Like most retroviruses and retrotransposons, the
retrotransposon Ty3 expresses its pol gene analog
(POL3) as a translational fusion to the upstream
gag analog (GAG3). The Gag3-Pol3 fusion
occurs by frameshifting during translation of the mRNA
that encodes the two separate but overlapping ORFs. We
showed previously that the shift occurs by out-of-frame
binding of a normal aminoacyl-tRNA in the ribosomal A site
caused by an aberrant codon[bull ]anticodon interaction
in the P site. This event is unlike all previously described
programmed translational frameshifts because it does not
require tRNA slippage between cognate or near-cognate codons
in the mRNA. A sequence of 15 nt distal to the frameshift
site stimulates frameshifting 7.5-fold. Here we show that
the Ty3 stimulator acts as an unstructured region to stimulate
frameshifting. Its function depends on strict spacing from
the site of frameshifting. Finally, the stimulator increases
frameshifting dependent on sense codon-induced pausing,
but has no effect on frameshifting dependent on pauses
induced by nonsense codons. Complementarity between the
stimulator and a portion of the accuracy center of the
ribosome, Helix 18, implies that the stimulator may directly
disrupt error correction by the ribosome.