The yeast Ty element: recent advances in the study of a model retro-element

The past three years have seen a dramatic increase in our understanding of the structural organization and expression strategies of the dispersed, repetitive yeast transposon, Ty. These studies have led to a logical comparison of Ty with retroviral proviruses and other mobile, repetitive elements. Such comparisons have culminated in the hypotheses that transposition occurs via the formation of Ty-encoded virus-like particles and that these particles represent a basic unit of all ‘retro-systems’.     

Nucleotide sequence characterization of Ty 1-17, a class II transposon from yeast

We have determined the nucleotide sequence of a class II yeast transposon (Ty 1-17) which is found just centromere-distal to the LEU2 structural gene on chromosome III of Saccharomyces cerevisiae. The complete element is 5961 bp long and is bounded by two identical, directly repeated, delta sequences of 332 bp each. The sequence organization indicates that Ty 1-17 is a retrotransposon, like the class I elements characterized previously. It contains two long open reading-frames, TyA (439 amino acids) and TyB (1349 amino acids). In this paper, the sequences of the two classes of yeast transposon are compared with one another and with analogous elements, such as retroviral proviruses, cauliflower mosaic virus and copia sequences. Features of the Ty 1-17 sequence which may be important to its mechanism of transposition and its genetic action are discussed.     

Expression strategies of the yeast retrotransposon Ty: a short sequence directs ribosomal frameshifting

The Ty element of yeast is a member of a class of eukaryotic transposons which bear a striking resemblance to retroviral proviruses in their structure and expression strategies (1,2). A direct comparison can be drawn between the production of a fusion protein encoded by Ty, resulting from a frameshift event which fuses two out-of-phase open reading frames TYA and TYB, and the production of Pr180gag-pol in a retrovirus such as Rous Sarcoma Virus (RSV) (3,4). We present data which shows, definitively, that RNA splicing is not responsible for the frameshift in Ty. By in vitro mutation of a class I element, Ty1-15, we demonstrate that 31 nucleotides contained within the region where the TYA and TYB open reading frames overlap direct the frameshift. Within this short sequence there is a region of homology with a class II element which we show is also able to frameshift.