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Previously, we had isolated by transposon mutagenesis aLegionella pneumophila mutant that appeared defective for intracellular iron acquisition. While sequencing in the proximity of the mini-Tn10 insertion, we found a locus that had a predicted protein product with strong similarity to PilB fromPseudomonas aeruginosa . PilB is a component of the type II secretory pathway, which is required for the assembly of type IV pili. Consequently, the locus was cloned and sequenced. Within this 4-kb region were three genes that appeared to be organized in an operon and encoded homologs ofP. aeruginosa PilB, PilC, and PilD, proteins essential for pilus production and type II protein secretion. Northern blot analysis identified a transcript large enough to include all three genes and showed a substantial increase in expression of this operon whenL. pneumophila was grown at 30°C as opposed to 37°C. The latter observation was then correlated with an increase in piliation when bacteria were grown at the lower temperature. Southern hybridization analysis indicated that thepilB locus was conserved withinL. pneumophila serogroups and otherLegionella species. These data represent the first isolation of type II secretory genes from an intracellular parasite and indicate that the legionellae express temperature-regulated type IV pili.

Identification and Temperature Regulation of Legionella pneumophila Genes Involved in Type IV Pilus Biogenesis and Type II Protein Secretion