[Influence of rifampicin, chloramphenicl and cycloheximide on uridine-incorporation into chloroplast ribosome precursors of Chlorella].

The unicellular green alga Chlorella incorporates labeled uridine mainly into the precursors of chloroplast ribosomes. After treatment with rifampicin for 60 min, the uridine incorporation into the particles is completely inhibited. Chloramphenicol treatment results in the same complete inhibition. In constrast, cycloheximide (actidione) slightly stimulates the incorporation of uridine into the chloroplast ribosome precursors.Short-time incorporation of inorganic phosphate into the ribosome fractions is nearly unaffected by rifampicin and chloramphenicol, but it is strongly inhibited by cycloheximide.Isolation and chromatographic separation of nucleic acids after treatment of cells with rifampicin shows that uridine incorporation into RNA is completely inhibited. Chloramphenicol causes only partial inhibition of uridine labeling in the high molecular weight RNA. Here again, cycloheximide stimulates the uridine incorporation.The results indicate that uridine is preferentially incorporated by Chlorella cells into the chloroplast ribosome precursors. Inorganic phosphate is introduced both into cytoplasmic and into chloroplasmic RNA, but because of the quantitative distribution, the cytoplasmic ribosomes are more extensively labeled. Since only inhibitors of bacterial and chloroplasmic RNA-and protein synthesis affect the formation of uridine-labeled ribosomes, this synthesis must take place in the chloroplast itself.