Type II protein secretion by Pseudomonas aeruginosa:: genetic suppression of a conditional mutation in the pilin-like component XcpT by the cytoplasmic component XcpR

Pseudomonas aeruginosa exports a number of hydrolytic enzymes and toxins using the type ii or general secretion pathway, found in a variety of Gram-negative bacteria and requiring the functions of at least 12 gene products (XcpP-Z and PilD/XcpA in P. aeruginosa). A number of these gene products are homologues of components of the type IV pilus biogenesis system, including four proteins, XcpT-W, which are highly similar to the pilin subunit in their size, localization and post-translational modifications. These proteins, in addition to the pilin subunit, are cleaved and methylated by the PilD/XcpA prepilin peptidase, but their interactions with other components of the export apparatus are unclear. Using a medium developed for the selection of export-proficient P. aeruginosa strains, we have isolated temperature-sensitive mutations in the xcpT gene and extragenic suppressors for one of the mutants. These suppressors fall into two classes, one that maps outside of the xcpP-Z gene cluster and may define additional cellular functions that are required for export, and a second that maps to the xcpR gene product and indicates a potential protein-protein interaction connecting two different cellular compartments and required for the assembly or function of the export apparatus.