Mg-dechelatase is involved in the formation of photosystem II but not in chlorophyll degradation in Chlamydomonas reinhardtii.

The STAY-GREEN (SGR) gene encodes a Mg-dechelatase that catalyzes the conversion of chlorophyll (Chl) a to pheophytin (Pheo) a. This reaction is the first and most important regulatory step in the chlorophyll degradation pathway. Conversely, Pheo a is an indispensable molecule in photosystem (PS) II, suggesting the involvement of SGR in the formation of PSII. To investigate the physiological functions of SGR, we isolated Chlamydomonas sgr mutants by screening an insertion-mutant library. The sgr mutants had reduced maximum quantum efficiency of PSII (Fv/Fm) and reduced Pheo a levels. These phenotypes were complemented by the introduction of the Chlamydomonas SGR gene. Blue-native polyacrylamide gel electrophoresis and immunoblotting analysis showed that although PSII levels were reduced in the sgr mutants, PSI and light-harvesting Chl a/b complex levels were unaffected. Under nitrogen starvation conditions, Chl degradation proceeded in the sgr mutants as in wild type, indicating that Chlamydomonas SGR is not required for Chl degradation and primarily contributes to the formation of PSII. In contrast, in the Arabidopsis sgr triple mutant (sgr1 sgr2 sgrL), which completely lacks SGR activity, PSII was synthesized normally. These results suggest that the Arabidopsis SGR participates in Chl degradation, while the Chlamydomonas SGR participates in PSII formation in spite of the same catalytic property. This article is protected by copyright. All rights reserved.