Chlorophyll biosynthesis in Chlamydomonas starts with the formation of glutamyl-tRNA

Abstract

An RNA moiety has been shown to be involved in the conversion of Glu to delta-aminolevulinic acid (ALA), the first committed intermediate of the chlorophyll pathway. We now have evidence suggesting that in Chlamydomonas, the first reaction for converting Glu to ALA is the aminoacylation of Glu to a Glu-specific tRNA. The Glu-tRNA thus formed could be the substrate for Glu-1-semialdehyde synthesis catalyzed by a postulated dehydrogenase. Glu-1-semialdehyde can be converted to ALA by an aminotransferase. Of the three reactions converting Glu to ALA, only the second reaction, catalyzed by a postulated dehydrogenase, is sensitive to inhibition by heme (a known inhibitor of ALA synthesis). We think the regulated enzyme of ALA synthesis is the postulated dehydrogenase. It is postulated that in the chloroplast of Chlamydomonas, the synthesis of ALA and the synthesis of proteins may share a common pool of glutamyl-tRNA.