Eukaryotic initiation factor (eIF)-4F. Implications for a role in internal initiation of translation

In order to study the eukaryotic translation initiation mechanisms of "internal initiation," "re-initiation," and/or "coupled internal initiation," a series of model mRNAs have been constructed which contain two non-overlapping open reading frames (ORFs) that encode different lengths of rabbit alpha globin. These mRNAs, along with the bicistronic constructs TK/CAT and TK/P2CAT developed by Pelletier and Sonenberg (Pelletier, J., and Sonenberg, N. (1988) Nature 334, 320-325, 1988), were used to program an in vitro rabbit reticulocyte lysate translation system. Cap-dependent and cap-independent translation were distinguished by monitoring translation in the presence or absence of exogenously added cap analog (m7GTP). Messenger RNAs which translate both ORF1 and ORF2 by a cap-dependent mechanism, as well as mRNAs that translate ORF2 by a cap-independent mechanism while still translating ORF1 in a cap-dependent fashion have been obtained. These same alpha globin mRNAs differ by no more than 45 nucleotides in intercistronic length. Initiation factor addition studies were performed in this same in vitro translation system. Both eukaryotic initiation factor (eIF)-4F and, to a lesser extent, eIF-4B can stimulate translation of an internally located ORF independent of upstream ORF translation and in a manner not dependent on mRNA cap recognition. This indicates that the cap-recognition initiation factor, eIF-4F, and eIF-4B facilitate cap-independent and internal initiation of an open reading frame.