The nucleotide sequence of the chromosomal DNA flanking the Actinomyces naeslundii (formerly A. viscosus) T14V type 1 fimbrial structural subunit gene (fimP) was determined. Six open reading frames (ORFs), in the order 5' ORF3, ORF2, ORF1,fimP, ORF4, ORF5, ORF6 3', were identified. ORF1 encoded a protein of 408 amino acid residues (Mr = 39,270) and had significant sequence homology with the A. naeslundii T14V type 1 and A. naeslundii WVU45 type 2 fimbrial structural subunits. An in-frame fusion of ORF1 to the malE gene of the expression vector, pMAL-c2, yielded a protein that was immunostained with antibodies raised against the maltose binding protein and A. naeslundii T14V whole bacteria. Digestion of the fusion protein with factor Xa released a protein (apparent molecular mass of 34 kDa) that was immunostained only with the antibody directed against A. naeslundii T14V whole bacterial cells. Integration plasmids carrying a kanamycin resistance gene (kan) that was used to substitute for ORF1 or for DNA fragments internal to the coding region of the other five ORFs were used to transform A. naeslundii T14V. Neither type 1 fimbriae nor the 65-kDa fimbrial structural subunit was detected in mutants obtained by allelic replacement of ORF1 or ORF2. Mutants obtained by allelic replacement of ORF3 or ORF4 expressed only the 65-kDa fimbrial structural subunit. These mutants did not bind, in vitro, to proline-rich proteins that serve as the receptors for Actinomyces type 1 fimbriae. In contrast, a mutant in which the integration plasmid DNA had been inserted at a site close to the carboxyl terminus of ORF6 expressed type 1 fimbriae and had adherence properties similar to those observed in the wild-type strain. These results demonstrate the existence of additional genes near fimP that are likely to be involved in the synthesis and function of cell surface fimbriae of A. naeslundii T14V.