Characterization of Rapidly Labelled Nucleic Acids in Tissues of Plant Seedlings

Abstract

The nucleic acid metabolism of plant tissues was examined by incubating seedlings of Phaseolus, Vicia, Pisum, and Soja or sections of them with 32P for short periods of time. The nucleic acids extracted from this material were fractionated on columns of methylated albumin coated on kieselgur, and the radioactivity and composition of the specific fractions were determined. Application of 32P to intact seedlings or to excised parts of seedlings resulted in the same pattern of labelled nucleic acids in all tissues, but the total amount of incorporated radioactivity was different. In all tissues investigated five rapidly labelled RNA fractions were found associated with the following components: (I) soluble RNAs, (II) DNA, (III—V) ribosomal RNA. Fraction I contained equally high amounts of CMP and GMP thus differing significantly from the soluble RNAs. The composition of fraction (II) which was probably bound to DNA in the form of a complex, was similar to that of fraction I provided the labelling period was short. Fractions III—V were of the ribosomal type. The rapidly labelled DNA fraction had a high guanine-cytosine content (60%) as compared with the bulk DNA (40%). Fractional centrifugation of the tissue homogenates revealed that the labelled RNA of the ribosomal type was partly associated with the ribosomes, partly with larger particles which were sedimented by low speed centrifugation. The RNA associated with the latter had a higher specific activity than the ribosomal RNA in the supernatant. Fraction I and the second component of the soluble RNA (s-2) were also confined to the sediment of larger particles. Actinomycin D (10 µg/ml) inhibited the incorporation of 32P in the nucleic acids. In chase experiments it caused a decrease in the specific activity of all RNA fractions, most prominently, however, in fraction I and II indicating their instability and resemblance to messenger RNA.