Efficient translational frameshifting occurs within a conserved sequence of the overlap between the two genes of a yeast Ty1 transposon

Abstract

The Ty (transposon yeast) family of retroviral-like transposons include two genes, TYA and TYB, analogous to the gag and pol genes of metazoan retroviruses. TYB lies downstream of TYA, the two genes overlapping by 38 base pairs. The primary translation product of TYB is a TYA/TYB fusion protein whose expression has been inferred to occur by translational frameshifting within the overlap region. We show that the event leading to expression of TYB is very efficient, resulting in 20% read-through into TYB from TYA. We demonstrate that the Ty mRNA is colinear with the DNA sequence of the element, eliminating any pretranslational model for TYB expression. Frameshifting requires no particular sequence of the upstream TYA gene, nor any global RNA structure. Surprisingly, it can be promoted by a 14-base-pair oligonucleotide of the overlap region. The ability of this oligonucleotide to function is inhibited when it is positioned immediately downstream of an initiator AUG. We conclude that the TYB gene is expressed by an efficient ribosomal frameshifting event requiring a small oligonucleotide sequence derived from the TYA/TYB overlap region.