SACCHAROMYCES CARLSBERGENSIS CONTAINS 2 FUNCTIONAL MET2 ALLELES SIMILAR TO HOMOLOGS FROM SACCHAROMYCES-CEREVISIAE AND S-MONACENSIS

The brewing yeast, Saccharomyces carlsbergensis, is allopolyploid, der ived from two diverged genomes. To obtain information about the possib le origin of this yeast, we cloned two different S. carlsbergensis MET 2 genes (encoding homoserine acetyltransferase). One has a nucleotide (nt) sequence identical or very similar to MET2 of Saccharomyces cerev isiae. The other has a different sequence, but was functional in S. ce revisiae. This allele was sequenced and revealed a coding region of 48 6 amino acids (aa). The nt sequence of the coding region showed 82% ho mology to S. cerevisiae MET2, while the derived aa sequences were 94% identical. Hybridization experiments to genomic DNA of different yeast strains revealed that the divergent MET2 gene had higher sequence hom ology to segments from type strains of S. monacensis, S. bayanus and S . uvarum than to MET2 from S. cerevisiae. Sequencing of 330 bp of a PC R-amplifred fragment of MET2 from these organisms shows that the non-S . cerevisiae-like sequence from S. carlsbergensis is identical to the corresponding sequence in S. monacensis, while it is 93% homologous wi th S. bayanus and S. uvarum. Our results are consistent with the propo sal that S. carlsbergensis originated as a hybrid between S. monacensi s and S. cerevisiae. The complete identity of the MET2 fragments from S. monacensis and the S. carlsbergensis-specific MET2 allele suggests that the hybridization must have been a quite recent event.