The number of growing polypeptide chains on reticulocyte polyribosomes

Abstract

Reticulocyte polyribosomes synthesizing hemoglobin can be separated physically from ribosomes inactive in protein synthesis. After labeling with [ 14 C]leucine or phenylalanine, the specific activity of the polyribosomes was determined. By measuring the intra-cellular specific activity of the amino acids, the number of leucine or phenylalanine residues per ribosome was calculated. The results are consistent with the conclusion that there is a single growing polypeptide chain per ribosome in the polyribosomes. This calculation is based upon the assumption that all of the reticulocyte polyribosomes are active in protein synthesis. Experiments with cell fractionation and varying magnesium ion concentrations support this assumption. The effect of a non-uniform rate of polypeptide chain assembly is discussed.