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

Cyanobacteria, red algae, and cryptophytes contain Phycobiliproteins which function as photosynthetic light-harvesting pigments. The chromophores of phycobiliproteins are phycobilins, open-chain tetrapyrroles that are synthesized from protoheme. The first step of phycobilin formation is the conversion of protoheme to biliverdin IXalpha in a reaction that is catalyzed by heme oxygenase. In the unicellular red alga, Cyanidium caldarium, light is required for the accumulation of phycobiliproteins. It has been reported previously that the synthesis of the apoprotein components of allophycocyanin and phycocyanin is induced by light in C. caldarium, that the phycobilin precursors, delta-aminolevulinic acid (ALA), protoporphyrin IX, and protoheme can substitute for light, and that the regulation is exerted at the level of mRNA synthesis. We have determined that a key enzyme of phycobilin formation is induced 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 activity decreases to a level approximately equal to the initial activity. Heme oxygenase is induced in unilluminated cells by administration of ALA. 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 cycloheximide, an inhibitor of cytoplasmic protein synthesis, but not by chloramphenicol, an inhibitor of chloroplast protein synthesis. Rifampicin, an inhibitor of algal chloroplast RNA synthesis, and gabaculine, a competitive inhibitor of ALA biosynthesis, block the induction of heme oxygenase by light but not by ALA. These results indicate that heme oxygenase in C. caldarium is induced by phycobilin precursors. The induction by light and the repression of the induction by D-glucose are probably indirect effects mediated by the effects of light and D-glucose on phycobilin precursor formation. The results also indicate that heme oxygenase is encoded by a nuclear gene and is synthesized on cytoplasmic ribosomes.