Uptake of Adenine-8-¹⁴C by Whole and Fractional Amœbæ

Abstract

RECENTLY, Goldstein and Plaut¹ have obtained direct evidence for the synthesis of cytoplasmic ribonucleic acid by the nucleus in Amoeba proteus. The demonstration by Brachet and Szafarz² that enucleate fragments of the unicellular alga Acetabularia mediterranea exhibit a distinct turnover of a precursor of ribonucleic acid (orotic acid) has been interpreted by Brachet³ as indicative of cytoplasmic synthesis of ribonucleic acid in the absence of a nucleus. If these two findings are both applicable to cells in general, we could conclude that there are two basic systems of synthesis of cytoplasmic ribonucleic acid : one centred in the nucleus, and the other a cytoplasmic system which is independent of the nucleus. In order to investigate this possibility, we have undertaken a study of the incorporation of the ribonucleic acid precursor adenine-8-¹⁴C into whole and fractional amœbæ. For a valid comparison of the rates of synthesis of cellular constituents it is necessary, first of all, to ascertain the intracellular availability for synthesis of a labelled precursor in whole and nucleate and enucleate half-cells⁴; otherwise an impaired ability to transfer a material across the cell membrane might be mistaken for a reduction or loss of synthetic capacity.