All DNA manipulations were performed according to standard methods,35 and all media were prepared by standard procedures.36 All yeast strains and plasmids used are described in Table 3. Chemicals were obtained from Ambion (Austin, TX), Sigma Chemical Co. (St. Louis, MO), US Biological (Swampscott, MA), or Fisher Scientific (Pittsburgh, PA) unless otherwise noted. Site-directed mutagenesis was performed using the Stratagene QuikChange mutagenesis kit. All mutants were sequenced to ensure that the intended change was introduced and no additional changes to the sequence were present.