Formation of DNA triplexes accounts for arrests of DNA synthesis at d(TC)n and d(GA)n tracts.

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Abstract
To study the mechanism of arrest of DNA synthesis at d(TC)n and d(GA)n sequences, single-stranded DNA molecules including d(TC)27 or d(TC)31 tracts or a d(GA)27 tract were used as templates for in vitro assays of complementary DNA synthesis performed by extension of a primer with the Klenow polymerase or the Taq polymerase (Thermus aquaticus DNA polymerase). Electrophoresis of the products revealed that arrests occurred around the middle of these tracts. The arrests in the d(TC)n sequences were eliminated when dATP or dGTP was replaced with the analogue 7-deaza dATP or 7-deaza dGTP, respectively, or when the templates were preincubated with the Escherichia coli single-strand binding protein (SSB). Preincubation of the template including a d(GA)27 tract with SSB has also eliminated the arrests at this sequence. Furthermore, arrests did not occur at d[G(7-deaza A)]27 or d[(7-deaza G)A]27 tracts when molecules including such tracts were used as templates. These results are compatible with the notion that the arrests were caused by formation of d(TC)i.d(GA)i.d(TC)i and d(GA)i.d(GA)i.d(TC)i triplexes, in which the bases in the uncopied portions of the d(TC)n tracts, or of the d(GA)27 tract, and the purine bases in the newly synthesized d(TC)i.d(GA)i duplexes were bound by hydrogen bonds. In the assays performed with the Taq polymerase, the pH dependence (in the range of 6.0-9.0) and the temperature dependence of the arrests were determined. As the pH was lowered, the arrests in the d(TC)27 tract were enhanced, in line with the expected properties of d(TC)i.d(GA)i.d(TC)i triplexes. The arrests in the d(GA)27 tract were enhanced by an increase in the pH. At pH 7.2 the arrests in the d(GA)27 tract persisted up to 80 degrees C, whereas the arrests in the d(TC)27 tract were eliminated at 50 degrees C; these results presumably reflect the relative stabilities of the two triplexes mentioned above at this physiological pH value and could be biologically significant.