VIEW ARTICLE

Fixation and Incorporation of CO₂ into Ribonucleic Acid by Germinating Uredospores of Uromyces phaseoli. G. F. Stallknecht, Formerly NSF Trainee, Department of Plant Pathology, University of Minnesota, now Assistant Professor, Department of Plant Science, University of Idaho; C. J. Mirocha, Associate Professor, Department of Plant Pathology, University of Minnesota, St. Paul 55101. Phytopathology 61:400-405. Accepted for publication 17 November 1970. DOI: 10.1094/Phyto-61-400.

The percentage of ¹⁴C incorporated into ribonucleic acid (RNA), assayed by radioactivity present in cytidylic acid (CMP), adenylic acid (AMP), guanylic acid (GMP), and uridylic acid (UMP), was greater in the purine nucleotides than in the pyrimidine nucleotides at all times sampled. The concentrations of mononucleotides increased from 15.3 µmole/250 mg spores 5 minutes after the start of germination to 17.5, 8 hr after germination, indicating an increase in RNA during germination. The mole per cent of GMP and UMP in the RNA hydrolysate was always highest, and CMP lowest. During germination, the purine:pyrimidine ratio decreased from 1.34 to 1.02 in 8 hr. When nucleic acids from germinating bean rust uredospores were separated on methylated albumin columns during the first 30 min of germination, significant radioactivity was present only in the oligonucleotide fraction. Radioactivity was present in both light and heavy ribosomal RNA 1.5 hr after initiation of germination; the highest activity was in the elution profile just past the heavy ribosomal RNA. Six hours after germination, radioactivity was present in the soluble RNA peak, in the ribosomal RNA. Six hours after germination, the highest activity was once again found in the region just beyond the heavy ribosomal RNA; radioactivity was also present in the soluble RNA peak and in the ribosomal RNA peaks. Radioactivity was highest in the oligonucleotide peak for each experiment, with no activity in either transfer RNA or deoxyribonucleic acid peaks. The sequence of synthesis of RNA species, as determined by ¹⁴C incorporation, appears to be (i) an adduct of both light and heavy ribosomal RNA (sometimes interpreted as mRNA); (ii) ribosomal RNA; and (iii) soluble RNA.