A RIBONUCLEIC ANTITERMINATOR SEQUENCE (RAT) AND A DISTANT PALINDROME ARE BOTH INVOLVED IN SUCROSE INDUCTION OF THE BACILLUS-SUBTILIS SACXY REGULATORY OPERON

The Bacillus subtilis sacXY regulatory operon is involved in sucrose i nduction of the levansucrase sacB gene by an antitermination mechanism , In the presence of sucrose, the activated SacY antiterminator protei n stabilizes the secondary structure of a ribonucleic antiterminator s equence (RAT) located in the leader region of the sacB transcript, and overlapping a rho-independent transcription terminator. Formation of the SacY-RAT complex prevents alternative formation of the terminator, allowing transcription of the downstream sequences. In the absence of sucrose, inhibition of SacY activity by SacX leads to termination of transcription. Expression of sacXY is also sucrose-inducible. This ind uction was previously shown to be mediated by SacY itself and/or SacT, another antiterminator involved in induction of genes belonging to a distinct sucrose pathway. These antiterminators are not activated at t he same concentration of sucrose. We show here that sacXY induction oc curs through activation of either SacY or SacT antiterminators, at the ir respective sucrose activation concentration. This result demonstrat es a link between SacY- and SacT-mediated metabolic pathways. In addit ion, the sacXY leader region carries a RAT-like sequence, which howeve r does not appear to overlap any apparent rho-independent transcriptio n terminator. Site-directed mutagenesis experiments on this RAT-like s equence demonstrated its involvement in sucrose induction. Deletions g enerated in the sacXY leader region showed that a palindrome, located 100 nt downstream from the RAT-like sequence, also acts as a cis-actin g element. Computer analysis of the leader RNA suggested that formatio n of the secondary structure of the RAT-like sequence and the palindro me could be mutually exclusive.