2 DIFFERENT MECHANISMS MEDIATE CATABOLITE REPRESSION OF THE BACILLUS-SUBTILIS LEVANASE OPERON

There are two levels of control of the expression of the levanase oper on in Bacillus subtilis: induction by fructose, which involves a posit ive regulator, LevR, and the fructose phosphotransferase system encode d by this operon (lev-PTS), and a global regulation, catabolite repres sion, The LevR activator interacts with its target, the upstream activ ating sequence (UAS), to stimulate the transcription of the E sigma(L) complex bound at the ''-12, -24'' promoter. Levanase operon expressio n in the presence of glucose was tested in strains carrying a ccpA gen e disruption or a ptsH1 mutation in which Ser-46 of HPr is replaced by Ala, In a levR(+) inducible genetic background, the expression of the levanase operon was partially resistant to catabolite repression in b oth mutants, indicating that the CcpA repressor and the HPr-SerP prote in are involved in the glucose control of this operon. In addition, a cis-acting catabolite-responsive element (CRE) of the levanase operon was identified and investigated by site-directed mutagenesis, The CRE sequence TGAAAACGCTT(a)ACA is located between positions -50 and -36 fr om the transcriptional start site, between the UAS and the -12, -24 pr omoter. However, in a background constitutive for levanase, neither HP r, CcpA, nor CRE is involved in glucose repression, suggesting the exi stence of a different pathway of glucose regulation, Using truncated L evR proteins, we showed that this CcpA-independent pathway required th e presence of the domain of LevR (amino acids 411 to 689) homologous t o the BglG family of bacterial antiterminators.