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J. Biol. Chem., Vol. 278, Issue 44, 42854-42866, October 31, 2003

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Analysis of DNA Replication Intermediates Suggests Mechanisms of Repeat Sequence Expansion^*

Janaki Veeraraghavan, Marie L. Rossi, and Robert A. Bambara

From the Department of Biochemistry and Biophysics and the Cancer Center, University of Rochester School of Medicine and Dentistry, Rochester, New York 14624

We previously developed a system to investigate the mechanism of repeat sequence expansion during eukaryotic Okazaki fragment processing. Upstream and downstream primers were annealed to a complementary template to overlap across a CAG repeat region. Annealing by the competing primers lead to structural intermediates that ligated to expand the repeat segment. When an equal number of repeats overlapped on the upstream and downstream primers, a 2-fold expansion was expected, but no expansion occurred. We show here that such substrates do not expand irrespective of their repeat length. To reveal mechanism, we tested different hairpin loop intermediates expected to form and facilitate ligation. Substrates configured to form large loops in either the upstream or downstream primer alone allowed expansion. Large or small fixed position single loops allowed expansion when located at least six nucleotides up- or downstream of the nick. Fixed loops in both primers, simulating a double loop intermediate, allowed expansion as long as each loop was nine nucleotides from the nick. Thus, neither the double loop configuration required to form with equal length overlaps nor the large single loop configuration are fundamental structural impediments to expansion. We propose a model for the expansion mechanism based on the relative stabilities of single loop, double loop, hairpin, and flap intermediates that is consistent with the observed expansion efficiency of equal and unequal overlap substrates. The model suggests that the equilibrium concentration of double loop intermediates is so vanishingly small that they are not likely contributors to sequence expansion.

Received for publication, May 16, 2003 , and in revised form, August 1, 2003.

^* This work was supported in part by National Institutes of Health Grant GM24441 (to R. A. B.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed: Dept. of Biochemistry and Biophysics, University of Rochester Medical Center, 601 Elmwood Ave., Box 712, Rochester, NY 14642. Tel.: 585-275-3269; Fax: 585-271-2683; E-mail: robert_bambara{at}urmc.rochester.edu.

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