Transcriptional regulation of the Streptococcus mutans gal operon by the GalR repressor

The galactose operon of Streptococcus mutans is transcriptionally regulated by a repressor protein (GalR) encoded by the galR gene, which is divergently oriented from the structural genes of the gal operon. To study the regulatory function of GalR, we partially purified the protein and examined its DNA binding activity by gel mobility shift and DNase I footprinting experiments. The protein specifically bound to the galR-galK intergenic region at an operator sequence, the position of which would suggest that GalR plays a role in the regulation of the gal operon as well as autoregulation. To further examine this hypothesis, transcriptional start sites of the gal operon and the galR gene were determined. Primer extension analysis showed that both promoters overlap the operator, indicating that GalR most likely represses transcription initiation of both promoters. Finally, the results from in vitro binding experiments with potential effector molecules suggest that galactose is a true intracellular inducer of the galactose operon.