Evolutionary and functional diversity of the Pseudomonas type IVa pilin island

In Pseudomonas aeruginosa, most proteins involved in type IVa pilus (T4aP) biogenesis are highly conserved except for the major pilin PilA and the minor pilins involved in pilus assembly. Here we show that each of the five major pilin alleles is associated with a specific set of minor pilins, and unrelated strains with the same major pilin type have identical minor pilin genes. The sequences of the minor pilin genes of strains with group III and V pilins are identical, suggesting that these groups diverged recently through further evolution of the major pilin cluster. Both gene clusters are localized on a single 'pilin island' containing putative tRNA recombinational hotspots, and a similar organization of pilin genes was identified in other Pseudomonas species. To address the biological significance of group-specific differences, cross-complementation studies using group II (PAO1) and group III (PA14) minor pilins were performed. Heterologous minor pilins complemented twitching motility to various extents except in the case of PilX, which was non-functional in non-native backgrounds. A recombinant PA14 strain expressing the PAO1 minor pilins regained motility only upon co-introduction of the PA14 pilX gene. Comparison of PilX and PilQ secretin sequences from group II, III and V genomes revealed discrete regions of sequence that co-varied between groups. Our data suggest that changes in PilX sequence have led to compensatory changes in the PilQ secretin monomer such that heterologous PilX proteins are no longer able to promote opening of the secretin to allow pili to appear on the cell surface.