Transport and processing of ribosomal RNA in plant cells after treatment with cycloheximide.

Abstract

In freely suspended cells of parsley (Petroselinum crispum) protein synthesis is inhibited nearly totally by 5 mug/ml cycloheximide within 10 min. This very low dose has a slight effect on the rRNA synthesis too. During an incubation period of 60 min with [32P] orthophosphate in the presence of the inhibitor (5 mug/ml) at least 65-70% mature rRNA are synthesized compared with the control. After 120 min the synthesis is progressively reduced to 60-65%, and after 240 min to 30-40%. Cycloheximide causes a delay of the prn addition to the 2.3 X 10(6) daltons RNA, normally detectable in pulse experiments, two further precursor molecules do emerge which undernormal circumstances apparently are shortlived. Their molecular weights are 2.0 X 10(6) and 0.9 X 10(6) daltons. The pulse-chase technique and cell fractionation into nuclear and ribosomal parts enables us to demonstrate a rapid transfer of labelled 18S and 25S RNA during a 15 min chase treatment to the mature cytoplasmic ribosomes. Under these conditions no differential transport of the two components takes place. The cells possess a pool of proteins. Therefore the formation of RNP-particles or ribosomes respectively and the transport from the nucleus to the cytoplasm are indipendent from protein synthesis for some time.