Characterization of rabbit reticulocyte factor(s) that stimulates the translation of mRNAs lacking 5'-terminal 7-methylguanosine.

Purified reticulocyte initiation factors were assayed for their ability to stimulate the translation of uncapped vesicular stomatitis virus (VSV) mRNA relative to capped VSV mRNA in reticulocyte lysates. Both eIF-3 and eIF-4B preparations contained such an activity. However, at least some of the activity was due to the presence of an as yet unidentified factor that contaminated the eIF-3 and eIF-4B preparations. A polypeptide of apparent molecular weight 24,000 that can be specifically cross-linked to the oxidized 5'-terminal cap structures of reovirus and other viral mRNAs co-purifies with this new factor, and may be a component of it. This stimulatory activity present in our eIF-4B preparations was found to be greater when lower concentrations of KCl were present in the reaction mixture, possibly explaining why uncapped VSV mRNA is translated relatively more efficiently at low K+ concentrations.