GLUTAMYL-TRANSFER RNA - AT THE CROSSROAD BETWEEN CHLOROPHYLL AND PROTEIN-BIOSYNTHESIS

5-Aminolevulinic acid (ALA) is the key precursor for chlorophyll biosy nthesis. 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 b y Glu-tRNA reductase (GluTR). The Glu-tRNA is common to the biosynthes es of both proteins and porphyrins. Glutamate-1-semialdehyde-2,1-amino mutase (GSA-ARI) then catalyzes the conversion of GSA to ALA. The gene s that encode these enzymes and glutamate-specific tRNA have now been characterized from a variety of plants, and it has therefore become po ssible to understand the components and regulation of this essential b iosynthetic process in higher plants in considerable detail.