Charging of Both, Plastidal tRNAg,n and tRNAglu with Glutamate and Subsequent Amidation of the Misacylated tRNAg,n by a Glutamyl-tRNA Amidotransferase in the Unicellular Green Alga Scenedesmus obliquus, Mutant C-2A’

U. C. Vothknecht* and D. Dörnemann Fachbereich Biologie/Botanik, Philipps-Universität Marburg, Karl-v.-Frisch-Straße, D-35032 Marburg, Bundesrepublik Deutschland Z. Naturforsch. 50c, 789-795 (1995); received June 16/August 28, 1995 C5-Pathway, Glutamyl-tRNAglu-Synthetase (E.C.6.1.1.17), Misacylation of tRNAglu, Amidotransferase, Scenedesmus obliquus In a previous paper we described the purification of a glutamyl-tRNA synthetase from the unicellular green alga Scenedesmus obliquus, mutant C-2A'. We now demonstrate that, firstly, this enzyme is capable of mischarging plastidal tRNA gln from barley with glutamate, as well as it regularly charges the plastidal tRNAglu from Scenedesmus. Secondly, we show that the mischarged glutamyl-tRNAgln is subsequently amidated by a glutamyl-tRNA amidotransfer­ ase to form the glutaminyl-tRNAglri required for plastidal protein biosynthesis. This phenom­ enon could already be demonstrated for higher plant chloroplasts, mitochondria, cyanobact­ eria and gram-positive bacteria, as far as investigated. As recently shown the applied glutamyl-tRNA synthetase from Scenedesmus is a plastidal enzyme. In this paper we prove by treatment with monobromobimane and cyanogen bromide that the „regular“ substrate of the enzyme, tRNAglu from Scenedesmus, is a plastidal tRNA with the plastid-specific sulfur modification in the anticodon. In the case of cyanogen bromide treatment, a total inactivation of the tRNA was achieved, revealing the presence of a sulfur modification in the plastidtRNAglu anticodon.