BIOCHEMICAL AND ELECTRON-MICROSCOPE ANALYSES OF THE DNA REVERSE TRANSCRIPTS PRESENT IN THE VIRUS-LIKE PARTICLES OF THE YEAST TRANSPOSON TY1- IDENTIFICATION OF A 2ND ORIGIN OF TY1DNA PLUS STRAND SYNTHESIS

Transposition of Saccharomyces cerevisiae Ty1 retroelements has been s hown to involve reverse transcription in intracytoplasmic virus-like p articles (Ty-VLPs). Ty DNA present in the particles specified by Tyl-H 3 element was found to consist of the full-length genomic DNA as well as incomplete cDNAs mainly of plus polarity. Our results indicate that identical sequences (TGGGTGGTA) are used as primers for the synthesis of plus strand cDNA, generating cDNAs of 0. 345 kb (analogous to the retroviral strong-stop plus cDNA) and of 2.1 kb. Electron microscopic analyses of Tyl-VLP DNA revealed two distinct classes, one full-length and the other corresponding to 0.34 kbp molecules, the size of a LTR sequence. The full-length molecules are either completely double-stran ded or only partially double- stranded at one end or at both ends. The se double-stranded regions are of a length corresponding to those of i ncomplete plus strands detected by biochemical techniques. Double-stra nded circular molecules mainly of a length corresponding to that of tw o-LTR circles were also detected on electron micrographs. These analys es allowed us to propose a scheme for reverse transcription in Ty part icles.