Mechanisms of sod2 gene amplification in Schizosaccharomyces pombe.
Gene amplification in eukaryotes plays an important role in drug resistance, tumorigenesis, and evolution. The Schizosaccharomyces pombe sod2 gene provides a useful model system to analyze this process. sod2 is near the telomere of chromosome I and encodes a plasma membrane Na(+)(Li(+))/H(+) antiporter. When sod2 is amplified, S. pombe survives otherwise lethal concentrations of LiCl, and >90% of the amplified sod2 genes are found in 180- and 225-kilobase (kb) linear amplicons. The sequence of the novel joint of the 180-kb amplicon indicates that it is formed by recombination between homologous regions near the telomeres of the long arm of chromosome I and the short arm of chromosome II. The 225-kb amplicon, isolated three times more frequently than the 180-kb amplicon, is a palindrome derived from a region near the telomere of chromosome I. The center of symmetry of this palindrome contains an inverted repeat consisting of two identical 134-base pair sequences separated by a 290-base pair spacer. LiCl-resistant mutants arise 200-600 times more frequently in strains deficient for topoisomerases or DNA ligase activity than in wild-type strains, but the mutant cells contain the same amplicons. These data suggest that amplicon formation may begin with DNA lesions such as breaks. In the case of the 225-kb amplicon, the breaks may lead to a hairpin structure, which is then replicated to form a double-stranded linear amplicon, or to a cruciform structure, which is then resolved to yield the same amplicon.
Albrecht
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Department of Molecular Biology, Lerner Research Institute, The Cleveland Clinic Foundation, Cleveland, Ohio 44195, USA.
Hunyady
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Stark
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Patterson
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D001483:Base Sequence; D003001:Cloning, Molecular; D004264:DNA Topoisomerases, Type I / Q000235:genetics; D004271:DNA, Fungal / Q000032:analysis; D005784:Gene Amplification; D008025:Ligases / Q000235:genetics; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D012091:Repetitive Sequences, Nucleic Acid / Q000235:genetics; D012568:Schizosaccharomyces / Q000235:genetics*; D012689:Sequence Homology, Nucleic Acid; D017923:Sodium-Hydrogen Exchangers / Q000235:genetics*; D016615:Telomere / Q000235:genetics
D004271:DNA, Fungal; D017923:Sodium-Hydrogen Exchangers; D004264:DNA Topoisomerases, Type I; D008025:Ligases