Biochemical and electron microscope analyses of the DNA reverse transcripts present in the virus-like particles of the yeast transposon Ty1. Identification of a second origin of Ty1DNA plus strand synthesis.

Transposition of Saccharomyces cerevisiae Ty1 retroelements has been shown to involve reverse transcription in intracytoplasmic virus-like particles (Ty-VLPs). Ty DNA present in the particles specified by Ty1-H3 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 Ty1-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-stranded or only partially double- stranded at one end or at both ends. These double-stranded regions are of a length corresponding to those of incomplete plus strands detected by biochemical techniques. Double-stranded circular molecules mainly of a length corresponding to that of two-LTR circles were also detected on electron micrographs. These analyses allowed us to propose a scheme for reverse transcription in Ty particles.