Control of proteobacterial central carbon metabolism by the HexR transcriptional regulator: a case study in Shewanella oneidensis

Bacteria exploit multiple mechanisms for controlling central carbon metabolism (CCM). Thus, a bioinformatic analysis together with some experimental data implicated the HexR transcriptional factor as a global CCM regulator in some lineages of Gammaproteobacteria operating as a functional replacement of the Cra regulator characteristic of Enterobacteriales. In this study, we combined a large scale comparative genomic reconstruction of HexR-controlled regulons in 87 species of Proteobacteria with the detailed experimental analysis of the HexR regulatory network in the Shewanella oneidensis model system. Although nearly all of the HexR-controlled genes are associated with CCM, remarkable variations were revealed in the scale (from 1 to 2 target operons in Enterobacteriales up to 20 operons in Aeromonadales) and gene content of HexR regulons between 11 compared lineages. A predicted 17-bp pseudo-palindrome with a consensus tTGTAATwwwATTACa was confirmed as a HexR-binding motif for 15 target operons (comprising 30 genes) by in vitro binding assays. The negative effect of the key CCM intermediate, 2-keto-3-deoxy-6-phosphogluconate, on the DNA-regulator complex formation was verified. A dual mode of HexR action on various target promoters, repression of genes involved in catabolic pathways and activation of gluconeogenic genes, was for the first time predicted by the bioinformatic analysis and experimentally verified by changed gene expression pattern in S. oneidensis DeltahexR mutant. Phenotypic profiling revealed the inability of this mutant to grow on lactate or pyruvate as a single carbon source. A comparative metabolic flux analysis of wild-type and mutant strains of S. oneidensis using [(13)C]lactate labeling and GC-MS analysis confirmed the hypothesized HexR role as a master regulator of gluconeogenic flux from pyruvate via the transcriptional activation of phosphoenolpyruvate synthase (PpsA).