THE NUCLEOTIDE-SEQUENCE OF CHROMOSOME-I FROM SACCHAROMYCES-CEREVISIAE

Chromosome I from the yeast Saccharomyces cerevisiae contains a DNA mo lecule of approximate to 231 kbp and is the smallest naturally occurri ng functional eukaryotic nuclear chromosome so far characterized. The nucleotide sequence of this chromosome has been determined as part of an international collaboration to sequence the entire yeast genome. Th e chromosome contains 89 open reading frames and 4 tRNA genes. The cen tral 165 kbp of the chromosome resembles other large sequenced regions of the yeast genome in both its high density and distribution of gene s. In contrast, the remaining sequences flanking this DNA that compris e the two ends of the chromosome and make up more than 25% of the DNA molecule have a much lower gene density, are largely not transcribed, contain no genes essential for vegetative growth, and contain several apparent pseudogenes and a 15-kbp redundant sequence. These terminally repetitive regions consist of a telomeric repeat called W', flanked b y DNA closely related to the yeast FLO1 gene. The low gene density, pr esence of pseudogenes, and lack of expression are consistent with the idea that these terminal regions represent the yeast equivalent of het erochromatin. The occurrence of such a high proportion of DNA with so little information suggests that its presence gives this chromosome th e critical length required for proper function.