Three Distinct Forms of Nuclear Poly(A) Polymerase.

Poly(A) polymerase activities have been solubilized from rat liver nuclei and purified by chromatography on Bio-Gel A-1.5m, DEAE-Sephadex and CM-cellulose. Three distinct forms of nuclear poly(A) polymerase have been resolved by chromatography on CM-cellulose. According to their sequence of elution from CM-cellulose these enzyme activities have been termed A, B and C. Enzymes A and B are Mn²⁺-dependent, enzyme C requires Mg²⁺. With the same chromatographic step on CM-cellulose the Mn²⁺-dependent poly(A) polymerase activities were separated from a dependent enzyme system capable of synthesizing RNA a primed poly(U), poly(G) and poly(C). The effect of different nuclear and cytoplasmic RNA primers on the rate of poly(A) formation suggests enzyme A to be responsible for the elongation of preexisting poly(A) chains. The phosphorylated derivative of cordycepin, 3'-deoxyadenosine 5'-triphosphosphate (3'-dATP), which is known to inhibit nuclear poly(A) synthesis in vivo, also impairs poly(A) formation in vitro. It is shown that 3'-dATP very probably is not incorporated into poly(A) in vitro, suggesting that 3'-dATP primarily affects the catalytic activities of the poly(A) polymerase species rather than directly blocking chain elongation. [ABSTRACT FROM AUTHOR]