Abstract
Affinity chromatography on columns containing globin mRNA, R17 phage mRNA, or double-stranded RNA linked to cellose is used to demonstrate unequivocally that the eukaryotic initiation factor (eIF-2) that forms a ternary complex with Met-tRNAf and GTP also binds tightly to these RNA species. Affinity chromatography of reticulocyte ribosomal wash yields over 100-fold purification of Met-tRNAf-binding factor. This factor is eluted as one of the most tightly bound proteins, and is active in protein synthesis even after passage over a column of double-stranded RNA-cellulose. eIF-2 binds mRNA and double-stranded RNA in distinctly different modes, protecting essentially all sequences in double stranded RNA, but very few in mRNA, against digestion with ribonuclease. Apparently, eIF-2 recognized the A conformation of double-stranded RNA, but not its sequence. By contrast, globin, Mengo virus, R17 and vesicular stomatitis virus mRNA are shown to possess a high-affinity binding site for eIF-2 that is absent in negative-strand RNA of vesicular stomatitis virus, an RNA that cannot serve as messenger. The results support the concept that eIF-2, the initiation factor that binds Met-tRNAf, recognizes an internal sequence in mRNA essential for protein synthesis.