The sensitive biological assay for reverse transcriptase was used to monitor potential perturbation effects of spin labels covalently bound to various nucleic acids or nucleic acid analogs to the extent of about one label per 100 residues. The inhibitory properties of the spin labeled and unlabeled biopolymers were compared for evaluating possible interference of the reporter group in protein-nucleic acid interaction studies. The amount of inhibitor required for 50% inhibition (ED50) was determined for the competitive inhibitors (U)n, l(U)n, (RUGT,U)n, (Um)n, (A)n, (Am)n, and l(A)n as well as for thenon- or uncompetitive inhibitors (dUfl)n, l(dUfl)n, (dUz)n, and l(dUz)n. The most pronounced inhibition was observed with spin labeled and unlabeled (dUfl)n. The results indicate that the ED50 and the kinetic patterns of inhibition are similar for the spin labeled and unlabeled inhibitors studied. Thus, the presence of a limited number of spin labels in a nucleic acid matrix has little effect, if any, on reverse transcriptase-nucleic acid complexes and most likely on other protein-nucleic acid complexes.
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