Rhie G, Beale S I
Division of Biology and Medicine, Brown University, Providence, Rhode Island 02912.
J Biol Chem. 1994 Apr 1;269(13):9620-6.
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 IX alpha 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.
蓝藻、红藻和隐藻含有藻胆蛋白，其作为光合捕光色素发挥作用。藻胆蛋白的发色团是藻胆素，即由原血红素合成的开链四吡咯。藻胆素形成的第一步是在血红素加氧酶催化的反应中原血红素转化为胆绿素IXα。在单细胞红藻嗜热栖热放线菌中，藻胆蛋白的积累需要光照。此前有报道称，在嗜热栖热放线菌中，别藻蓝蛋白和藻蓝蛋白的脱辅基蛋白成分的合成受光照诱导，藻胆素前体δ-氨基乙酰丙酸（ALA）、原卟啉IX和原血红素可以替代光照，且这种调控作用于mRNA合成水平。我们已经确定，嗜热栖热放线菌中藻胆素形成的一种关键酶受光照诱导。在黑暗中生长的细胞中，可提取的血红素加氧酶活性较低，在细胞光照后的最初24小时内，其活性增加约6倍。24小时后，活性降至约等于初始活性的水平。通过给予ALA可在未光照的细胞中诱导血红素加氧酶。已知D-葡萄糖会抑制嗜热栖热放线菌中藻蓝蛋白的积累，它会抑制光照或ALA对血红素加氧酶的诱导。光照或ALA对血红素加氧酶的诱导会被环己酰亚胺（一种细胞质蛋白合成抑制剂）阻断，但不会被氯霉素（一种叶绿体蛋白合成抑制剂）阻断。利福平（一种藻类叶绿体RNA合成抑制剂）和加巴喷丁（一种ALA生物合成的竞争性抑制剂）会阻断光照对血红素加氧酶的诱导，但不会阻断ALA对其的诱导。这些结果表明，嗜热栖热放线菌中的血红素加氧酶由藻胆素前体诱导。光照诱导以及D-葡萄糖对诱导的抑制可能是光照和D-葡萄糖对藻胆素前体形成的影响所介导的间接效应。结果还表明，血红素加氧酶由核基因编码，并在细胞质核糖体上合成。
