Successful fermentations to produce ethanol require microbial strains that have a high tolerance to glucose and ethanol. Enhanced glucose/ethanol tolerance of the laboratory yeast Saccharomyces cerevisiae strain BY4741 under certain growth conditions as a consequence of the expression of a dominant mutant allele of the SPT15 gene (SPT15-300) corresponding to the three amino acid changes F177S, Y195H, and K218R has been reported (H. Alper, J. Moxley, E. Nevoigt, G. R. Fink, and G. Stephanopoulos, Science 314:1565-1568, 2006). The SPT15 gene codes for the TATA-binding protein. This finding prompted us to examine the effect of expression of the SPT15-300 allele in various yeast species of industrial importance. Expression of SPT15-300 in leucine-prototrophic strains of S. cerevisiae, Saccharomyces bayanus, or Saccharomyces pastorianus (lager brewing yeast), however, did not improve tolerance to ethanol on complex rich medium (yeast extract-peptone-dextrose). The enhanced growth of the laboratory yeast strain BY4741 expressing the SPT15-300 mutant allele was seen only on defined media with low concentrations of leucine, indicating that the apparent improved growth in the presence of ethanol was indeed associated with enhanced uptake and/or utilization of leucine. Reexamination of the microarray data published by Alper and coworkers likewise suggested that expression of genes coding for the leucine permeases, Tat1p and Bap3p, were upregulated in the SPT15-300 mutant, as was expression of the genes ARO10, ADH3, ADH5, and SFA1, involved in leucine degradation.