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We characterized a new quorum-sensing regulator, PlcRa, which is present in various members of the  B. cereus  group and identified a signaling heptapeptide for PlcRa activity: PapRa 7 . We demonstrated that PlcRa is a 3D structural paralog of PlcR using sequence analysis and homology modeling. A comparison of the transcriptomes at the onset of stationary phase of a Δ plcRa  mutant and the wild-type  B. cereus  ATCC 14579 strain showed that 68 genes were upregulated and 49 genes were downregulated in the Δ plcRa  mutant strain (>3-fold change). Genes involved in the cysteine metabolism (putative CymR regulon) were downregulated in the Δ plcRa  mutant strain. We focused on the gene with the largest difference in expression level between the two conditions, which encoded -AbrB2- a new regulator of the AbrB family. We demonstrated that purified PlcRa bound specifically to the  abrB2  promoter in the presence of synthetic PapRa 7 , in an electrophoretic mobility shift assay. We further showed that the AbrB2 regulator controlled the expression of the  yrrT  operon involved in methionine to cysteine conversion. We found that the Δ plcRa  mutant strain was more sensitive to hydrogen peroxide- and disulfide-induced stresses than the wild type. When cystine was added to the culture of the Δ plcRa  mutant, challenged with hydrogen peroxide, growth inhibition was abolished. In conclusion, we identified a new RNPP transcriptional regulator in  B. cereus  that activated the oxidative stress response and cysteine metabolism in transition state cells.

PlcRa, a New Quorum-Sensing Regulator from Bacillus cereus, Plays a Role in Oxidative Stress Responses and Cysteine Metabolism in Stationary Phase