The two-component system BfrAB regulates expression of ABC transporters in Streptococcus gordonii and Streptococcus sanguinis

The putative two-component system BfrAB is involved in Streptococcus gordonii biofilm development. Here, we provide evidence that BfrAB regulates the expression of bfrCD and bfrEFG, which encode two ATP-binding cassette (ABC) transporters, and bfrH(1) which encodes a CAAX amino-terminal protease family protein. BfrC and BfrE are ATP-binding proteins, and BfrD, BfrF and BfrG are homologous membrane-spanning polypeptides. Similarly, BfrAB(ss), the BfrAB homologous system in Streptococcus sanguinis, controls the expression of two bfrCD-homologous operons (bfrCD(ss) and bfrXY(ss)), a bfrH-homologous gene (bfrH1(ss)) and another CAAX amino-terminal protease family protein gene (bfrH2(ss)). Furthermore, we demonstrate that the purified BfrA DNA-binding domain from S. gordonii binds to the promoter regions of bfrCD(1) bfrEFG, bfrH, bfrCD(ss), bfrXY(ss) and bfrH1(ss) in vitro. Finally, we show that the BfrA DNA-binding domain recognizes a conserved DNA motif with a consensus sequence of TTTCTTTAGAAATATTTTAGAATT. These data suggest, therefore, that S. gordonii BfrAB controls biofilm formation by regulating multiple ABC-transporter systems.