Differential effects of cycloheximide and other inhibitors of protein synthesis on in vivo ribosomal RNA synthesis in Achlya bisexualis

Abstract

Inhibitors of protein synthesis (cycloheximide, dl-p- fluorophenylalanine , puromycin, and blasticidin-S) were tested for their effect on the in vivo synthesis of both whole cell and ribosomal RNA by the water mold Achlya bisexualis. Whereas none of the drugs affected the uptake of [5- 3H]uridine, all resulted in a decrease in the rate of accumulation of this precursor into acidinsoluble polynucleotides. Analysis of uridine incorporation into rRNA showed that, while dl-p- fluorophenylalanine , puromycin and blasticidin-S inhibited synthesis gradually over a period of 30–40 min, cycloheximide resulted in complete cessation of synthesis within 2.5 min. In the presence of each of the inhibitors except cycloheximide the capacity of the cells to synthesize RNA, as measured by incorporation of uridine into either acid-insoluble nucleic acid or into ribosomes, decays rapidly following pseudo-first-order kinetics, with a half-life of between 10 and 23 min. In the presence of cycloheximide, synthetic capacity decays with a half-life of 1.4 min. These results are interpreted to mean that in Achlya (1) there is a rapidly turning over protein requisite to rRNA synthesis, and (2) the effect on rRNA synthesis elicited by cycloheximide is not exclusively the result of inhibition of protein synthesis.