CitI, a Transcription Factor Involved in Regulation of Citrate Metabolism in Lactic Acid Bacteria

Abstract

A large variety of lactic acid bacteria (LAB) can utilize citrate under fermentative conditions. Although much information concerning the metabolic pathways leading to citrate utilization by LAB has been gathered, the mechanisms regulating these pathways are obscure. In Weissella paramesenteroides (formerly called Leuconostoc paramesenteroides), transcription of the citMDEFCGRP citrate operon and the upstream divergent gene citI is induced by the presence of citrate in the medium. Although genetic experiments have suggested that CitI is a transcriptional activator whose activity can be modulated in response to citrate availability, specific details of the interaction between CitI and DNA remained unknown. In this study, we show that CitI recognizes two A؉T-rich operator sites located between citI and citM and that the DNA-binding affinity of CitI is increased by citrate. Subsequently, this citrate signal propagation leads to the activation of the cit operon through an enhanced recruitment of RNA polymerase to its promoters. Our results indicate that the control of CitI by the cellular pools of citrate provides a mechanism for sensing the availability of citrate and adjusting the expression of the cit operon accordingly. In addition, this is the first reported example of a transcription factor directly functioning as a citrate-activated switch allowing the cell to optimize the generation of metabolic energy.Variability and adaptability are crucial characteristics of organisms possessing the ability to survive and prosper under a wide variety of environmental conditions. In order for bacteria to effectively compete and survive, they have to sense and respond to changes in the availability of specific nutrients. Citrate is an abundant nutrient in nature since it is a natural constituent of all living cells. It is therefore not surprising that a large variety of bacterial species can utilize citrate under aerobic or anaerobic conditions. Although the citrate degradation pathways in bacteria are well established, less information is available on the mechanisms for sensing the availability of citrate and controlling its utilization. These mechanisms were studied mainly in Klebsiella pneumoniae and Bacillus subtilis. In these bacteria, two-component systems regulate the expression of citrate utilization genes. In particular, the CitA/B two-component system of K. pneumoniae has been extensively studied (4)(5)(6)16). This system regulates the expression of two operons, citCDEFG and CitS-oadGAB-citAB, which are involved in the transport and metabolism of citrate. Expression of these operons is induced under anoxic conditions in the presence of citrate and is subject to catabolic repression (6, 16). In Bacillus subtilis, the CitS/T two-component system (a member of the CitA/B family) is involved in the regulation of the expression of genes encoding citrate transporters required for the utilization of citrate in aerobic conditions (22,23).Citrate metabolism has been extensively studied in lactic acid bacteria (LAB) because ...

RNA-seq reveals the critical role of CspA in regulating Brucella melitensis metabolism and virulence

“…These included BMNI_II0026-0028, BMNI_II0309, BMNI_II0310, BMNI_II0357, BMNI_ II0358, BMNI_II0604, BMNI_II0794, BMNI_II0894-0896, and BMNI_II1119 which participate in amino acid transport and metabolism, BMNI_II0741 and BMNI_II0742 encoding acyl-COA dehydrogenase participate in fatty acid metabolism. In addition, two genes, BMNI_II0741 and BMNI_I1231, are involved in pyruvate metabolism, which plays an important role in balancing energy metabolism (Martin et al, 2005), and were both up-regulated in NIcspA. Interestingly, eight genes, BMNI_II0247, BMNI_ II0261, BMNI_II0262, and BMNI_II0284-0288 encoding nitrite reductase that participate in the denitrification pathway, were all up-regulated in NIcspA.…”

Characterization of the Ers Regulon of Enterococcus faecalis

“…For Weissella paramesenteroides (previously named Leuconostoc paramesenteroides), the transcription of the operon containing genes involved in citrate fermentation is induced by the presence of citrate in the medium (22). The expression of the cit operon depends on posttranscriptional regulation and on the transcriptional activator CitI (24). This regulator also seems to be responsible for the acid-inducible transcription of the operon encoding the citrate lyase complex of L. lactis (23).…”

Transcriptional Regulation of the Citrate Gene Cluster of Enterococcus faecalis Involves the GntR Family Transcriptional Activator CitO

“…CitI, a member of DeoR family, is highly conserved in different lactic acid bacteria such as W. paramesenteroides, Leuconosctoc mesenteroides, Lactococcus lactis, and Lactobacillus plantarum (31)(32)(33)(34). Interestingly, the presence of an alternative transcriptional factor associated with citrate breakdown emerged from gene context analysis of citrate fermentation gene clusters.…”

“…Recently, the transcriptional factor CitI from Weissella paramesenteroides was reported to function as a switch of the cit genes, which are directly activated by citrate (34). CitI, a member of DeoR family, is highly conserved in different lactic acid bacteria such as W. paramesenteroides, Leuconosctoc mesenteroides, Lactococcus lactis, and Lactobacillus plantarum (31)(32)(33)(34).…”

“…These mechanisms ensure that selected genes are expressed under the appropriate conditions and at the right time. The expression of the specific transporters and the citrate lyase complex, required for citrate fermentation, are regulated through the control of transcription initiation by the two-component sensor-regulator system CitA/B (6,37) and by the transcriptional activator protein CitI (34).…”