mRNA secondary structure as a determinant in cap recognition and initiation complex formation. ATP-Mg2+ independent cross-linking of cap binding proteins to m7I-capped inosine-substituted reovirus mRNA.

Polypeptides of Mr = 50,000 and 80,000 in rabbit reticulocyte initiation factor preparations can be specifically cross-linked to the oxidized t' cap structure of native reovirus mRNA in an ATP-Mg2+-dependent manner (Sonenberg, N., Guertin, D., Cleveland, D., and Trachsel, H. (1981) Cell 27, 563-572). However, specific cross-linking of these polypeptides can occur in the absence of ATP-Mg2+ when m7I-capped inosine substituted mRNA, which contains less secondary structure than native reovirus mRNA, is used. We also found, using wheat germ extract, that inhibition of initiation complex formation by high salt concentrations is directly related to the degree of secondary structure of the mRNA. Binding of ribosomes to bromouridine-substituted reovirus mRNA is severely inhibited at high K+ concentrations, while binding to inosine-substituted mRNA is only slightly inhibited and binding of native reovirus mRNA is inhibited to an intermediate degree. The results are consistent with the hypothesis that cap recognition factors mediate an ATP-dependent melting of secondary structures involving 5' proximal sequences to the initiation codon in order to facilitate binding of ribosomes during translation initiation.