The genetic organization of the yeast ty element

The genetic organization of the yeast transposon Ty resembles that of higher eukaryotic retroviruses and other elements such as the copia-like sequences of Drosophila. The Ty genome is 5·9 kb (10³ bases) long. It has 340 bp (base pairs) terminal repeats known as delta sequences and it produces a terminally redundant 5·7kb RNA that starts in the 5’ delta and ends in the 3′ delta. Ty transcription is directed by signals upstream and downstream of the major RNA start site and is regulated by the mating-type configuration of the cell. The 5·7 kb transcriptional unit is divided into two overlapping open reading frames, TYA and TYB. TYA occupies approximately the first quarter of the transcriptional unit while TYB occupies the rest. TYB overlaps TYA by either 38 or 44 nucleotides, depending on the element, and is in the plus one reading frame with respect to TYA. TYA is expressed to produce protein pl (50×10³M_r) and TYB is expressed as a fusion protein, p3 (190× 10³M_r,). Both of these proteins are subsequently cleaved to produce proteins p2, p4, p5, p6, reverse transcriptase and a protease that is responsible for some of these cleavage events. These proteins are assembled into virus-like particles (Ty-VLPs) that contain Ty RNA and reverse transcriptase activity. It is likely that the Ty-VLPs are units of transposition as Ty transposes via an RNA intermediate.