The coordinated regulatory roles of two LysR-Type Transcriptional Regulators balance chorismate and protocatechuate partition in Listeria organisms

Listeria monocytogenes is an economically deleterious foodborne pathogens that continually challenges the global food supply chain. Listeria species in general, synthesize protocatechuate from saprophytically-derived quinate and shikimate utilizing a novel class of bacterial dehydroshikimate dehydratase. Paradoxically, Listeria species are unable to metabolically utilize protocatechuate, as such, it was proposed that this compound is used as a currency to Listeria interactions with other microorganisms to improve their environmental proliferation. Therefore, an understanding of the regulatory mechanism for the metabolic pathway for protocatechuate biosynthesis is of great importance. Two LysR Type Transcriptional Regulators (LTTR), annotated QuiR and in this study QuiR2, are found upstream of genomic operons, qui1 and qui2, which transcribe genes for protocatechuate synthesis. QuiR, has been shown to activate the expression of genes from both operons with shikimate as a coinducer. However, the role of QuiR2, Lmo2233, is not clear. In this study, we conducted structural, biochemical and bioinformatics analyses of QuiR2 and demonstrated that it functions as a negative regulator of protocatechuate biosynthesis in Listeria species. Moreover, we determined that protocatechuate functions in modulating QuiR2 repressive properties through our mobility shift assay and LacZ reporter activity studies. Furthermore, phylogenetic analyses reveal that QuiR2 clusters closely but independently from QuiR thus supporting their distinct regulatory roles. We propose that QuiR2 prevents metabolic commitment of dehydroshikimate to protocatechuate when elevated and in limiting shikimate condition. In this study we revisited the biological role of the shikimate pathway in microbes and demonstrated that in addition to it producing chorismite for aromatic compound metabolism it is also important in allowing organisms to shuttle shikimate and quinate to produce protocatechuate which can be used as an energy source and more importantly in Listeria it is used to facilitate microbial interactions.