The formation of a stable complex between glutamyl-tRNA synthetase and the first enzyme of chlorophyll biosynthesis glutamyl-tRNA reductase was investigated in the green alga Chlamydomonas reinhardtii. Apparently homogenous enzymes, purified after previously established purification protocols were incubated in various combinations with ATP, glutamate, tRNAGlu and NADPH and formed complexes were isolated via glycerol gradient centrifugation. Stable complexes were detected only after the preincubation of glutamyl-tRNA synthetase, glutamyl-tRNA reductase with either glutamyl-tRNA or free tRNAGlu, ATP and glutamate, indicating the obligatory requirement of aminoacylated tRNAGlu for complex formation. The further addition of NADPH resulting in the reduction of the tRNA-bound glutamate to glutamate 1-semialdehyde led to the dissociation of the complex. Once complexed to the two enzymes tRNAGlu was found to be partially protected from ribonuclease digestion. Escherichia coli, Bacillus subtilis and Synechocystis 6803 tRNAGlu were efficiently incorporated into the protein—RNA complex. The detected complexes provide the chloroplast with a potential channeling mechanism for Glu-tRNAGlu into chlorophyll synthesis in order to compete with the chloroplastic protein synthesis machinery.
