MerR and RpiR Antagonistically Regulate the alsS Promoter to Tune Diacetyl Formation in Lactiplantibacillus plantarum

Diacetyl is a key aroma compound in fermented dairy products, but its transcriptional regulation in lactic acid bacteria remains unclear. Here, two transcription factors, MerR and RpiR, were identified as candidate regulators of the alsS promoter in Lactiplantibacillus plantarum WJ108. DNA pull-down coupled with LC-MS/MS, EMSA, ChIP-qPCR, and ChIP-seq supported their association with the alsS regulatory region and revealed distinct promoter-recognition patterns. Genetic deletion and overexpression showed opposite effects on alsS transcription and diacetyl production, with MerR displaying activating potential and RpiR displaying repressive potential. During milk fermentation, changes in oxidation-reduction potential were temporally associated with shifts in merR, rpiR, and alsS transcript levels. These findings support a working model in which MerR- and RpiR-associated regulation contributes to stage-dependent control of alsS and diacetyl formation in L. plantarum, while broader regulatory effects remain to be defined.