Effects of cordycepin and cordycepintriphosphate on polyadenylic and ribonucleic acid-synthesising enzymes from eukaryotes

Abstract

GENETIC information encoded in the nuclear DNA of a eukaryotic cell is selectively transcribed into an array of nuclear RNA molecules heterogeneous in size, base sequence and half-life. The α-amanitin-sensitive, DNA-dependent RNA polymerase isolated from nucleoplasm is thought to catalyse the transcription of heterogeneous nuclear RNAs (HnRNA). While in the nucleus, the length of HnRNAs is modified in two ways: first, HnRNAs are lengthened by addition of a poly(A) sequence. The ATP–polynucleotidylexotransferase isolated and purified from several eukaryotic tissues is presumed to catalyse the sequential addition of AMP moieties to the 3′ terminus of the HnRNA. Secondly, HnRNAs bearing poly(A) tails are thought to be shortened by cleavage of a portion of the nucleotide sequence at the 5′ terminus and the remainder transported to the cytoplasm. After association with ribosomal subunits, the processed transcripts form polysomes and function as mRNAs in the translation of specific polypep-tides. Inferences as to the processing of HnRNA to give rise to mRNA are drawn from experiments using cordycepin (3′-deoxyadenosine) as an inhibitor of RNA synthesis^1–2. Interpretation of many of these experiments rests on the assumption that the cordycepin selectively inhibits the addition of polyadenylic acid to HnRNA^4–6. On the other hand, Klenow⁷ has shown that cordycepin is phosphorylated to 3′-dATP by Erlich ascites tumour cells, and using a bacterial DNA-dependent RNA polymerase, Shigeura and Gordon⁸ demonstrated inhibition of in vitro RNA synthesis by 3′dATP. They postulated that the mechanism of inhibition involved incorporation of the 3′-dATP into the growing chain of RNA with subsequent failure to provide a 3′-hydroxyl for the next incoming nucleosidetriphosphate to terminate elongation prematurely.