REGULATION OF HEME OXYGENASE ACTIVITY IN CYANIDIUM-CALDARIUM BY LIGHT, GLUCOSE, AND PHYCOBILIN PRECURSORS

Cyanobacteria, red algae, and cryptophytes contain Phycobiliproteins w hich function as photosynthetic light-harvesting pigments. The chromop hores of phycobiliproteins are phycobilins, open-chain tetrapyrroles t hat are synthesized from protoheme. The first step of phycobilin forma tion is the conversion of protoheme to biliverdin IXalpha in a reactio n that is catalyzed by heme oxygenase. In the unicellular red alga, Cy anidium caldarium, light is required for the accumulation of phycobili proteins. It has been reported previously that the synthesis of the ap oprotein components of allophycocyanin and phycocyanin is induced by l ight in C. caldarium, that the phycobilin precursors, delta-aminolevul inic acid (ALA), protoporphyrin IX, and protoheme can substitute for l ight, and that the regulation is exerted at the level of mRNA synthesi s. We have determined that a key enzyme of phycobilin formation is ind uced by light in C. caldarium. Extractable heme oxygenase activity is low in dark-grown cells, and it increases approximately 6-fold during the first 24 h after the cells are illuminated. After 24 h, the activi ty decreases to a level approximately equal to the initial activity. H eme oxygenase is induced in unilluminated cells by administration of A LA. D-Glucose, which is known to inhibit phycocyanin accumulation in C . caldarium, inhibits the induction of heme oxygenase by light or ALA. Induction of heme oxygenase by light or ALA is blocked by cycloheximi de, an inhibitor of cytoplasmic protein synthesis, but not by chloramp henicol, an inhibitor of chloroplast protein synthesis. Rifampicin, an inhibitor of algal chloroplast RNA synthesis, and gabaculine, a compe titive inhibitor of ALA biosynthesis, block the induction of heme oxyg enase by light but not by ALA. These results indicate that heme oxygen ase in C. caldarium is induced by phycobilin precursors. The induction by light and the repression of the induction by D-glucose are probabl y indirect effects mediated by the effects of light and D-glucose on p hycobilin precursor formation. The results also indicate that heme oxy genase is encoded by a nuclear gene and is synthesized on cytoplasmic ribosomes.