Transcription in the acetoin catabolic pathway is regulated by AcoR and CcpA in Bacillus thuringiensis

Acetoin (3-hydroxy-2-butanone) is an important physiological metabolic product in many microorganisms. Acetoin breakdown is catalyzed by the acetoin dehydrogenase enzyme system (AoDH ES), which is encoded by acoABCL operon. In this study, we analyzed transcription and regulation of the aco operon in Bacillus thuringiensis (Bt). RT-PCR analysis revealed that acoABCL forms one transcriptional unit. The Sigma 54 controlled consensus sequence was located 12 bp from the acoA transcriptional start site (TSS). β-galactosidase assay revealed that aco operon transcription is induced by acetoin, controlled by sigma 54, and positively regulated by AcoR. The HTH domain of AcoR recognized and specifically bound to a 13-bp inverted repeat region that participates in 30-bp fragment mapping 81 bp upstream of the acoA TSS. The GAF domain in AcoR represses enhancer transcriptional activity at the acoA promoter. Transcriptions of the aco operon and acoR were repressed by glucose via CcpA, and CcpA specifically bound to sequences within the acoR promoter fragment. In the acoABCL and acoR mutants, acetoin use was abolished, suggesting that the aco operon is essential for utilization of acetoin. The data presented here improve our understanding of the regulation of the aco gene cluster in bacteria.