Initiation of endogenous messenger RNA translation on hen oviduct polysomes

The eIF-2A fraction of reticulocyte ribosomal salt wash is capable of maximally stimulating the translation of endogenous messenger RNA by hen oviduct polysomes. The factor increases the initiation of protein synthesis 2--3-fold when measured by the factor-dependent synthesis of NH2-terminal peptides. The addition to these polysomes of elongation factor, EF-1, also increases protein synthesis but at a distinctly different rate and Mg2+ concentration optimum than the eIF-2A fraction. Moreover, there is no stimulation of NH2-terminal peptide synthesis with EF-1 alone. In contrast, all the known initiation factors are required for the translation of exogenous globulin mRNA on oviduct polysomes. Reticulocyte polysomes isolated by an identical procedure to that used for oviduct polysomes or by standard methods also require all the initiation factors for the translation of either endogenous mRNA or exogenous ovalbumin mRNA. Addition of 7-methylguanosine 5'-monophosphate does not inhibit the factor-dependent stimulation of oviduct polysomes except at high concentrations (1.0 mM) indicating that the sites with which 7-methylguanosine 5'-monophosphate normally competes are already occupied. These findings suggest that the messenger RNA remains bound to the oviduct polysomes or initiation factors. Hence the addition of exogenous factors which are involved with mRNA recognition and binding to the ribosome are not required. It has been previously shown that eIF-2A is capable of binding in vitro the initiatior tRNA to an existing Ado-Urd-Gua-40 S complex and initiating protein synthesis when such a complex is present. These present studies indicate that such an initiation complex may exist within the oviduct cell on membrane-associated polysomes. Under these circumstances eIF-2A mediates binding of the initiator tRNA and initiates protein synthesis.