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 TraFF. 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, TrbBF, DsbCR27, or HtdTR27, but not TraFF 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 TrbBF 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 TrbBF and E. coli DsbC. Our results indicate that TrbBF, DsbCR27, and HtdTR27 are putative disulfide bond isomerases for their respective transfer systems. TraFF is essential for conjugation but appears to have a function other than disulfide bond chemistry. Copyright © 2005, American Society for Microbiology. All Rights Reserved.