| My dissertation research addresses the signal transduction mechanisms through which the cyanobacterium Fremyella diplosiphon regulates its photosynthetic pigments called phycobiliproteins in response to light quality and nutrient levels. Red light induces production of phycocyanin while green light induces phycoerythrin. This light color induced differential accumulation of phycobiliproteins allows the cells to maximize photosynthesis and growth rate. In red light the induction of phycocyanin genes is controlled by the phytochrome-based Rca system, which also partially represses the phycoerythrin genes. Further repression of phycoerythrin genes in red light is performed by a second pathway, called the Cgi system. To identify components of this pathway, I performed transposon mutagenesis and found several genes required for proper regulation of phycoerythrin. One encodes a translation initiation factor 3 (IF3). F. diplosiphon has two IF3 encoding genes and both appear to be dispensable. This raises the possibility that in cyanobacteria, core components of the protein synthesis process were duplicated and recruited for regulatory purposes.;The phycobiliproteins are also regulated by the availability of nutrients in their environments. When F. diplosiphon experiences low sulfate levels, it drastically down-regulates two phycocyanins known as PC1 and PC2 and expresses another form of phycocyanin (PC3), which has almost no sulfur containing amino acids. Using gene knockouts and gene expression assays I found that a non-coding portion of the RNA encoding PC3 destabilizes the RNA produced by the PC1 and PC2. This is a novel form of regulation through which the cells use the non-coding portion of a structural RNA to repress the expression of functionally related genes. |
