Environmental regulation of fimbrial gene expression in Porphyromonas gingivalis.

Porphyromonas gingivalis fimbriae are an important virulence factor involved in attachment and invasion. Fimbrillin, encoded by the fimA gene, is the major subunit protein of the fimbriae. To elucidate the influence of environmental signals on the expression of the fimA gene, a strain of P. gingivalis (designated PLE) containing a chromosomal transcriptional fusion between a promoterless lacZ gene and the fimA promoter region was constructed. Promoter activity was assessed by measurement of beta-galactosidase activity of PLE. An 11-fold increase in activity of fimA promoter was found as growth temperature declined from 39 to 34 degrees C. Promoter activity decreased by approximately 50% in response to hemin limitation and upon culture on solid medium. In addition, the presence of serum or saliva in the growth medium decreased fimA promoter activity by similar amounts. A correlation between fimA promoter activity and phenotypic properties dependent upon fimbriae was established. P. gingivalis grown at 34 degrees C, compared to 39 degrees C, showed an increased ability to adhere to Streptococcus gordonii and to invade primary cultures of gingival epithelial cells. These studies indicate that expression of the P. gingivalis fimA gene is regulated at the transcriptional level in response to several environmental conditions and that altered fimA expression can also modulate the adherence and invasion abilities of P. gingivalis.