PicR as a MarR Family Transcriptional Repressor Multiply Controls the Transcription of Picolinic Acid Degradation Gene Cluster in Alcaligenes faecalis JQ135.

Picolinic acid (PA) is a natural toxic pyridine derivative as well as an important intermediate used in the chemical industry. In a previous study, we identified a gene cluster, , that responsible for the catabolism of PA in Alcaligenes faecalis JQ135. However, the transcriptional regulation of the cluster remains known. This study showed that the entire cluster was composed of 17 genes and transcribed as four operons: , , , and . Deletion of , encoding a putative MarR-type regulator, greatly shortened the lag phase of PA degradation. An electrophoretic mobility shift assay and DNase I footprinting showed that PicR has one binding site in the - intergenic region and two binding sites in the - intergenic region. The DNA sequences of the three binding sites have the palindromic characteristics of TCAG-N-CTNN: the space consists of four nonspecific bases, and the four palindromic bases on the left and the first two palindromic bases on the right are strictly conserved, while the last two bases on the right vary among the three binding sites. An β-galactosidase activity reporter assay indicated that 6-hydroxypicolinic acid but not PA acted as a ligand of PicR, preventing PicR from binding to promoter regions and thus derepressing the transcription of the cluster. This study revealed the negative transcriptional regulation mechanism of PA degradation by PicR in JQ135 and provides new insights into the structure and function of the MarR-type regulator. The gene cluster was found to be responsible for PA degradation and widely distributed in , , and . Thus, it is very necessary to understand the regulation mechanism of the cluster in these strains. This study revealed that PicR binds to three sites of the promoter regions of the cluster to multiply regulate the transcription of the cluster, which enables JQ135 to efficiently utilize PA. Furthermore, the study also found a unique palindrome sequence for binding of the MarR-type regulator. This study enhanced our understanding of microbial catabolism of environmental toxic pyridine derivatives.