Genomic analysis highlights a novel class of DPCF-type cyanobacteriochrome

Cyanobacteriochromes (CBCRs) are light-sensitive photoreceptors that regulate diverse cyanobacterial responses to light. In this study, we combined large-scale genome mining, phylogenetics, heterologous expression, structural modeling, and spectroscopy to identify and characterize a previously unrecognized family of CBCR photoreceptors. CBCRs are light-sensing proteins that bind a chromophore through a GAF (cGMP-specific phosphodiesterases, adenylyl cyclases, and the bacterial transcription factor FhlA) domain. This family is defined by a conserved N-terminal DUF4118 domain (Domain of Unknown Function 4118) and a DPCF (Asp-Pro-Cys-Phe) motif-containing GAF domain. Interestingly, these GAF domains are phylogenetically much closer to recently described DPCI-2 (Asp-Pro-Cys-Ile) GAF domains than to conventional DPCF (Asp-Pro-Cys-Phe) GAF domains, therefore named as DPCF-2 GAF domains. These proteins, primarily distributed within the Nostocales order, are consistently associated with conserved two-component signaling (TCS) elements, suggesting their integration into light-regulated signaling cascades that might coordinate transcriptional responses in filamentous cyanobacteria. Spectral characterization of the DPCF-2 GAF domain from Tolypothrix sp. PCC 7910 revealed a stable, fully reversible blue-teal photocycle, shifting from a blue-absorbing dark state (Pb, 417 nm) to a teal-absorbing state (Pt, 514 nm) upon blue light exposure. This distinctive spectral response opens new avenues for exploring both the biological functions of DPCF-2 CBCRs and their potential biotechnological applications.