Radioactive uridine is incorporated by Chlorella strain 211-8b/p into ribosomal subunits and their rapidly labelled RNA comigrates with chloroplast RNA on polycrylamide gels.
Ribosomal particles which can be labelled by short pulses of orotic acid cosediment with the particles labelled by uridine pulses and contain the same RNA species as these when separated either on sucrose gradients or on polycrylamide gels. This incorporation is, like that of uridine, sensitive to rifampin and chloramphenicol, but insensitive to cycloheximide.
A comparative study of short-time incorporation of uridine, orotic acid and guanosine into the RNA of Chlorella showed that all three precursors were incorporated mainly into RNA of chloroplastic origin. However, guanosine was also partly incorporated into cytoplasmic rRNA. Nitrogen-deficient cells always incorporated part of all three precursors into cytoplasmic rRNA, but the proportions of these were different anaong the different precursors. These results are consistent with the hypothesis that the described differences in the incorporation of the above mentioned precursors into I~NA of different cellular compartments are largely attributable to effects of pool sizes.