Multilevel regulation of lactose operon in Bacillus licheniformis: Coordination of LacR, CcpA and TnrA regulators

Abstract

Previous research has confirmed the involvement of the lac1 gene cluster in lactose metabolism in Bacillus licheniformis, which is able to co-utilize lactose and low concentrations of glucose or certain carbon sources. However, the internal regulatory mechanisms underlying specific lactose utilization remain elusive. In this study, we conducted a comprehensive investigation to elucidate the modulation mechanism of the lac1 gene cluster. We discovered a 162 bp bidirectional promoter located between the lacDCAB and lacR genes, which encodes a GntR family transcriptional regulatory factor. A 38 bp perfectly palindromic sequence (BOX1) within this promoter is critical for inducing lac1 expression. LacR and CcpA were shown to bind specifically to BOX1, and their novel binding motifs were identified. In BL2ΔlacR, BL2ΔccpA, and BL2ΔlacRΔccpA mutant strains, we confirmed the differential inhibitory levels of LacR and CcpA on bidirectional promoters. Furthermore, it was demonstrated that TnrA also binds BOX1 and exerts a positive regulatory effect on the lac1 cluster. Finally, we proposed a novel model to unravel the complex molecular interactions controlling the expression of the lac1 gene. These insights provide a foundation for exploring rational design strategies to optimize lactose fermentation processes and improve the efficiency of industrial lactose utilization.