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.