CELL-CYCLE-REGULATED HISTONE GENE-EXPRESSION IN SYNCHRONIZED PLANT-CELLS

The regulation of histone gene expression during the cell cycle has be en studied in a synchronized tobacco (BY2) cell suspension. Maximal ex pression was found in S phase but unlike other eukaryotic organisms th e amount of histone mRNA was not coupled to DNA synthesis. Using aphid icolin as an inhibitor of DNA synthesis either before or at mid-S phas e an accumulation of histone mRNAs was found in the absence of any DNA synthesis which was not due to a stabilization of the mRNAs but to ac tual control at the transcriptional level. At the completion of unpert urbed or delayed S phase, histone, mRNAs were selectively degraded by a post-transcriptional mechanism requiring de novo transcription.