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We have recently described the expression of two pili of different lengths on the surface ofLegionella pneumophila (B. J. Stone and Y. Abu Kwaik, Infect. Immun. 66:1768–1775, 1998). Production of long pili requires a functionalpilE L locus, encoding a type IV pilin protein. Since type IV pili inNeisseria gonorrhoeae are associated with competence for DNA transformation, we examined the competence ofL. pneumophila for DNA transformation under conditions that allowed the expression of type IV pili. We show thatL. pneumophila is naturally competent for DNA transformation by isogenic chromosomal DNA and by plasmid DNA containingL. pneumophila DNA. Many differentL. pneumophila loci are able to transformL. pneumophila after addition of plasmid DNA, includinggspA ,ppa ,asd , andpilE L . The transformation frequency is reduced when competing DNA containing eitherL. pneumophila DNA or vector sequences is added to the bacteria, suggesting that uptake-specific sequences may not be involved in DNA uptake. Competence for DNA transformation correlates with expression of the type IV pili, and apilE L mutant defective in expression of type IV pili is not competent for DNA transformation. Complementation of the mutant for competence is restored by the reintroduction of a cosmid that restores production of type IV pili. Minimal competence is restored to the mutant by introduction ofpilE L alone. We conclude that competence for DNA transformation inL. pneumophila is associated with expression of the type IV pilus and results in recombination ofL. pneumophila DNA into the chromosome. Since expression of type IV pili also facilitates attachment ofL. pneumophila to mammalian cells and protozoa, we designated the type IV pili CAP (for competence- and adherence-associated pili).

Natural Competence for DNA Transformation by Legionella pneumophila and Its Association with Expression of Type IV Pili