A putative phytochrome-like photoreceptor regulates the expression of red light-induced psbA RNA binding protein genes in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii is a photosynthetic green alga that is readily used as a model system for the study of photosynthesis. Our research focuses on the regulation of a set of nuclear genes (rb38, rb47 and rb60), which encode proteins that bind to the 5’-untranslated region of the chloroplast-encoded psbA mRNA, directing its translation to the photosynthetic D1 protein. We showed using northern blot analyses that these genes, and the nuclear psbO gene (encodes the Oxygen Evolving Enhancer I protein), are induced by red light. These results are novel, as very few red light-induced processes have been characterized in C. reinhardtii, and no red light photoreceptor has been identified. Analysis of the C. reinhardtii genome showed the presence of a putative red light photoreceptor, similar to the CPH2 phytochrome-like photoreceptor, in the cyanobacterium Synechocystis sp. PCC 6803. The hallmark of an active phytochrome is the reversal of the red light response by far red light. Preliminary results obtained through RT-PCR indicate that the red light-induced expression of rb38 and rb60 is reversed by far red light. Based on these observations, we hypothesize that a phytochrome-like photoreceptor is responsible for initiating the intracellular signal-transduction cascade that culminates in rb gene expression. Currently, genomic PCR and Southern blot analyses are being performed in order to isolate the phytochrome-like gene in C. reinhardtii. The results obtained should elucidate the nature of the genomic sequence for this novel red light photoreceptor, aiding in the construction of the rb (and psbO) signal-transduction pathway.