Transport and Processing of Ribosomal RNA in Plant Cells after Treatment with Cycloheximide

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Abstract
In freely suspended cells of parsley (Petroselinum crispum ) protein synthesis is inhibited nearly totally by 5 /ug/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 /ugjml) 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 processing of the precursor RNA species leading to an accumulation of these components. In addition to the 2.3 x 10® daltons RNA, normally detectable in pulse experiments, two further precursor molecules do emerge which under normal circumstances apparently are shortlived. Their molecular weights are 2.0 x 10® and 0.9 x 10® 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 independent from protein synthesis for some time

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