NUCLEOTIDE-SEQUENCE OF A GENE CODING FOR A SALIVA-BINDING PROTEIN (SSAB) FROM STREPTOCOCCUS-SANGUIS 12 AND POSSIBLE ROLE OF THE PROTEIN IN COAGGREGATION WITH ACTINOMYCES

The nucleotide sequence of a 2.9-kb streptococcal DNA fragment which codes for two proteins with M(r)s of 36,000 (Streptococcus sanguis adhesin B [SsaB]) and 20,000 has been determined. The ssaB gene is 927 bp and codes for a 34,684-Da protein. The open reading frame coding for the 20-kDa protein is 489 bp and codes for a protein of 17,885 Da. The SsaB protein has a putative hydrophobic 19-amino-acid signal sequence resulting in a 32,620-M(r) secreted protein, whereas the 20-kDa protein has no signal sequence. Both proteins are hydrophilic, and neither appears to have a hydrophobic membrane anchor sequence in the carboxy-terminal region. A DNA sequence homology of 73% exists between the cloned fragment containing the ssaB gene from S. sanguis 12 and the cloned fragment containing the type 1 fimbrial gene of S. sanguis FW213 (J. C. Fenno, D. J. LeBlanc, and P. Fives-Taylor, Infect. Immun. 57:3527-3533, 1989). Amino acid comparisons of the SsaB and type 1 fimbrial proteins show 87% homology, indicating a close similarity of the two proteins. Antiserum raised against the cloned SsaB protein cross-reacts with a 38-kDa protein identified from Streptococcus gordonii (S. sanguis) PK488 which was proposed to mediate coaggregation with Actinomyces naeslundii PK606 (P. E. Kolenbrander and R. N. Andersen, Infect. Immun. 58:3064-3072, 1990). The SsaB adhesin may play a role in oral colonization by binding either to a receptor on saliva or to a receptor on actinomyces.