A CONSERVED AUG TRIPLET IN THE 5' NONTRANSLATED REGION OF POLIOVIRUS CAN FUNCTION AS AN INITIATION CODON IN-VITRO AND IN-VIVO

Poliovirus translation is initiated at AUG(743), 154 nt downstream of a conserved heptanucleotide CUUAUGG at the 3' border of the internal r ibosome entry site. AUG(586) is part of this motif and is normally not an initiation codon, but was activated following alteration of its co ntext from CUUAUGG to ACCAUGG. Initiation at AUG(586) was efficient an d yielded a 7.2-kDa polypeptide translated in an open reading frame th at overlapped AUG,, by 38 nt, but the presence of this activated codon reduced initiation at AUG(743) by only 50%. Growth of a mutant poliov irus W1-5NC-1 containing the CUU --> ACC substitutions was impaired an d was not alleviated by a termination codon placed four triplets downs tream of AUG(586) in the virus W1-5NC-2. The virus W1-5NC-6 contained the substitution U(584)A and had a similar sp phenotype; the phenotype of W1-5NC-1 is thus probably due to substitution within the conserved CUUAUGG motif per se rather than to activation of AUG(586). A sp muta nt virus W1-5NC-3 was derived from W1-5NC-1 by deletion of nt 588-745, indicating that AUG(586) could initiate translation in vivo. These ob servations indicate that although AUG(586) can be activated by upstrea m substitutions, it is nevertheless readily bypassed by ribosomes in m RNAs containing wt downstream elements, resulting in initiation at AUG (743). (C) 1994 Academic Press, Inc.