Tandem repeats of simple doublet and triplet sequences occur with high frequency in the DNA of eucaryotes. Among the most frequent is the repeat of dTG, which has unusual structural properties. We show here that HMG1 (modeled by the second HMG box motif from HMG1 of the rat, HMGb) binds to complexes formed from annealing unequal lengths of dTG·dCA and inhibits the in vitro elongation of these complexes by the Klenow fragment of DNA polymerase 1 at 37 °C. At 46 °C, HMGb enhances the elongation. Polylysine inhibits elongation at both temperatures. These results show that the stability of this repeat in vivo can be influenced by the presence of basic proteins in general, and more selectively by the abundant nuclear protein HMG1.
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