38. Genome Sequencing of Schizosaccharomyces pombe Chromosome III
Mitsuoki Morimyo, Kazuei Mita, Etsuko Hongo, Tomoyasu Higashi, Kimihiko Sugaya, Shuichi Hiraoka, Masahiro Ajimura, Go Kenu, Shunichi Sasanuma, Miyako Gotou, Takumi Era, Yukari Ito Yoshie Ishihara and Etsuko Shiroma
Keywords: S. pombe, cDNA, BAC clone, DNA sequenctng
In a study of the structure and function of the housekeeping genes of eukaryotic cells, we chose Schizosaccharomyces pombe (S. pombe) as a model organism. It is assumed to have 6,000 genes and the genome size of 14 megabases (Mb) compared to 100,000 genes and 3,000 Mb for humans. In spite of the greater numbers for humans, S. pombe can be used as a model organism for humans, because its genes are similar to human genes which can be nor mally expressed in yeast, its housekeeping genes are considered to be conserved through eukaryottc evolution and many of its genes have introns which make cDNA sequences essential to determine physi cat organization of genes from the genomic DNA sequence. Moreover, the functions of S. pombe genes are easily identified by disrupting the genes with homologous recom bination.
We have analyzed 12,000 cDNA clones made by mRNA prepared from late log phase cells of S.pombe and identified over 2,500 genes (900 similar clones with known genes and 1800 newly found clones). Among the cDNA clones, 8,118 were deposited with the DDBJ and put on the WWW homepage of NIRS (http://www.nirs.go.jp). By using these cDNA clones mapped on the chromosome III of S. pombe, we started genome sequencing. We made a BAC contig map of chromosome III and selected a minimum set of 18 BAC clones covering the whole chromosome III. The BAC DNA was partially di gested by an endonuelease followed by sedumentatton in a sucrose gradient and DNA fragments ranging from 1 to 3 kb were recovered for a shotgunsequencing. We finished sequencing of all 18 BAC clones and determined DNA sequence of chromosome III except three gaps, the centromere and both chromosome ends carrying tandem repeats of rDNA region. We identified an ORF found in an assembled DNA sequence as a gene by a homology search in protein databases and our S. pombe cDNA database (http://188.8.131.52). On average, we found a gene without introns every 4 kb, which is about half the prediction. This is because about 50% genes are known to have introns. A systematic disruption of genes related to radiation senstttvtty is in progress to identify their functions.
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