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Abstract
We have examined the hypothesis that the highly selective recombination of an active mating type locus (MAT) with either HMLα or HMRa is facilitated by the spatial positioning of relevant sequences within the budding yeast (Saccharomyces cerevisiae) nucleus. However, both position relative to the nuclear envelope (NE) and the subnuclear mobility of fluorescently tagged MAT, HML, or HMR loci are largely identical in haploid a and α cells. Irrespective of mating type, the expressed MAT locus is highly mobile within the nuclear lumen, while silent loci move less and are found preferentially near the NE. The perinuclear positions of HMR and HML are strongly compromised in strains lacking the Silent information regulator, Sir4. However, HMLα, unlike HMRa and most telomeres, shows increased NE association in a strain lacking yeast Ku70 (yKu70). Intriguingly, we find that the yKu complex is associated with HML and HMR sequences in a mating-type-specific manner. Its abundance decreases at the HMLα donor locus and increases transiently at MATa following DSB induction. Our data suggest that mating-type-specific binding of yKu to HMLα creates a local chromatin structure competent for recombination, which cooperates with the recombination enhancer to direct donor choice for gene conversion of the MATa locus.
ACKNOWLEDGMENTS
The Gasser laboratory acknowledges the Swiss Cancer League, the Swiss National Science Foundation, NCCR Frontiers in Genetics, and the Novartis Research Foundation, as well as fellowships from the European Molecular Biology Organization (EMBO) to H.V.A. and V.D. and from the Human Frontiers Science Program to H.V.A. K.B. further acknowledges financial support from the University Paul Sabatier, Toulouse, and the ANR (project JC05-42116).
We thank M. Tsai for assistance with Myc-tagged strain construction, V. Kalck for assistance with positioning data acquisition, and O. Gadal for fruitful discussions.