CITRATE UTILIZATION GENE-CLUSTER OF THE LACTOCOCCUS-LACTIS BIOVAR DIACETYLACTIS - ORGANIZATION AND REGULATION OF EXPRESSION

The transport of citrate in Lactococcus lactis biovar diacetylactis is mediated by the citrate permease P. This polypeptide is encoded by th e citP gene carried by plasmid pCIT264. In this report, we characteriz e the citP transcript, identify a cluster of two genes cotranscribed w ith citP and describe their post-transcriptional regulation. The trans criptional promoter is located 1500 nucleotides upstream of the citP g ene and the transcriptional terminator is positioned next to the 3'-en d of this gene. The DNA sequence was determined of the region upstream of the citP gene, including the promoter. Two partially overlapping o pen reading frames, citQ and citR were identified, which could encode polypeptides of 3.9 and 13 kDa respectively. These two genes, together with citP, constitute the cit cluster. Moreover, an IS-like element l ocated between the cit promoter and the citQ open reading frame was id entified. This element includes an open reading frame ORF1, which coul d encode a 33 kDa polypeptide. A translational fusion between the citP and a cat reporter gene showed that translation of citR and citP is c oupled, and regulated by CitR. The cit mRNA was subjected to specific cleavage after addition of rifampicin to the bacterial cultures. We pr opose that expression of the cit cluster is controlled at the post-tra nscriptional level by mRNA processing at a putative complex secondary structure and by translational repression mediated by CitR.