About yeast retrotransposons...

Ty elements (transposon-yeast) are DNA sequences that move from one chromosomal site to another via an RNA intermediate (3). The DNA segment is transcribed into RNA, then reverse-transcribed into cDNA which is reinserted into the nuclear genome, usually at a different site. The original retrotransposon stays in place, and the new retrotransposon takes with it copies of any adjacent genes or regions that may have been duplicated during the process, making transposition a major source of gene expansion during genome evolution. The insertion event can also influence genomic evolution by disrupting coding or transcriptional control elements, or by promoting chromosomal rearrangements via homologous recombination (1).

There are approximately 50 retrotransposons in the yeast genome, comprising 5 types (Ty1 through Ty5), which represent two distinct groups of eukaryotic LTR-retrotransposons known as Ty1-copia (Pseudoviridae) and Ty3-gypsy (Metaviridae) elements. Ty1, Ty2, Ty4, and Ty5 are all Ty1-copia retrotransposons, whereas the Ty3 retrotransposons are the only representatives of the Ty3-gypsy group (1). Individual types can vary greatly in copy number and also in sequence, indicating different episodes of element amplification and subsequent divergence over time. Transposition is a nonrandom process, and insertion hotspots vary with Ty element type (4). Ty1, Ty2, Ty3 and Ty4 elements tend to integrate near tRNAs or other genes transcribed by RNA polymerase III (5, 6, 1), and Ty5 elements preferentially integrate into heterochromatin at telomeres and silent mating loci (7).

All five types of Ty elements share the same basic structure, consisting of TYA and TYB open reading frames (analogous to the retroviral gag and pol genes) flanked by long terminal repeats (LTRs) (1). TYA encodes structural proteins of the virus-like particle (VLP), where reverse transcription takes place. TYB encodes a polyprotein with protease (PR), integrase (IN), reverse transcriptase (RT) and ribonuclease H (RH) catalytic domains, all of which are essential for retrotransposition (2).

Ty elements are transcribed from their genomic DNA by the host cell's RNA polymerase II, producing RNAs that serve as both genomic RNA and mRNA. The mRNAs have caps and polyadenylated tails, and are translated on cytoplasmic ribosomes. After translation, the RNAs are packaged into virus-like particles (VLPs) in the cytoplasm and are reverse-transcribed by the self-encoded reverse transcriptase (RT) into double-stranded cDNAs (8). The replication cycle is completed by the movement of the cDNA back into the nucleus where it is inserted into new chromosomal sites by the retrotransposon-encoded integrase (9).

Last updated: 2007-03-26