Characterization of cyanobacteriochrome photoreceptors in fremyella diplosiphon.

摘要

Photosensory proteins that are capable of sensing and responding to many different wavelengths of light are essential for light color perception among prokaryotic microorganisms such as cyanobacteria. In the freshwater cyanobacterium Fremyella diplosiphon, I have identified a significant number of putative light sensing proteins encoded within its genome. Classification of these photosensory proteins has revealed 27 phytochrome superfamily photoreceptors, and of these 23 are classified as cyanobacteriochromes, which are specific to cyanobacteria and require only a single photosensory GAF domain (cGMP phosphodiesterase/adenylate cyclase/FhlA) to perceive light. The first cyanobacteriochrome characterized in F. diplosiphon was the green/red sensor histidine kinase RcaE due to its light color sensing role in regulating the process of type III chromatic acclimation, which affects the accumulation of light harvesting components, called phycobilisomes, in response to red and green light. RcaE is able to regulate the expression of genes involved in this process via a complex two component phosphorelay, through which the phosphorylation of the transcription factor RcaC in red light causes the activation and repression of specific genes through binding to a direct-repeat binding site, called the L box. In my dissertation research I characterize the function of IflA, a four color sensing cyanobacteriochrome that is regulated by RcaE, via RL repression of iflA mRNA by RcaC, and whose transcript accumulates to a 6-fold higher level in green versus red light. This research provides the first example of the hierarchical regulation of one phytochrome-class photoreceptor by another phytochrome-class photoreceptor in prokaryotes. IflA also has complex light sensing capabilities that allow it to sense blue, green, red, and far-red light through the combined actions of two photosensory GAF domains. My dissertation research details the inter- and intra-molecular interactions of these photosensory GAF domains in response to these four colors of light and demonstrates, by the characterization of the physiology of wild type and an iflA null mutant strain, that IflA plays a role in acceleration of the growth of cells under low cell density conditions.