Studies on ribonucleic acid-polyphosphate in plants

Condensed phosphates in Lemna minor L. and their relationship to nucleic acids

After previous incubation in a P-free culture medium whole plants of Lemna minor L. were given [(32)P]-orthophosphate in the logarithmic and in the stationary growth phase. The condensed phosphates could be separated from the nucleic acids by fractionation on methylated serum albumin and Kieselgur (MAK). The various inorganic condensed phosphates were eluted before the nucleic acids. The former were dialyzed and separated by two-dimensional thin layer chromatography on a mixture of cellulose and microcrystalline cellulose using an acid and an alkaline solvent in succession. (32)P-radioactivity was present in all linear oligophosphates containing 2 to 7 residues, in cyclic metaphosphates (tri-, tetra-, penta- and hexametaphosphates) and in high-molecular condensed phosphates which remained at the starting point.Among the low-molecular inorganic oligophosphates the trimetaphosphate contained significantly more radioactivity than the other compounds. The labeling rate of (32)P-incorporation depended on the previous depletion of phosphate. In the logarithmic phase maximal labeling occurred after 4 h following a 10 h period of phosphate depletion; in the stationary phase the maximum rate was reached 5 h after the end of a 4.5 h period of phosphate depletion. The results indicate that the phosphate pool was restored first and that the nucleic acids were labeled subsequently. That is to say: in contrast to previous findings by other authors, our results show that net synthesis of inorganic condensed phosphates does takes place during the logarithmic phase.After cultivation of Lemna for 24 h or longer in a P-free medium the DNA incorporated more (32)P than the other nucleic acids during the logarithmic phase. During the stationary phase [(32)P]-orthophosphate incorporation into nucleic acids was markedly lower than during the logarithmic phase.

Biosynthesis of nucleotides in wheat. IV. Metabolism of specifically 14-C-labeled orotic acid

The biosynthesis of pyrimidine nucleotides in wheat leaves was investigated, with specifically14C-labeled orotic acids as precursors. The utilization of orotic acid, unlike uracil, for the synthesis of pyrimidine nucleotides is far more predominant than the degradative process. The labeled orotic acid was rapidly metabolized to uracil nucleotides and the sugar nucleotide, uridine-5′-diphosphate glucose. However, cytosine nucleotides were not significantly labeled within 60 minutes. In spite of the fact that no detectable radioactive OMP was observed, the results presented indicate strongly that the biosynthetic pathway for pyrimidine nucleotides from orotic acid in this plant proceeds according to the following reactions: orotic acid + 5-phosphoribosyl-1-pyrophosphate → orotidine-5′-monophosphate → uridine-5′-monophosphate → uridine-5′-triphosphate → uridine-5′-diphosphate glucose, and uridine-5′-triphosphate → cytidine-5′-triphosphate.