SYNTHESIS AND BINDING OF PHYCOERYTHRIN AND ITS ASSOCIATED LINKERS TO THE PHYCOBILISOME IN RHODELLA-VIOLACEA (RHODOPHYTA) - COMPARED EFFECTSOF HIGH LIGHT AND TRANSLATION INHIBITORS

We studied the synthesis and binding of phycoerythrin and its associat ed linkers to the phycobilisome (PBS) in Rhodella violacea (Kornmann) Wehrmeyer and compared the effects of high light and translation inhib itors on these processes. Rhodella violacea has a simple hemidiscoidal PBS structure with a well-known composition. The number of PBSs per c ell decreases when irradiance is increased, and at higher irradiances the rods are shortened with a specific loss of the terminal hexamer of phycoerythrin (PE) and its associated linker. To test whether or not the observed variations were due to a coordination between the express ion of the chloroplast-encoded PE and the nuclear-encoded linkers, we inhibited the expression of the chloroplast genes by the translation i nhibitor chloramphenicol. In the few PBSs synthesized, the linker asso ciated to the terminal PE hexamer was missing while that associated wi th the intermediate PE hexamer was still present. The inhibition by cy cloheximide of the translation of the nuclear-encoded linkers did not influence the synthesis of the chloroplast-encoded phycobiliproteins. The absence of linkers prevented the formation of PE hexamers and thei r binding to the PBSs. We therefore propose the existence of two level s of regulation for PE and associated linkers: the intermediate PE hex amer and associated linker are always present even though their amount is reduced when irradiance is increased. In contrast, the terminal he xamer of PE and its associated linker are no longer present under high light. Their absence can. be due to a feedback control between the le vel of PE and the synthesis of the linker: when the level of PE is low ered below a given value by the action of light on, the chloroplast, a signal coming from the chloroplast reaches the nucleus and the synthe sis of the linker is repressed. There is no sign of nuclear regulation of the synthesis of PE, but the nuclear-encoded linkers have a struct ural role in the formation of PE hexamers.