Role and Function of Class III LitR, a Photosensor Homolog from Burkholderia multivorans.

The LitR/CarH protein family is an adenosyl B (AdoB)-dependent photoreceptor family with DNA-binding activity, and its homologs are widely distributed in the genomes of diverse bacterial genera. In this investigation, we studied the role and functions of a LitR homolog from a Gram-negative soil bacterium, , which does not possess an AdoB-binding domain. Transcriptome analysis indicated the existence of 19 light-induced genes, including , , , photolyase gene , and , located in the region flanking Disruption of caused constitutive expression of all the light-inducible genes, while mutation in the light-induced sigma factor gene, , abolished the transcription of the operon and the operon, indicating that LitR and LitS play a central role in light-inducible transcription. A gel shift assay showed that recombinant protein LitR specifically binds to the promoter regions of and the operon, and its binding was weakened by UV-A illumination. LitR absorbs light at maximally near 340 nm and exhibited a photocyclic response and light-dependent dissociation of multimer into tetramer. The mutant produced a 20-fold-higher intracellular level of folate than that of the wild-type strain. Thus, the evidence suggests that LitR light-dependently regulates the transcription of itself and the operon, resulting in the production of folate, and then the expressed RNA polymerase complex containing σ directs the transcription of the operon and the operon. These light-dependent characteristics suggest that class III LitR, in complex with a UV-A-absorbing molecule, follows a novel light-sensing mechanism. Members of the LitR/CarH family are adenosyl B-based photosensory transcriptional regulator involved in light-inducible carotenoid production in nonphototrophic bacteria. Our study provides the first evidence of the involvement of a class III LitR, which lacks an adenosyl B-binding domain in the light response of belonging to betaproteobacteria. Our biochemical analysis suggests that class III LitR protein exhibits features as a photosensor including absorption of light at the UV-A region (λ = ca. 340 nm), photocyclic response, and light-dependent dissociation. This suggests that class III LitR associates with a UV-A-absorbing molecule, and it has a photosensing mechanism distinguishable from that of the B-based type.