Hotspots for unselected Ty1 transposition events on yeast chromosome III are near tRNA genes and LTR sequences.

A collection of yeast strains bearing single marked Ty1 insertions on chromosome III was generated. Over 100 such insertions were physically mapped by pulsed-field gel electrophoresis. These insertions are very nonrandomly distributed. Thirty-two such insertions were cloned by the inverted PCR technique, and the flanking DNA sequences were determined. The sequenced insertions all fell within a few very limited regions of chromosome III. Most of these regions contained tRNA coding regions and/or LTRs of preexisting transposable elements. Open reading frames were disrupted at a far lower frequency than expected for random transposition. The results suggest that the Ty1 integration machinery can detect regions of the genome that may represent "safe havens" for insertion. These regions of the genome do not contain any special DNA sequences, nor do they behave as particularly good targets for Ty1 integration in vitro, suggesting that the targeted regions have special properties allowing specific recognition in vivo.