The entry of sugar beet cells into the G 0 state involves extensive re-programming of gene expression mechanisms via transcriptional and translational controls

Abstract

Sugar beet cells grown in liquid suspension culture entered the stationary phase on day 17 of the batch culture cycle as a consequence of nutrient depletion. Between days 17 and 19 the cells exhibited a number of marked changes in RNA and protein metabolism characteristic of entry into a quiescent or G 0 state. Protein synthesis declined considerably and very few polypeptides incorporated sufficient radioactively labelled methionine to be detected by fluorography following polyacrylamide gel electrophoresis (SDS-PAGE). There was a marked decrease in protein half life during the stationary phase and this was accompanied by a rise in protease activity. RNA synthesis also decreased to <10% of the level in dividing cells and the residual transcription was insensitive to inhibition by 0.4 μg ml −1 actinomycin D. However, rRNA remained intact in the quiescent cells, only minor changes were detected in the profiles of stained polypeptides separated by SDS-PAGE, and the cells remained viable. Despite the marked decline in protein synthesis in the stationary phase, a pool of translatable mRNA remained in the cells which produced similar translation products in vitro to the abundant mRNAs from dividing cells. On the other hand, comparison of the mRNA populations of cell samples taken from cells in the stationary phase and after subculture into fresh medium by the techniques of differential display and Northern blotting showed that there were a substantial number of changes in gene expression during the transition from quiescence to division. Only one-third of mRNA sequences were expressed constitutively, a number of stationary phase-specific mRNAs were detected whilst others appeared only after subculture. In conclusion, the G 0 state which results from nutrient depletion in these sugar beet cells is characterised by the transcription of a number of G 0-specific genes during a period in which there is large overall reduction in transcriptional and translational activity.