Transcriptional control of the citrate-inducible citMCDEFGRP operon, encoding genes involved in citrate fermentation in Leuconostoc paramesenteroides

In this study we describe the expression pattern of the Leuconostoc parames enteroides citMCDEFGRP operon in response to the addition of citrate to the growth medium. An 8.8-kb polycistronic transcript, which includes the citM CDEFGRP genes, was identified; its synthesis was dramatically induced upon addition of citrate to the growth medium. We also found that expression of the cit operon is subjected to posttranscriptional regulation, since proces sing sites included in four complex secondary structures (I, II, III, and I V) were identified by Northern blot analysis and mapped by primer extension . Upstream of the citMCDEFGRP operon a divergent open reading frame, whose expression was also increased by citrate, was identified by DNA sequencing and designated citI. The start and end sites of transcription of the cit op eron and citI gene were mapped. The start sites are separated by a stretch of 188 bp with a very high A+T content of 77% and are preceded by transcrip tional promoters. The end sites of the transcripts are located next to the 3' end of two secondary structures characteristic of rho-independent transc riptional terminators. The effect of the citI gene on expression of the cit operon was studied in Escherichia coli. The presence of the citI gene in c is and in trans resulted in increased activity of the cit promoter. These d ata provide the first evidence that citrate fermentation in Leuconostoc is regulated at the transcriptional level by a transcriptional activator rathe r than by a repressor.