Abstract
In plants, phytochromes play roles in various photoresponses as red-farred light photoreceptor. Recently, novel photoreceptors which have a phytochrome-like GAF domain were found in cyanobacteria and named "Cyanobacteriochrome". CcaS is a putative cyanobacteriochrome which controls light-dependent expression of phycobilisome protein (CpcG2). CcaS is a His kinase protein which contains the putative cyanobacteriochrome GAF domain and flavin-binding LOV-like domain.
In this study, we prepared His-tagged CcaS-GAF from Synechocystis 6803 and phycocyanobilin (PCB)-producing Escherichia coli. Both proteins showed reversible photoconversion between green-absorbing and red-absorbing forms with difference spectra very similar to each other. This suggests that CcaS-GAF binds PCB in Synechocystis 6803. Then, His-tagged CcaS without transmembrane region was purified from PCB-producing E. coli. The absorption spectra showed that CcaS bound both PCB and flavin, suggestive of a composite photoreceptor. Autophosphorylation activity was detected. Effects of light illumination and phosphotransfer from CcaS to a cognate response-regulator will be presented.
