Type IV pilus genes pilA and pilC of Pseudomonas stutzeri are required fornatural genetic transformation, and pilA can be replaced by corresponding genes from nontransformable species

Pseudomonas stutzeri lives in terrestrial and aquatic habitats and is capab le of natural genetic transformation. After transposon mutagenesis, transfo rmation-deficient mutants were isolated from a P. stutzeri JM300 strain. In one of them a gene which coded for a protein with 75% amino acid sequence identity to PilC of Pseudomonas aeruginosa, an accessory protein for type I V pilus biogenesis, was inactivated. The presence of type IV pili was demon strated by susceptibility to the type N pilus-dependent phage PO4, by occur rence of twitching motility, and by electron microscopy. The pilC mutant ha d no pili and was defective in twitching motility. Further sequencing revea led that pilC is clustered in an operon with genes homologous to pilB and p ilD of P. aeruginosa, which are also involved in pilus formation. Next to t hese genes but transcribed in the opposite orientation a pilA gene encoding a protein with high amino acid sequence identity to pilin, the structural component of type IV pili, was identified. Insertional inactivation of pilA abolished pilus formation, PO4 plating, twitching motility, and natural tr ansformation. The amounts of H-3-labeled P. stutzeri DNA that were bound to competent parental cells and taken up were strongly reduced in the pilC an d pilA mutants. Remarkably, the cloned piLA genes from nontransformable org anisms like Dichelobacter nodosus and the PAK and PAO strains of P. aerugin osa fully restored pilus formation and transformability of the P. stutzeri pilA mutant (along with PO4 plating and twitching motility). It is conclude d that the type IV pili of the soil bacterium P. stutzeri function in DNA u ptake for transformation and that their role in this process is not confine d to the species-specific pilin.