The Role of N-Formyl-methionyl-sRNA in Protein Biosynthesis

Bacterial sRNA has been fractionated into two methionine-accepting species, only one of which could be formylated. The binding of each charged species to ribosomes under the direction of a variety of trinucleoside diphosphates has been studied. The codeword AUG could be assigned to both species, whereas the related trinucleoside diphosphates GUG and UUG caused binding only of the species which could be formylated. The presence of the formyl group on this methionyl-sRNA did not alter its binding characteristics. The results of the binding studies were supported by investigations using a cell-free system directed by poly UAG or poly UG. Methionine was incorporated from the charged species, which could be formylated into the N-terminal position of polypeptides by both polymers whether this species was formylated or not. Poly UAG, but not poly UG, stimulated the incorporation of methionine into polypeptide from the methionyl-sRNA species, which could not be formylated. The analysis of this polypeptide product identified methionine in internal positions.