Ty3 integrates within the region of RNA polymerase III transcription initiation

Over 190 independent insertions into target plasmids of the retrovirus-like element Ty3 were recovered and mapped. Ty3 was shown to insert upstream of tRNA, 5S, and U6 genes, all of which are transcribed by RNA polymerase III. Integration sites were within 1-4 nucleotides of the position of transcription initiation, even for one mutant gene where the polymerase III initiation site was shifted to a completely new context. Mutagenesis of a SUP2 tRNA gene target showed that integration required functional promoter elements but that it did not correlate in a simple way with target transcription. This is the first report directly linking a discrete genomic function with preferential insertion of a retrotransposon.