Transcriptional regulation of the Tn5276-located Lactococcus lactis sucrose operon and characterization of the sacA gene encoding sucrose-6-phosphate hydrolase.

The Lactococcus lactis sucrose operon was located on the conjugative transposon Tn5276 and the nucleotide sequence of the sacA gene, encoding sucrose-6-phosphate hydrolase, and its surrounding regions was determined. Northern blot analysis showed that the sucrose operon contains two divergent transcriptional units of 3.2 and 3.6 kb, the expression of which is considerably higher in cells grown on sucrose than in cells grown on glucose. This was confirmed by primer extension studies which demonstrated that transcription is initiated at two sucrose-inducible promoters with a back-to-back organization. The 3.2-kb transcriptional unit includes the sacB gene which most probably encodes the sucrose-specific enzyme II of the phosphotransferase system, and may contain the gene encoding fructokinase. The 3.6-kb transcriptional unit includes genes sacA and sacR. The protein encoded by the sacR gene is likely to be involved in the regulation of the sac operon expression, since its deduced N terminus is homologous to helix-turn-helix DNA-binding domains found in several regulatory proteins.