Proteins with molecular masses of 50 and 80 kilodaltons encoded by genes downstream from the fimbrilin gene (fimA) are components associated with fimbriae in the oral anaerobe Porphyromonas gingivalis

Abstract

Flanking DNA regions of the fimbrilin gene (designated fimA), which encodes the major subunit protein of Porphyromonas (Bacteroides) gingivalis fimbriae, were cloned in several manners from the P. gingivalis chromosome into Escherichia coli by screening with probes derived from a 2.5-kb SacI DNA fragment previously cloned. A total of 10.4 kb of DNA fragments from the P. gingivalis genome was cloned in the pUC plasmid. Expression of the fimA gene and possible flanking genes in the fragments cloned was examined in a pUC plasmid vector system and in a bacteriophage T7 RNA polymerase-promoter expression vector system. The results show that in the pUC plasmid system, a 45-kDa protein, a product of fimA, was only poorly expressed as a precursor of the fimbrilin protein (FimA) and could be detected from cell extracts in Western blotting (immunoblotting) analysis as a sharp band but not in colony immunoblotting analysis. On the other hand, in the T7 RNA polymerase-promoter system, the product of fimA and products of the possible flanking genes responsible for fimbriation were overproduced as thick bands of the 45-kDa protein and as 63-, 50-, and 80-kDa proteins, respectively, in stained electrophoresis gels. All of the recombinant proteins were insoluble and seemed to be expressed as precursors with leader peptides. The 63-kDa, 45-k*Da (a truncated protein of the 50-kDa protein), and 80-kDa proteins were purified after solubilization with sodium dodecyl sulfate. N-terminal amino acid sequences of the 45-k*Da and 80-kDa proteins were analyzed up to the first 35 residues with a gas-phase sequencer. Monospecific antibodies directed to the recombinant proteins, i.e., the 63-kDa, 45-k*Da, and 80-kDa proteins, were raised in rabbits. By using the antibodies, localization of their matured proteins in P. gingivalis was investigated by Western blotting analysis. Immunoblotting analysis suggests that at least the 50- and 80-kDa proteins, encoded by genes downstream from the fimA gene, are minor components associated with fimbriae.