Metabolism of naturally occurring homopolymers

RNA metabolism and poly(A) distribution in mouse liver following administration of dimethylnitrosamine

Dimethylnitrosamine (DMNA) strongly inhibited RNA synthesis in mouse liver under conditions when the nucleotide pattern, rate of nucleotide synthesis and phosphorylation ratio were unaffected. (An unidentified, probably non-nucleotide, component in the acid-soluble liver fraction was selectively reduced.) The inhibition of RNA synthesis was associated with a decrease in the RNA polymerase activity of isolated liver nuclei, well established already 45 min after DMNA administration. The reduced activity included both Mg2+- and Mn2+/(NH4)2SO4-stimulated polymerase functions. The inhibition in vivo involved the whole complement of RNA, including poly (A)-containing RNA and isolated poly(A) sequences. The transfer of labelled RNA from the nucleus to the cytoplasm was not impaired. There was no detachment of poly(A)-containing RNA from the microsomes, and the proportion of tightly membrane-bound microsomal RNA and poly(A) sequences was not reduced as determined by use of a flotation technique. No breakage or shortening of the poly(A) chains was indicated by sedimentation analysis.

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 Mn+ -dependent poly(A) polymerase activities were separated from a Mn2+ -dependent enzyme system capable of synthesizing RNA-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 derivated 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)invitro, suggesting that 3'-dATP primarily affects the catalytic activities of the poly(A) polymerase species rather than directly blocking chain elongation.

Release of nuclear DNA template restrictions by specific polyribonucleotides

Certain synthetic homo- and copolyribonucleotide polymers are capable of releasing the DNA template restriction of isolated nuclei of rat liver cells when assayed with an excess of or in the absence of exogenous DNA polymerase. The purine homopolyribonucleotides are far more effective than are the pyrimidine polymers.