On the role of the global regulator RlcA in red-light sensing in Aspergillus nidulans

A large proportion of fungal genomes are under the control of light. Most fungi employ complex light sensing systems, consisting of red-, blue-, and in some cases green-light photoreceptors. Here we studied the light response in Aspergillus nidulans. In a genetic screen, followed by whole-genome sequencing we identified a global regulator, which appears to be involved in chromatin structure modification. We therefore named the protein RlcA (regulator of light sensing and chromatin remodeling). The protein comprises a nuclear localization signal, a PHD (plant homeodomain) finger, a TFSII (found in the central region of the transcription elongation factor S-II), and a SPOC domain (Spen paralog and ortholog C-terminal domain). In the mutant, where light-controlled genes were constitutively active, the SPOC domain is missing. RlcA localized to the nucleus and interacted with the phytochrome FphA. The PHD-finger domain probably binds to trimethylated lysine 4 of histone H3, whereas the TFSII domain binds RNA polymerase II. The SPOC domain could mediate interaction with a global repressor protein. In the mutant, repressor recruitment would be hindered, whereas in the wild type repressor release would be induced after light stimulation. Our results add another layer of complexity to light sensing in filamentous fungi.