The absence of a m7G cap on beta-globin mRNA and alfalfa mosaic virus RNA 4 increases the amounts of initiation factor 4F required for translation

Abstract

beta-Globin mRNA and alfalfa mosaic virus (AMV) RNA 4, two naturally capped mRNAs, and satellite tobacco necrosis virus (STNV) RNA, a naturally uncapped mRNA, were prepared by in vitro transcription with and without a 5' m7G cap structure (m7G(5')ppp(5')N). The translation of the capped and uncapped forms of these mRNAs was measured in a crude S30 system and a partially purified system from wheat germ. In the S30 system the uncapped forms of beta-globin mRNA and AMV RNA 4 are much less active (greater than or equal to 10%) than their capped forms, whereas the uncapped and capped forms of STNV RNA are equally active. The low activity of uncapped beta-globin mRNA and AMV RNA 4 in the S30 system is due, in part, to inactivation of the uncapped mRNAs in this system. Additional studies, carried out in the partially purified system in which very little inactivation of the mRNAs occurs, show that the uncapped and capped forms of beta-globin mRNA or AMV RNA 4 differ markedly with respect to the amount of eukaryotic initiation factor (eIF)-4F required for translation. For beta-globin mRNA the absence of the 5' cap structure increases the concentration of eIF-4F required for half-maximal translation about 6-fold (from 10 to 60 nM) and for AMV RNA 4 it increases the concentration of eIF-4F about 12-fold (from 5 to 60 nM). The concentrations of eIF-3, eIF-4A, and eIF-4B required for half-maximal translation of the uncapped forms of beta-globin mRNA and AMV RNA 4 are either the same or only slightly higher (1.5- to 2-fold) than the concentrations required for the capped forms. With STNV RNA the concentration of eIF-4F required for half-maximal translation of either uncapped or capped STNV RNA is 3 nM, and the concentrations of eIF-3, eIF-4A, and eIF-4B required for the two forms are also the same. The translation of the capped and uncapped forms of beta-globin mRNA and AMV RNA 4 is inhibited strongly by low concentrations of m7GTP in the partially purified system containing low concentrations of eIF-4F. Under the same conditions, the translation of capped or uncapped STNV RNA is inhibited only slightly by m7GTP. These findings suggest the possibility that the mechanism by which eIF-4F interacts and initiates translation with naturally uncapped mRNAs may not be identical to the mechanism by which eIF-4F interacts and initiates translation of naturally capped mRNAs.