Chromosomal structures of bottom fermenting yeasts

A genomic comparison of bottom fermenting yeasts was performed by pulsed-fi eld gel electrophoresis and Southern blot analysis with some S. cerevisiae gene probes. We confirmed that strains of bottom fermenting yeast have four chromosomes originating from S. bayanus. Since the structures of these chr omosomes were recombined with S. cerevisiae chromosomes, these S. bayanus c hromosomes could be differentiated from S. cerevisiae chromosomes using Sou thern hybridization. Our Southern hybridization results indicate that botto m fermenting yeasts have both chromosomes originating from both S, cerevisi ae and S. bayanus. It was reconfirmed that top fermenting yeast should be c lassified as S. cerevisiae, based on the chromosomal structure. The chromos omal structure of S. pastorianus CBS1538, the type stain of S. pastorianus, was also investigated. This strain has chromosomes originating only from S . bayanus. S. carlsbergensis CBS1513 has chromosomes originating from both S. cerevisiae and S. bayanus. From these results, we contend that bottom fe rmenting yeasts should be classified as S. carlsbergensis.