Some clinical isolates of the gram-negative periodontal pathogen Actinobacillus actinomycetemcomitans are naturally competent for DNA uptake. In this study, we examined the sequence and the function of a type IV pilus-like pilABCD gene cluster and its downstream region in a naturally transformable A. actinomycetemcomitans strain D7S. Specific knockout mutants of pilABCD of strain D7S were constructed by replacing individual genes with an antibiotic resistance cassette. The transformation frequency of chromosome markers in the wildtype strain D7S was approximately 10(-3) per CFU. In contrast, the delta pilA, delta pilB, delta pilC, delta pilBC or delta pilD mutants were non-transformable (transformation frequency <10(-8)). Disruption of an ORF downstream of pilD had no apparent effect on the transformability of this bacterium. The pilA or pilBC deletion did not seem to affect fimbria expression or cell surface structure in either rough or smooth strains as determined by scanning and transmission electron microscopy examinations. RT-PCR analysis showed that pilA was expressed in strain D7S under a competence-inducing growth condition. The expression of pilA was barely detectable in strain D7S cultured under a non-competence-inducing condition or in the non-transformable strain JP-2. The results indicate that pilABCD are required for competence but are apparently not involved in fimbria expression of A. actinomycetemcomitans.