Role of mRNA competition in regulating translation: further characterization of mRNA discriminatory initiation factors.

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Abstract
Host and reovirus mRNA competed with one another for translation in infected cells. Kinetic analysis suggested that the site of competition is a message discriminatory initiation factor which must bind to the mRNA before it can interact with the 40S ribosomal unit. The present communication describes an in vitro assay which can detect message discriminatory activities. A competitive situation is established by using reovirus and globin mRNA and then the specificity with which this competition is relieved by added components is measured. Among the various initiation factors survyeed with this assay, 2 have the properties expected of the mRNA discriminatory factor. These are eukaryotic initiation factor 4A and a cap binding protein complex. Inasmuch as the cap binding protein complex contains a subunit similar or identical to the initiation factor eIF-4A, it seems likely that only one form of the latter factor may be active in vivo. In vitro, both factors relieve competition among both capped and uncapped reovirus mRNA according to similar hierarchies. Some feature other than the m7G cap, such as nucleotide sequence or secondary structure, is recognized by the discriminatory factor.