SCBA FROM STREPTOCOCCUS-CRISTA-CC5A - AN ATYPICAL MEMBER OF THE LRAI GENE FAMILY

A new member of the lraI family of putative adhesin genes was cloned, from Streptococcus crista CC5A, and sequenced. The gene, scbA, appears to be part of an ABC transport operon and encodes a putative peptide of 34.7 kDa. The protein contains a signal sequence with residues 17 t o 21 (L-A-A-C-S) matching the consensus sequence for the prolipoprotei n cleavage site of signal peptidase II. ScbA is 57 to 93% identical, a t the amino acid level, with the five previous sequenced members of th e LraI family. Surprisingly, ScbA does not exhibit adhesion properties characteristic of the other LraI proteins. Strain CC5A bound poorly t o saliva-coated hydroxyapatite and did not coaggregate with Actinomyce s naeslundii PK606. An scbA insertion-duplication mutation that abolis hed expression of ScbA was created. There was no difference in fibrin binding between this mutant and wild-type CC5A. Since it is possible t hat ScbA could play a role in corncob formation between S. crista and Fusobacterium nucleatum, this property was examined. The mutant strain retained the ability to form corncobs. On the basis of the lack of ad hesin properties it appears that ScbA is an atypical member of the Lra I family.