The type IV pre-pilin leader peptidase of Xanthomonas campestris pv campestris is functional without conserved cysteine residues

Type IV pre-pilin leader peptidase was demonstrated to be required for protein secretion, in addition to its involvement in biogenesis of type IV pill. The type IV pre-pilin leader peptidase gene of Xanthomonas campestris pv. campestris was located on a 3 kb Accl fragment on account of its hybridization with the DNA fragment containing the type IV pre-pilin leader-peptidase gene pilD/xcpA of Pseudomonas aeruginosa. Sequencing of the cloned fragment revealed an open reading frame (ORF) (designated xpsO) of 287 amino acid residues. A protein with an apparent molecular mass of approximately 32.5 kDa was synthesized in vitro from a DNA fragment containing the xpsO gene. The amino acid sequence shares 50% identity with that of PilD throughout the entire sequence. Among other type IV pre-pilin leader peptidases, XpsO is unique in not having the two conserved -CXXC-motifs in a cytoplasmic domain. Instead, new motifs were noted when the protein was compared with XpsE, which is another member of the extracellular protein-secretion machinery. When the xpsO gene was introduced into the pilD mutant of P. aeruginosa, both the sensitivity against infection with the pilus-specific phage PO4 and the ability to secrete extracellular protein were recovered. Furthermore, immunoblot analysis indicated that the P. aeruginosa pilin was apparently processed in vivo by the xpsO gene product.