N-Formylmethionyl-tRNA_f of Wheat Chloroplasts. Its Synthesis by a Wheat Transformylase

In a study of the role of methionine in protein chain initiation in wheat, a transformylase was isolated and partially purified from commercial wheat germ and from wheat leaves grown under sterile conditions. This enzyme catalyzed the formylation of a methionyl-tRNA_f which was present in the wheat leaf chloroplasts. The enzyme used N¹⁰-formyltetrahydrofolate as formyl donor and required 5 mM Mg²⁺ and 20 mM K⁺ ions for maximum activity. The molecular weight of the transformylase was estimated at 45,000 by gel filtration on Sephadex G-150. The enzyme was present in isolated chloroplasts at a higher specific activity than in the remainder of the leaf extract. This transformylase and the methionyl-tRNA_f are components of a protein chain initiating system in wheat chloroplasts similar in many ways to the system in Escherichia coli.