Regulation of the human globin gene switching mechanisum based on unusual DNA structure

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The human beta-globin locus contains five active genes (s-, Gy-, Ay-, S-and beta-globins) and a j3-globin pseudogene (PSIbeta-globin) in a region larger than 70 kb. The sequence homologies between these genes were mostly lost and insertions of AIu and Li repetitive sequences have occurred since their separation, resulting in a total mixture of the nucleotide sequence. However, these genes show marked coordination when they switch expression during development and differentiation. This requires strict control of the transcription machinery including RNA polymerases and the interaction between the promoters located as far as 42 kb. The locus control region that governs this coordination is located 6 kb to more than 20 kb upstream of the e-globin gene. Therefore, the regulation of expression of these genes provides a good model in which to study the biological significance of unusual DNA structure in genomic DNA.Periodicity of DNA bend sites was firstly reported in the human epsilon-globin gene region (J.Biol. Chem. 269.1994) and subsequently in other regions of the same locus. Based on the results, we proposed that periodic bent DNA are associated with Long range coordinations of genomic DNA.In this study we mapped a total of 98 bend sites in the about 70 kb region of the beta-globin locus by circular permutation assay with average interval of approximately 680 bp between them. Most of their locations relative to the cap sites were conserved during evolution. There were the 75 potentialbend core sequences A/A/A (A2N8A2N8A2) found in the 51 bend sites, 64 sequences from 48 sites showed bending profiles by oligonucleotide-based assay. These lines of evidence suggested that DNA bending is a basic and universal structural component of genomic DNA and has a direct or indirect influence on biological phenomena such as regulation of the golobin gene switching mechanism.