Inhibitory effects of 'cap' analogues on globin mRNA and encephalomyocarditis RNA translation in a reticulocyte cell-free system.

The cap analogues 7-methylguanosine 5'-phosphate [m7G(5')p], 7-methylguanosine 5'-triphosphate [m7G(5')ppp] and 2'-O-methylguanosine 5'-triphosphate [Gm(5')ppp] inhibit the translation of capped globin mRNA and encephalomyocarditis (EMC) RNA (a naturally uncapped mRNA) in a reticulocyte cell-free system. This inhibition occurs at the level of protein synthesis initiation and is of a competitive type since it can be overcome by increasing the mRNA concentration. However, the translation of globin mRNA is more sensitive to the inhibitory effects of the cap analogues m7G(5')p and m7G(5')ppp than translation of EMC RNA. The same spectra of specific inhibition is also observed with some other initiation inhibitors such as aurintricarboxylic acid, which inhibits mRNA binding, but not with pactamycin which does not affect mRNA interaction. A model is presented suggesting that this preferential inhibition by cap analogues could be explained mainly by the different affinities of globin mRNA and EMC RNA for the initiation complexes between 40-S subunits and Met-tRNAf. Moreover Gm(5')ppp cannot be considered simply as a cap analogue since it also affects some step prior to mRNA binding.