Glutamyl-transfer RNA: At the crossroad between chlorophyll and protein biosynthesis

5-Aminolevulinic acid (ALA) is the key precursor for chlorophyll biosynthesis. It is made in the chloroplast in a process that is dependent on transfer RNA (tRNA), and that involves two main steps. In the first pivotal step, tRNA charged with glutamate (glutamyl-tRNA) (Glu-tRNA) is reduced to glutamate-l-semialdehyde (GSA) in a reaction catalyzed by Glu-tRNA reductase (GluTR). The Glu-tRNA is common to the biosyntheses of both proteins and porphyrins. Glutamate-1-semialdehyde-2,1-aminomutase (GSA-ARI) then catalyzes the conversion of GSA to ALA. The genes that encode these enzymes and glutamate-specific tRNA have now been characterized from a variety of plants, and it has therefore become possible to understand the components and regulation of this essential biosynthetic process in higher plants in considerable detail.