CHARACTERIZATION OF PHOTORECEPTOR AND SIGNALING PATHWAY FOR LIGHT INDUCTION OF THE CHLAMYDOMONAS HEAT-SHOCK GENE HSP70A

Light induces heat-shock gene HSP70A by a heat stress-independent path way. Analysis of mutants defective in plastid-localized chlorophyll sy nthesis as well as feeding of chlorophyll precursors have previously p rovided evidence for the participation of the chloroplast in this ligh t induction. An involvement of photosynthesis appears unlikely because an inhibitor of photosystem II and various mutations causing defects in photosystems I and II or the cytb(6)/f complex did not affect light inducibility. The competence of a mutant defective in carotenoid bios ynthesis for induction of HSP70A by light also ruled out the involveme nt of photoreceptors with a carotenoid-based chromophore like chlamyrh odopsin. Analysis of the wavelength dependence of HSP70A mRNA accumula tion revealed a major peak around 600 mn and a minor one around 450 mn . This suggests that a novel photoreceptor mediates this induction. Co ntinuous irradiation during the induction phase was required for a sus tained accumulation of HSP70A mRNA, indicating that continuous trigger ing of the signaling pathway is needed. A prerequisite for this light induction is a state of competence achieved by incubation of the cells in the dark for at least 1 h.