Chromosome V of the Saccharomyces carlsbergensis lager yeast strain 244, a yeast not amenable to tetrad analysis, was analysed genetically in S. cerevisiae genetic standard strains. This was achieved by crossing meiotic progeny of the lager yeast with S. cerevisiae strains carrying kar1 as well as the chromosome V markers can1, ura3, his1, ilv1, and rad3. From the transitory heterokaryons formed we selected strains retaining the characteristics of the recipient strain but having become prototrophic for uracil, histidine, and isoleucine. The resulting strains were disomic for chromosome V, having acquired a chromosome V from S. carlsbergensis in addition to the normal S. cerevisiae chromosome complement (chromosome addition strains). They were of two classes: In one class the transferred chromosome hardly recombines with the S. cerevisiae chromosome V in the region CAN1-RAD3, which covers almost the entire known map. In the other class, the transferred chromosome recombined at normal levels. We conclude that S. carlsbergensis harbors two structurally different chromosomes V; one being homologous and one homoeologous to the S. cerevisiae chromosome. By use of the CAN1 locus, strains were selected which by mitotic chromosome loss had their normal chromosome V substituted by either the homologous or the homoeologous S. carlsbergensis chromosome, showing that these chromosomes are fully functional in S. cerevisiae. Tetrad analysis of the chromosome substitution strains confirmed the very different genetic behavior of the two S. carlsbergensis chromosomes V. In spite of the almost complete absence of recombination between the homoeologous chromosome and the S. cerevisiae chromosome, disjunction at meiosis appears normal, as indicated by high spore viability. Genomic Southern hybridizations with the probes CAN1, URA3, CYC7, and ILV1 could not detect any nucleotide sequence differences between these loci on the recombining S. carlsbergensis chromosome and the S. cerevisiae alleles. Under standard stringency (68?C, 0.1?SSC), hybridization of the probes to DNA from the strain with the homoeologous chromosome was only observed in the case of ILV1, where weak hybridization was found, indicating a considerable difference in nucleotide sequence. To further study the extent of nucleotide sequence inhomology, the two different ILV1 genes of S. carlsbergensis were cloned in λ vectors. Mapping of 16 restriction enzyme sites showed identity between the allele located on the recombining chromosome and the ILV1 gene of S. cerevisiae. The nucleotide sequence of the ILV1 gene of the non-recombining chromosome was by restriction site mapping found to be very different from that of the S. cerevisiae allele.

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Journal Article

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Nilsson-Tillgren T, Gjermansen C, Holmberg S, Petersen JGL, Kielland-Brandt MC

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