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 pathway. Analysis of mutants defective in plastid-localized chlorophyll synthesis as well as feeding of chlorophyll precursors have previously provided evidence for the participation of the chloroplast in this light 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 cytb₆/f complex did not affect light inducibility. The competence of a mutant defective in carotenoid biosynthesis for induction of HSP70A by light also ruled out the involvement of photoreceptors with a carotenoid-based chromophore like chlamyrhodopsin. Analysis of the wavelength dependence of HSP70A mRNA accumulation revealed a major peak around 600 nm and a minor one around 450 nm. This suggests that a novel photoreceptor mediates this induction. Continuous irradiation during the induction phase was required for a sustained accumulation of HSP70A mRNA, indicating that continuous triggering 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 1h.