, 1995 C5-Pathway, Glutamyl-tRNAglu-Synthetase (E.C.220.127.116.11), Misacylation of tR N A glu, Amidotransferase, Scenedesmus obliquus In a previous paper we described the purification of a glutamyl-tRNA synthetase from the unicellular green alga Scenedesmus obliquus, m utant C-2A'. We now dem onstrate that, firstly, this enzyme is capable of mischarging plastidal tR N A gln from barley with glutamate, as well as it regularly charges the plastidal tR N A glu from Scenedesmus. Secondly, we show that the mischarged glutamyl-tRNAgln is subsequently am idated by a glutamyl-tRNA am idotransfer ase to form the glutaminyl-tRNAglri required for plastidal protein biosynthesis. This phenom enon could already be dem onstrated 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 treatm ent with m onobromobim ane and cyanogen bromide that the "regular" substrate of the enzyme, tR N A glu from Scenedesmus, is a plastidal tRNA with the plastid-specific sulfur modification in the anticodon. In the case of cyanogen bromide treatm ent, a total inactivation of the tRNA was achieved, revealing the presence of a sulfur modification in the plastid-tRNA glu anticodon.