Chlorophyll a is a favorable substrate for Chlamydomonas Mg-dechelatase encoded by STAY-GREEN

Abstract Mg removal from chlorophyll by Mg-dechelatase is the first step of chlorophyll degradation. Recent studies showed that in Arabidopsis, Stay Green ( SGR ) encodes Mg-dechelatase. Though the Escherichia coli expression system is advantageous for investigating the properties of Mg-dechelatase, Arabidopsis Mg-dechelatase is not successfully expressed in E. coli . Chlamydomonas reinhardtii SGR (CrSGR) has a long, hydrophilic tail, suggesting that active CrSGR can be expressed in E. coli . After the incubation of chlorophyll a with CrSGR expressed in E. coli , pheophytin a accumulated, indicating that active CrSGR was expressed in E. coli . Substrate specificity of CrSGR against chlorophyll b and an intermediate molecule of the chlorophyll b degradation pathway was examined. CrSGR exhibited no activity against chlorophyll b and low activity against 7-hydroxymethyl chlorophyll a , consistent with the fact that chlorophyll b is degraded only after conversion to chlorophyll a . CrSGR exhibited low activity against divinyl chlorophyll a and chlorophyll a ′, and no activity against chlorophyllide a , protochlorophyll a , chlorophyll c 2 , and Zn-chlorophyll a . These observations indicate that chlorophyll a is the most favorable substrate for CrSGR. When CrSGR was expressed in Arabidopsis cells, the chlorophyll content decreased, further confirming that SGR has Mg-dechelating activity in chloroplasts.