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Abstract

The chloroplast glutamyl-tRNA (tRNAGlu) is unique in that it has two entirely different functions. In addition to acting in translation, it serves as substrate of glutamyl-tRNA reductase, the enzyme catalyzing the committed step in the tetrapyrrole biosynthetic pathway. How the tRNAGlu pool is distributed between the two pathways and whether tRNAGlu allocation limits tetrapyrrole biosynthesis and/or protein biosynthesis remains poorly understood. We have generated a series of transplastomic tobacco (Nicotiana tabacum) plants to alter tRNAGlu expression levels and also introduced a point mutation into the plastid trnE gene, which was reported to uncouple protein biosynthesis from tetrapyrrole biosynthesis in chloroplasts of the protist Euglena gracilis. We show that, rather than comparable uncoupling of the two pathways, the trnE mutation is lethal in tobacco as it inhibits tRNA processing, thus preventing translation of glutamate codons. Ectopic expression of the mutated trnE gene uncovered an unexpected inhibition of glutamyl-tRNA reductase by immature tRNAGlu. We further demonstrate that, whereas overexpression of tRNAGlu does not affect tetrapyrrole biosynthesis, reduction of GluTR activity through inhibition by tRNAGlu precursors causes tetrapyrrole synthesis to become limiting in early plant development when active photosystem biogenesis provokes a high demand for de novo chlorophyll biosynthesis. Taken together, our work provides insight into the roles of tRNAGlu at the intersection of protein biosynthesis and tetrapyrrole biosynthesis.