Studies on compartmentation of uridine 5'-triphosphate during synthesis of cytoplasmic and plastid ribosomal RNA's in Euglena gracilis

Abstract

1. 1. Compartmentation of uridine 5'-triphosphate (UTP) was studied during synthesis of cytoplasmic ribosomal RNA (cyt-rRNA) and plastid ribosomal RNA (pl-rRNA) in photoorganotrophically grown cells of Euglena gracilis Z. 2. 2. Using the approach of Wiegers et al. (1976) the steady state specific radioactivity of UTP was compared with that ofcyt-20S rRNA, cyt-25S rRNA, pl-16S rRNA and pl-23S rRNA under low and at 100-fold higher specific radioactivity of exogenously fed pHl-uracil. 3. 3. The equal steady state specific radioactivities of all rRNAs at both feeding conditions argue against compartmentation of UTP during their synthesis. 4. 4. At high specific radioactivity of exogenous [ 3H]-uracil the salvage-derived labelled UMP was shown to be diluted 15,000-fold by unlabelled UMP formed de novo, whereas this dilution factor was 100-fold lower at low specific radioactivity of [ 3H]-uracil indicating inhibition of the de novo synthesis of UMP. 5. 5. Transport is suggested of uridine nucleotides into chloroplasts by the 15-fold higher specific radioactivity of intracellular [ 3H]-uracil than that of UTP as well as UMP residues in pl-rRNA.