Ribonucleic Acid Polymerase from Micrococcus lysodeikticus

The homopolymer complex, (dAn·dTn), will direct the synthesis of r(A)n or r(U)n, or both, in the presence of highly purified RNA polymerase from Micrococcus lysodeikticus. When either 14C-ATP or 3H-UTP is present with saturating amounts of (dAn·dTn), AMP incorporation is considerably greater than UMP incorporation. This difference appears to reflect a product inhibition which is observed only in the case of r(U)n synthesis. When both ATP and UTP are present with (dAn·dTn), each substrate exerts an influence on the incorporation rate of the other. UTP nonspecifically decreases AMP incorporation, whereas ATP specifically enhances UMP incorporation resulting in similar rates of incorporation for both substrates. This phenomenon occurs at all Mn++ and Mg++ concentrations, although the UMP:AMP incorporation ratio is highly dependent upon the metal ion concentration. Since the specific enhancement of UMP incorporation by ATP is not related to the template activity of nascent r(A)n, the coupling phenomenon is interpreted in terms of an interaction of the nascent r(A)n product with the nascent r(U)n product. Nascent r(A)n probably enhances the rate of UMP incorporation by facilitating the release of r(U)n, which would otherwise form a triple stranded complex with (dAn·dTn). This hypothesis is supported by the finding that r(U)n, added to the (dAn·dTn) template before the addition of RNA polymerase, completely blocks UMP incorporation but does not affect AMP incorporation. Since this interaction of the two complementary product strands must be immediate, it is suggested that both are simultaneously synthesized at the same site by a dimeric enzyme. A similar coupling phenomenon does not occur with the double stranded ribosyl template, r(A)n·r(U)n, and comparative studies indicate large kinetic differences between ribosyl and deoxyribosyl templates.