Inhibition of the transition of a 40S ribosome–Met-tRNA_i^Met complex to an 80S ribosome–Met-tRNA_i^Met complex by 7-methylguanosine-5′-phosphate

Abstract

7-METHYLGUANOSINE-5′-PHOSPHATE (m⁷G^5′p) inhibits in vitro amino acid incorporation catalysed by several eukaryotic mRNAs1. The analogues, 7-methylguanosine-2(3)′-phosphate and unmethylated guanosine-5′-phosphate have no effect on these reactions, suggesting that the specific inhibition by m⁷G^5′p is related to the m⁷G^5′ppp cap known to be present at the 5′ end of many eukaryotic and viral mRNAs^2–12. As a more direct test of this idea, we report here experiments with a fractionated protein chain initiating system from wheat germ in which mRNA is required for the conversion of a 40S–Met-tRNA_i^Met complex to an 80S–Met-tRNA_i^Met complex. Using several viral RNAs, we show that m⁷G^5′p inhibits only the mRNA-dependent step, and that this inhibition is demonstrable only with mRNAs that require a 5′ cap for translational activity.