Role and function of Class III LitR, a photosensor homolog from Burkholderia multivorans ATCC 17616 in the regulation of folate production.

The LitR/CarH protein family is a adenosyl B12 (AdoB12)-dependent photoreceptor with DNA-binding activity and its homologs are widely distributed in the genome of diverged bacterial genera. Here, we studied a role and function of a LitR homolog from a Gram-negative soil bacteria Burkholderia multivorans, which has no AdoB12-binding domain. Transcriptome analysis revealed the existence of 19 light-induced genes (folE, cfa, litS, a Photolyase gene phrB2, cryB2) located in the flanking region of litR. Gene disruption of litR caused a constitutive expression of all light-inducible genes, while the mutation of the light-dependent expressed sigma factor litS abolished the transcription in cryB2, phrB2 operon and cfa operon, indicating that LitR and LitS play a central role in the light-inducible transcription. Gel-shift assay showed that a recombinant protein LitR specifically binds to the promoter region of litR and folE2 operon, and its ability was lowered by UV-A illumination. LitR absorbs maximally near 340 nm, and exhibited a photocyclic response and light-dependent subunit dissociation. Furthermore, the litR mutant produced a 20-fold high intracellular folate compared to the wild-type strain. Overall, the line of evidence suggest that Class III LitR light-dependently regulates the transcription of litR itself and folE2-litS operon, leading the production of folate, and then expressed RNA polymerase-LitS directs the transcription for phrB2 operon and cfa operon. Light-dependent characteristics suggests that Class III LitR in complex with a UV-A absorbing molecule has a novel light-sensing mechanism. Chromosomal RNA maps of wild-type cells cultured under the light conditions were compared with those of wild-type cells cultured under the dark conditions