5-Mercaptouridine-5′-diphosphate (hs5UDP) has been synthesized and investigated as a substrate of the polynucleotide phosphorylase of Micrococcusluteus. While hs5UDP is not utilized alone, It can be copolymerized with UDP; however, unusually for this enzyme, the ratio of 5-mercaptouridylate vs. uridylate residues in the polynucleotlde product (MPU) is always lower than the ratio of hs5UDP vs. UDP in the substrate mixture. Furthermore, hs5UDP decreases the rate of the enzymic polymerization reaction. The MPU productforms two-stranded and three-stranded complexes with poly(A). The circular dichroic spectra of these complexes are similar to those formed between poly(U) and poly(A), but their melting profiles indicate somewhat lower stability. The physicochemical and biochemical properties of the enzymic product are qualitatively similar to those of MPU prepared by chemical modification; both are potent inhibitors of a DNA-dependent RNA polyrmerase.
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