Molecular Biology and Evolution 17:179-188 (2000)
© 2000 Society for Molecular Biology and Evolution
Article |
Polymorphism and Divergence in the ß-Globin Replication Origin Initiation Region
*Institute of Molecular Evolutionary Genetics, Department of Biology, Pennsylvania State University;
and
Medical Research Council Molecular Haematology Unit, Institute of Molecular Medicine, University of Oxford;
and
Institute of Biological Anthropology, University of Oxford
DNA sequence polymorphism and divergence was examined in the vicinity of the human ß-globin gene cluster origin of replication initiation region (IR), a 1.3-kb genomic region located immediately 5' of the adult-expressed ß-globin gene. DNA sequence variation in the replication origin IR and 5 kb of flanking DNA was surveyed in samples drawn from two populations, one African (from the Gambia, West Africa) and the other European (from Oxford, England). In these samples, levels of nucleotide and length polymorphism in the IR were found to be more than two times as high as adjacent non-IR-associated regions (estimates of per-nucleotide heterozygosity were 0.30% and 0.12%, respectively). Most polymorphic positions identified in the origin IR fall within or just adjacent to a 52-bp alternating purine-pyrimidine ((RY)n) sequence repeat. Within- and between-population divergence is highest in this portion of the IR, and interspecific divergence in the same region, determined by comparison with an orthologous sequence from the chimpanzee, is also pronounced. Higher levels of diversity in this subregion are not, however, primarily attributable to slippage-mediated repeat unit changes, as nucleotide substitution contributes disproportionately to allelic heterogeneity. An estimate of helical stability in the sequenced region suggests that the hypervariable (RY)n constitutes the major DNA unwinding element (DUE) of the replication origin IR, the location at which the DNA duplex first unwinds and new strand synthesis begins. These findings suggest that the ß-globin IR experiences a higher underlying rate of neutral mutation than do adjacent genomic regions and that enzyme fidelity associated with the initiation of DNA replication at this origin may be compromised. The significance of these findings for our understanding of eukaryotic replication origin biology is discussed.
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