Differential inhibitory effects of 5-substituted 1-beta-D-xylofuranosyluracil 5'-triphosphates and related nucleotides on DNA-dependent RNA polymerases I and II from the cherry salmon (Oncorhynchus masou).

Various 5-substituted 1-beta-D-xylofuranosyluracil 5'-triphosphates (hydrogen, methyl-, ethyl-, n-propyl, n-butyl, fluoro-, chloro-, bromo-, and iodo derivatives) and some of the 3'-deoxyribofuranosyl nucleotides (3'-deoxy UTP and 3'-deoxy TTP) were synthesized chemically and their inhibitory effects on DNA-dependent RNA polymerases I and II of the cherry salmon (Oncorhynchus masou) were studied systematically. These 3'-modified UTP analogues could not be utilized as substrates in place of UTP, but they did inhibit the incorporation of UMP into RNA in vitro. In contrast, 2'-modified UTP analogues, such as 2'-dTTP and Ara TTP, were neither substrates nor inhibitors. Kinetic analysis showed that the inhibition by these compounds was essentially competitive with substrate UTP. The K1 values of RNA polymerase I for the analogues were smaller (2-6 microM) than the Km value for UTP (8 microM), but those for xylo-EtUTP, xylo-PrUTP, and xylo-BuUTP were larger (about 20 microM) than the Km for UTP. In contrast to these alkyl groups with steric and electron-donating effects, halogen groups have electron-withdrawing effects on the uracil nucleus. Therefore, it was concluded that the inhibitory activity of these analogues on RNA polymerase I was not affected by the inductive effects of substituent groups at the 5-position of uracil nucleus but by their steric effects. On the other hand, all of the K1 values of RNA polymerase II for UTP analogues were smaller (0.4-3 microM) than the Km value for UTP (4 microM). In this case, neither steric effect nor an inductive effect of substituents on UTP analogues influenced the inhibitory activity towards RNA polymerase II.