Formation of chloroplast ribosomes and ribosomal RNA in Euglena incubated with protein inhibitors

It has been shown previously (cf [26]) that virginiamycin M blocks temporarily chlorophyll formation and chloroplast multiplication; these effects are made permanent by virginiamycin S.Virginiamycin M inhibits reversibly the assembly of the 30S and 50S subunits of chloroplast ribosomes. The association of the two virginiamycin components, M and S, causes a permanent arrest of 70S ribosome formation. On the contrary the antibiotics have no apparent effect on the 87S cytoplasmic ribosomes.Formation of the 16S chloroplastic ribosomal RNA is also prevented by virginiamycin: in a transient manner by the M component, and in a permanent way by the association of M and S. Biosynthesis of the 26S and 21S cytoplasmic rRNAs does not undergo appreciable changes in the presence of the antibiotic.The alterations of rRNA synthesis by virginiamycin in eucaryotic organelles, thus, differ from those induced by the drug in procaryotes; these findings might have evolutionary implications. Moreover, they might explain the temporary bleaching caused by virginiamycin M, and the permanent bleaching effect produced by the two virginiamycin components.