTT) as monitored by mating ability. Overproduction of the E. coli disulfide bond isomerase DsbC, TrbB(F), DsbC(R27), or HtdT(R27), but not TraF(F) or TrhF(R27), reverses this hypersensitivity to DTT. Site-directed mutagenesis established that the C-X-X-C motif was necessary for this activity. Secretion into the periplasm of the C-terminal regions of TrbB(F) and DsbC(R27), containing putative thioredoxin folds, but not TrhF(R27), partially complemented the host dsbA mutation. A trbB(F) deletion mutant showed a 10-fold-lower mating efficiency in an E. coli dsbC null strain but had no phenotype in wild-type E. coli, suggesting redundancy in function between TrbB(F) and E. coli DsbC. Our results indicate that TrbB(F), DsbC(R27), and HtdT(R27) are putative disulfide bond isomerases for their respective transfer systems. TraF(F) is essential for conjugation but appears to have a function other than disulfide bond chemistry.
authors.affiliation: Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9.
authors.forename: Trevor C
authors.initials: TC
authors.lastname: Elton
authors.forename: Samantha J
authors.initials: SJ
authors.lastname: Holland
authors.forename: Laura S
authors.initials: LS
authors.lastname: Frost
authors.forename: Bart
authors.initials: B
authors.lastname: Hazes
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D001426:Bacterial Proteins / Q000235:genetics* / Q000502:physiology; D003227:Conjugation, Genetic / Q000235:genetics; D004220:Disulfides / Q000378:metabolism; D004229:Dithiothreitol / Q000494:pharmacology; D004926:Escherichia coli / Q000235:genetics*; D029968:Escherichia coli Proteins / Q000235:genetics / Q000502:physiology; D005144:F Factor / Q000235:genetics*; D010861:Fimbriae, Bacterial / Q000378:metabolism; D018628:Gene Dosage; D008954:Models, Biological; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D033903:Periplasmic Proteins / Q000235:genetics / Q000502:physiology; D010957:Plasmids / Q000235:genetics*; D019704:Protein Disulfide-Isomerases / Q000235:genetics / Q000502:physiology*; D011815:R Factors / Q000235:genetics*; D013879:Thioredoxins / Q000235:genetics*
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D004220:Disulfides; D029968:Escherichia coli Proteins; D033903:Periplasmic Proteins; C500309:TraF protein, E coli; D013879:Thioredoxins; D019704:Protein Disulfide-Isomerases; D004229:Dithiothreitol