The novel Pseudomonas aeruginosa type IV pilin accessory genes tfp and tfpZ affect pilus assembly dynamics

Abstract

Pseudomonas aeruginosa uses type IV pili (T4P) to colonize various materials and for surface-associated twitching motility. We previously identified five phylogenetically-distinct alleles of pi/A in P. aeruginosa, four of which occur in genetic cassettes with specific accessory genes (Kus et al., Microbiology 150:1315-1326, 2004). Each of the five pilin alleles, with and without its associated pilin accessory gene, was used to complement a group II PA01 pi/A mutant. Expression of group I or IV pi/A genes restored twitching motility to the same extent as the PA01 group II pilin. In contrast, complementation with group Ill or group V pi/A genes resulted in poor twitching that increased significantly when the cognate tfp Y or tfpZ accessory genes were cointroduced. The enhanced motility was linked to an increase in recoverable surface pili, and not to alterations in total pilin pools. Expression of the pilin genes, with or without accessory genes, in a PA01 pi/A-pi/T double mutant background resulted in expression of large amounts of surface pili, suggesting that the accessory proteins function to modulate pilin retraction dynamics. Reduction of twitching motility and surface piliation was also observed a tfpYknockout mutant of group Ill strain PA14, confirming that the accessory proteins enhance pilus assembly on the cell surface. The accessory proteins are specific for their cognate pilins; a PilAv-TfpY chimera produced few surface pili, resembling the phenotype of PA01 complemented with pi!Av alone. The linkage between specific pilin and accessory genes may be evolutionarily conserved because the accessory proteins antagonize pilus retraction, increasing pilus expression on the cell surface and thereby enhancing function.