Natural competence for DNA transformation by Legionella pneumaphila and its association with expression of type IV pili

We have recently described the expression of two pili of different lengths on the surface of Legionella pneumophila (B. J. Stone and Y. Abu Kwaik, Inf ect. Immun. 66:1768-1775, 1998). Production of long pili requires a functio nal pilE(L) locus, encoding a type IV pilin protein. Since type IV pili in Neisseria gonorrhoeae are associated with competence for DNA transformation , we examined the competence of L. pneumophila for DNA transformation under conditions that allowed the expression of type IV pili. We show that L. pn eumophila is naturally competent for DNA transformation by isogenic chromos omal DNA and by plasmid DNA containing L. pneumophila DNA. Many different L . pneumophila loci are able to transform L. pneumophila after addition of p lasmid DNA, including gspA, ppa, asd, and pilE(L). The transformation frequ ency is reduced when competing DNA containing either L. 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 transformat ion correlates with expression of the type IV pili, and a pilE(L) mutant de fective in expression of type IV pili is not competent for DNA transformati on. Complementation of the mutant for competence is restored by the reintro duction of a cosmid that restores production of type IV pili. Minimal compe tence is restored to the mutant by introduction of pilE(L) alone. We conclu de that competence for DNA transformation in L. pneumophila is associated w ith expression of the type IV pilus and results in recombination oft. pneum ophila DNA into the chromosome. Since expression of type IV pili also facil itates attachment of L. pneumophila to mammalian cells and protozoa, we des ignated the type IV pili CAP (for competence- and adherence-associated pili ).