Fractionation and partial characterization of the protein synthesis system of wheat germ. I. Resolution of two elongation factors and five initiation factors.

Wheat germ S100 supernatant was resolved into 10 components, all of which are required for tobacco mosaic virus RNA-directed incorporation of amino acid into protein. Two of the components, C2b and C2a, are, respectively, elongation factors 1 and 2, having molecular weights of 51,000 and 72,000. A third factor (C2e) binds Met-tRNA in a GTP-requiring reaction and is absolutely required for the formation of both 40 S ribosome.Met-tRNAiMet and 80 S ribosome.Met-tRNAiMet complexes. This factor is considered to be eucaryotic initiation factor 2. Formation of 80 S ribosome Met-tRNAiMet complexes is augmented 2 to 3-fold by the simultaneous addition of an mRNA, factor C1, and fraction D2(a + b). Factor C1 also reverses the inhibition of translation by an increased concentration of monovalent cations and promotes the translation of a "competitively inhibited" mRNA. These observations suggest that factor C1 and either factor D2a and D2b (or both) function in mRNA attachment reactions. Factor C1 has a molecular weight of 115,000 and appears to be made up of two or three subunits. Factor D2b has been purified to essential homogeneity and has a molecular weight of 55,000. Its interaction with mRNA and its molecular weight suggest that this factor is the wheat germ equivalent of reticulocyte eucaryotic initiation factor 4A.