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

Abstract

The LitR/CarH protein family is an adenosyl B12 (AdoB12)-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, Burkholderia multivorans, which does not possess an AdoB12-binding domain. Transcriptome analysis indicated the existence of 19 light-induced genes, including folE2, cfaB, litS, photolyase gene phrB2, and cryB, located in the region flanking litR Disruption of litR caused constitutive expression of all the light-inducible genes, while mutation in the light-induced sigma factor gene, litS, abolished the transcription of the phrB2 operon and the cfa 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 litR and the folE2 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 litR 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 litR itself and the folE2 operon, resulting in the production of folate, and then the expressed RNA polymerase complex containing σLitS directs the transcription of the phrB2 operon and the cfa operon. These light-dependent characteristics suggest that class III LitR, in complex with a UV-A-absorbing molecule, follows a novel light-sensing mechanism.IMPORTANCE Members of the LitR/CarH family are adenosyl B12-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 B12-binding domain in the light response of Burkholderia multivorans 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 (λmax = 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 B12-based type.