Two LysR Family Transcriptional Regulators, McbH and McbN, Activate the Operons Responsible for the Midstream and Downstream Pathways, Respectively, of Carbaryl Degradation in Pseudomonas sp. Strain XWY-1.

Previously, a LysR family transcriptional regulator, McbG, that activates the gene cluster involved in the upstream pathway (from carbaryl to salicylate) of carbaryl degradation in Pseudomonas sp. strain XWY-1 was identified by us (Z. Ke, Y. Zhou, W. Jiang, M. Zhang, et al., Appl Environ Microbiol 87:e02970-20, 2021, https://doi.org/10.1128/AEM.02970-20). In this study, we identified McbH and McbN, which activate the cluster (responsible for the midstream pathway, from salicylate to gentisate) and the cluster (responsible for the downstream pathway, from gentisate to pyruvate and fumarate), respectively. They both belong to the LysR family of transcriptional regulators. Gene disruption and complementation study reveal that McbH is essential for transcription of the cluster in response to salicylate and McbN is indispensable for the transcription of the cluster in response to gentisate. The results of electrophoretic mobility shift assay (EMSA) and DNase I footprinting showed that McbH binds to the 52-bp motif in the promoter area and McbN binds to the 58-bp motif in the promoter area. The key sequence of McbH binding to the promoter is a 13-bp motif that conforms to the typical characteristics of the LysR family. However, the 12-bp motif that is different from the typical characteristics of the LysR family regulator binding site sequence is identified as the key sequence for McbN to bind to the promoter. This study revealed the regulatory mechanisms for the midstream and downstream pathways of carbaryl degradation in strain XWY-1 and further our knowledge of (and the size of) the LysR transcription regulator family. The enzyme-encoding genes involved in the complete degradation pathway of carbaryl in Pseudomonas sp. strain XWY-1 include , , and . Previous studies demonstrated that the gene, responsible for hydrolysis of carbaryl to 1-naphthol, is constitutively expressed and that the transcription of was regulated by McbG. However, the transcription regulation mechanisms of and have not been investigated yet. In this study, we identified two LysR-type transcriptional regulators, McbH and McbN, which activate the cluster (responsible for the degradation of salicylate to gentisate) and the cluster (responsible for the degradation of gentisate to pyruvate and fumarate), respectively. The 13-bp motif is critical for McbH to bind to the promoter of , and 12-bp motif different from the typical characteristics of the LysR-type transcriptional regulator (LTTR) binding sequence affects the binding of McbN to the promoter. These findings help to expand the understanding of the regulatory mechanism of microbial degradation of carbaryl.