Bimodal p53-mediated suppression of cell proliferation and growth. (A) Examination of the effect of energy and oncogenic stress, p53 activation by nutlin-3a (6 h and 19 h after treatment) and mTOR inhibition (by PP242 and rapamycin) on the expression level of cell-cycle related transcripts. p53 activation resulted in a very significant attenuation of the expression of cell-cycle genes. P-values were calculated for each condition by comparing the distribution of fold-change measured for cell-cycle and all the other genes in the dataset using Wilcoxon test. We included in this analysis a subset of 298 genes selected from the set of GO cell-cycle genes (GO:007049) which showed a variation of at least 1.5-fold in expression level (either induction or repression) across our entire dataset. (B) Examination of the effect of energy and oncogenic stress, p53 activation by nutlin-3a and mTOR inhibition on the translation efficiency of the ribosomal protein transcripts. p53 activation resulted in a very significant repression of the translation of ribosomal gene transcripts (P-values were calculating as in A). (C) Validation of the effect of p53 activation on translation efficiency of ribosomal gene transcripts measured by their ribosome occupancy. MCF-7 cells were treated with nutlin-3a for 20 h and subjected to isolation of polysome-associated mRNAs (see Methods). Polysomal fractionation followed by RT-PCR for two ribosomal genes (and GAPDH as a control) were used to quantify mRNA levels in the different polysomal fractions.