Polypeptide chain initiation in E. coli: isolation of homogeneous initiation factor E2 and its relation to ribosomal proteins.

Previous work has shown that F(2), one of several ribosomal factors involved in polypeptide chain initiation, functions in the binding of formylmethionyl-transfer RNA (fMet approximately tRNA(f)) to a messenger RNA-ribosome complex. F(2) was isolated from 1.0 M ammonium chloride washes of E. coli Q13 ribosomes as a protein homogeneous on polyacrylamide gel electrophoresis at both pH 4.5 and 7.8. Its molecular weight is approximately 80,000. Comparison of electrophoretic patterns of ribosomal proteins from NH(4)Cl-washed and unwashed ribosomes and F(2), at pH 4.5, shows that F(2) corresponds to the slowest-moving component of the proteins derived from unwashed ribosomes. This component is missing from the NH(4)Cl-washed ribosomes. The activity of F(2) is stimulated by two additional factors, initiation factor F(1) and a factor(s) present in a narrow ammonium sulfate fraction of the ribosomal NH(4)Cl wash. The nature of the latter is unknown.