Oligothymidylate analogues having stereoregular, alternating methylphosphonate/phosphodiester backbones, d-Tp(TpTp)4T isomers I and II and d-Tp(TpTp)3T(pT)1-5 isomers I and II, were prepared by methods analogous to the phosphotriester synthetic technique. The designations isomer I and isomer II refer to the configuration of the methylphosphonate linkage, which is the same throughout each isomer. Analogues with the type I methylphosphonate configuration form very stable duplexes with poly(dA) while those with the type II configuration form either 2T:1A triplexes or 1T:1A duplexes with poly(dA) of considerably lower stabilities. The oligothymidylate analogues were tested for their ability to initiate polymerizations catalyzed by Escherichia coli DNA polymerase I or calf thymus DNA polymerase a on a poly(dA) template. Neither d-Tp(TpTp)4T nor d-Tp(TpTp)3TpT served as initiators of polymerization while d-Tp(TpTp)3T(pT)2-5 showed increasing priming ability as the length of the 3'- oligothymidylate tail increased. Analogues with type I methylphosphonate configuration were more effective initiators than the type II analogues at 37 °C. The apparent activation energies of polymerizations initiated by d-T/>(TpTp)3T- (pT)4and5 isomer I were greater than those for reactions initiated by isomer II or d-(Tp)1T. The results suggest that DNA polymerase interacts with the charged phosphodiester groups of the primer molecule and may help stabilize primer/ template interaction. At least two contiguous phosphodiester groups are required at the 3’ end of the analogue primers in order for polymerization to occur. Interactions between the polymerase and primer also appear to occur with phosphodiester groups located at sites remote from the 3'-OH polymerization site and may be influenced by the configuration of the methylphosphonate group.
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