Abstract
The genome of the gram-positive bacterium Enterococcus faecalis contains the genes that encode the citrate lyase complex. This complex splits citrate into oxaloacetate and acetate and is involved in all the known anaerobic bacterial citrate fermentation pathways. Although citrate fermentation in E. faecalis has been investigated before, the regulation and transcriptional pattern of the cit locus has still not been fully explored. To fill this gap, in this paper we demonstrate that the GntR transcriptional regulator CitO is a novel positive regulator involved in the expression of the cit operons. The transcriptional analysis of the cit clusters revealed two divergent operons: citHO, which codes for the transporter (citH) and the regulatory protein (citO), and upstream from it and in the opposite direction the oadHDB-citCDEFX-oadA-citMG operon, which includes the citrate lyase subunits (citD, citE, and citF), the soluble oxaloacetate decarboxylase (citM), and also the genes encoding a putative oxaloacetate decarboxylase complex (oadB, oadA, oadD and oadH). This analysis also showed that both operons are specifically activated by the addition of citrate to the medium. In order to study the functional role of CitO, a mutant strain with an interrupted citO gene was constructed, causing a total loss of the ability to degrade citrate. Reintroduction of a functional copy of citO to the citO-deficient strain restored the response to citrate and the Cit(+) phenotype. Furthermore, we present evidence that CitO binds to the cis-acting sequences O(1) and O(2), located in the cit intergenic region, increasing its affinity for these binding sites when citrate is present and allowing the induction of both cit promoters.