Regulation and Localization of Isoforms of the Aerobic Oxidative Cyclase in Chlamydomonas reinhardtii

Tetrapyrrole synthesis is complex and is regulated at several points. The aerobic oxidative cyclase catalyzes the oxidative closure of the fifth ring characteristic of all chlorophylls. Chlamydomonas reinhardtii encodes two paralogous protein components of the cyclase, which are differentially accumulated based on copper nutrition and oxygen supply. CRD1 is accumulated in copper-deficient conditions, whereas CTH1 is present in copper replete conditions. Here, we show that CRD1 expression is regulated through a copper-responsive element located upstream of the transcription start site. The differential production of CRD1 transcript accounts for the differential accumulation of the corresponding polypeptide. The CTH1 locus produces two transcripts: a 2.1 kb one under copper-replete conditions and a 3.1 kb one under copper-deficient conditions. We show that the 2.1 kb transcript can be translated into protein in vitro whereas the 3.1 kb transcript cannot. Differential accumulation of the 2.1 vs the 3.1 kb transcript therefore accounts for the copper-responsive accumulation of CTH1. Biochemical fractionation reveals that both CRD1 and CTH1 are localized to chloroplast envelope as well as thylakoid membranes.