Light Response of KT2440 Mediated by Class II LitR, a Photosensor Homolog

KT2440 retains three homologs (PplR1 to PplR3) of the LitR/CarH family, an adenosyl B-dependent light-sensitive MerR family transcriptional regulator. Transcriptome analysis revealed the existence of a number of photoinducible genes, including , (encoding DNA photolyase), (furan-containing fatty acid synthase), (GTP cyclohydrolase I), (cryptochrome-like protein), and multiple genes without annotated/known function. Transcriptional analysis by quantitative reverse transcription-PCR with knockout mutants of to showed that a triple knockout completely abolished the light-inducible transcription in , which indicates the occurrence of ternary regulation of PplR proteins. A DNase I footprint assay showed that PplR1 protein specifically binds to the promoter regions of light-inducible genes, suggesting a consensus PplR1-binding direct repeat, 5'-T(G/A)TACANTGTA(C/T)A-3'. The disruption of B biosynthesis cluster did not affect the light-inducible transcription; however, disruption of (where LOV indicates "light, oxygen, or voltage") and , encoding blue light photoreceptors adjacently located to and , respectively, led to the complete loss of light-inducible transcription. Overall, the results suggest that the three PplRs and two PpSB-LOVs cooperatively regulate the light-inducible gene expression. The wide distribution of the / cognate pair homologs in spp. and related bacteria suggests that the response and adaptation to light are similarly regulated in the group of nonphototrophic bacteria. The LitR/CarH family is a new group of photosensor homologous to MerR-type transcriptional regulators. Proteins of this family are distributed to various nonphototrophic bacteria and grouped into at least five classes (I to V). retaining three class II LitR proteins exhibited a genome-wide response to light. All three paralogs were functional and mediated photodependent activation of promoters directing the transcription of light-induced genes or operons. Two LOV (light, oxygen, or voltage) domain proteins, adjacently encoded by two genes, were also essential for the photodependent transcriptional control. Despite the difference in light-sensing mechanisms, the DNA binding consensus of class II LitR [T(G/A)TA(C/T)A] was the same as that of class I. This is the first study showing the actual involvement of class II LitR in light-induced transcription.