By culturing of callus tissue originating from root explants ofPetroselinum sativumin a synthetic liquid medium under aeration, freely suspended single cells and small clusters consisting of mostly five cells were obtained. The rapidly dividing cells did not exhibit any morphogenesis. Their nucleic acid metabolism was investigated by pulse experiments with32P-orthophosphate. Rapidly labelled RNA was prominently found associated with high molecular RNA. During the fractionation of the total nucleic acids on MAK columns it was eluted after the ribosomal RNA components. Its base ratio, however, differed from the latter in that the AMP content was higher than the GMP content. Sucrose gradient centrifugation and polyacrylamide gel electrophoresis resulted in the separation of the ribosomal RNA from the rapidly labelled RNA, thus proving the higher molecular weight of the latter. Based upon the migration in the gel a sedimentation coefficient of approximately 32S was calculated. The possible function of the heavy rapidly labelled RNA component as precursor of ribosomal RNA is discussed.
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