Effects of U1 Nuclear RNA on Translation of Messenger RNA

Abstract

U 1 nuclear RNA has the 5’-terminal sequence m32,2,7Gpp(p)AmUmACUUACCUGGCAGGGAGAUACCAGG- [Reddy, R., et al. (1974), J. Biol. Chem. 249, 6486; Ro-Choi, T. S., et al. (l974), Fed. Proc., Fed. Am. Soc. Exp. Biol. 33, 15481. The cap, m32,2,7Gpp(p)AmA, CUU, and AUG are common to some mRNA species. Despite the presence of the cap and AUG initiator codon, U1 RNA is not translated in the wheat germ system but instead UI RNA markedly inhibits translation of poly[A(+)] mRNA in the wheat germ system. On a molar basis the inhibitory effect of U1 RNA is 15 times greater (IC50 = 2.1 pM) than either pm7G or m7GpppAm (IC50 pm7G and m7GpppAm = 30 pM). Inhibition of translation by U1 RNA was noncompetitive with respect to mRNA and not reversed by increasing mRNA concentrations. Inhibition by U1 RNA was reversed by increasing amounts of wheat germ s-30 fraction. ‘251-labeled tiation complex formation. The inhibitory effect of pm7G on translation is correlated with its inhibition of the formation of the 80s initiation complex but U1 RNA did not inhibit the formation of the 80s complex at concentrations inhibitory to translation. Much higher concentrations of U1 RNA (7 pM) only inhibited formation of the 80s initiation complex by 40%. U I RNA immediately stopped protein synthesis when added 20 min after the incubation started; a 10-min lag occurs with pm7G. UI RNA does not inhibit poly(U)-directed polyphenylalanine synthesis. These results show that, at low concentrations, U 1 RNA binds to ribosomes and inhibits translation. This inhibition may result from interaction of oligonucleotides adjacent to the 5’cap of U1 RNA with the messenger binding sites on ribosomes or with special factors necessary for protein synthesis.