Cloning of fibA, encoding an immunogenic subunit of the fibril-like surface structure of Peptostreptococcus micros

Although we are currently unaware of its biological function, the fibril-li ke surface structure is a prominent characteristic of the rough (Rg) genoty pe of the gram-positive periodontal pathogen Peptostreptococcus micros, The smooth (Sm) type of this species as well as the smooth variant of the Rg t ype (Rg(Sm)) lack these structures on their surface. A fibril-specific seru m, as determined by immunogold electron microscopy, was obtained through ad sorption of a rabbit anti-Rg type serum with excess bacteria of the Rg(Sm) type. This serum recognized a 42-kDa protein, which was subjected to N-term inal sequencing, Both clones of a lambda TriplEx expression library that we re selected by immunoscreening with the fibril-specific serum contained an open reading frame, designated fibA, encoding a 393-amino-acid protein (Fib A). The 15-residue N-terminal amino acid sequence of the 42-kDa antigen was present at positions 39 to 53 in FibA; from this we conclude that the matu re FibA protein contains 355 amino acids, resulting in a predicted molecula r mass of 41,368 Da. The putative 38-residue signal sequence of FibA strong ly resembles other gram-positive secretion signal sequences. The C termini of FibA and two open reading frames directly upstream and downstream of fib A exhibited significant sequence homology to the C termini of a group of se creted and surface-located proteins of other gram-positive cocci that are a ll presumably involved in anchoring of the protein to carbohydrate structur es. We conclude that FibA is a secreted and surface-located protein and as such is part of the fibril-like structures.