Translational Control by mRNA Competition

The molar translation yield of a given mRNA, that is, the frequency of translation of this mRNA, is not a constant property but is subject to regulation. Experimentally, this is observed by a shift in the relative amounts of individual proteins synthesized as a function of (1) the rate of overall protein synthesis, and (2) the amounts and types of mRNA present. 

In order to identify initiation factors involved in mRNA competition, a number of studies have employed reconstituted cell-free systems (Golini et al., 1976 Ray et al., 1983). A major problem with such systems is that certain components may be present in excess, while others may be limiting or partially inactivated, precluding efficient initiation. However, meaningful studies of translation initiation frequency in vitro can only be done in systems where ribosomes can cycle rapidly and repeatedly over mRNA. To date, the only cell-free systems that translate mRNA at high efficiency are the reticulocyte lysate (see Jackson, 1982) and the micrococcal nuclease-treated reticulocyte lysate (Pelham and Jackson, 1976). The latter system offers several advantages over reconstituted cell-free systems. It responds to translational control signals (see below), it is capable of extensive and efficient initiation in conditions more likely to be representative of protein synthesis in intact cells, and except for mRNA, it contains all other components for translation in a proportion much closer to that of the intact cell. 

Any decrease in the rate of initiation of translation leads to a drop in the a/ 3-globin synthetic ratio, while any increase causes a-globin mRNA translation to rise more than translation of p-globin mRNA (Lodish, 1974). This is the result expected if one assumes that each mRNA has its own rate constant for binding to ribosomes at initiation any nonspecific reduction in the rate of initiation at or before binding of mRNA will then result in a preferential inhibition of the translation of the mRNA species with the lower rate constant, in this case, a-globin mRNA (Lodish, 1974). 

In conditions of competition, translation of ferritin mRNA responds more readily to added eIF-2 than does translation of albumin mRNA the response of ferritin mRNA is that expected for an mRNA species that competes more weakly for eIF-2 (Kaempfer and Konijn, 1983). 

The cases discussed in this and the preceding two sections are clear-cut examples of mRNA competition for a general initiation factor that results in the preferential translation of certain mRNA species 

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In contrast to Millward and his colleagues, Detjen et al. (1982) find that protein synthesis in SC-1 cells remains cap dependent throughout infection and do not find a transition from cap-dependent to cap-independent translation in reovirus-infected L cells. These findings are consistent with their proposed mechanism of translation control by mRNA competition for a discriminatory factor. They suggest that a partial explanation for the difference of their results with those of Skup and Millward (1980u,Z>) may be the difference in experimental technique. They used m GTP inhibition to assess the extent of capping of mRNA species, which measures only mRNAs actively translated. Thus, their method would not detect the presence of uncapped mRNAs unless they were capable of translation. Thus, they cannot exclude the possibility that uncapped mRNAs are present but not translatable for some unclear reason. Millward and his coworkers also find that the translation of late reovirus mRNA is sensitive to m GTP but do not have an explanation for this observation (Zarbl and Millward, 1983). Clearly, further studies are necessary to explain the conflicts in the data reported by the laboratories of Mill-ward and Thach. 

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