Conservation of the PAPI-1 pil gene clusters in different microorganisms. The putative function of each PAPI-1 pilus protein is presented in Table 2. Homologous proteins in various bacteria are indicated by the same color. The numbers indicate the sequence similarity of each protein with its PAPI-1 homolog. The black arrows represent those genes lacking homologs in the PAPI-1 island. The gaps in PilQ2 and PilR2 of the P. aeruginosa (Pa) PAGI-5 represent the disruptions of ORFs by insertions. The pil cluster sequences were retrieved from GenBank: PAPI-1, accession no. AY273869; pKLC102, AY257538; PAGI-5, EF611301; R64 from Salmonella enterica serovar Typhimurium (St), NC_005014; and YAPI from Yersinia pseudotuberculosis (Yp), AJ627388. The pil cluster sequence of P. aeruginosa strain PA7, P. syringae (Ps) strain B782a, and P. fluorescens (Pf) strain Pf-5 were obtained from the Pseudomonas Genome Database (38).