Chromosome compaction is triggered by an autonomous DNA-binding module within condensin.

The compaction of chromatin into mitotic chromosomes is essential for faithful transmission of the genome during cell division. In eukaryotes, chromosome morphogenesis is regulated by the condensin complex, though the exact mechanism used to target condensin to chromatin and initiate condensation is not understood. Here, we reveal that condensin contains an intrinsically disordered region (IDR) that modulates its association with chromatin in early mitosis and exhibits phase separation. We describe DNA-binding motifs within the IDR that, upon deletion, inflict striking defects in chromosome condensation and segregation, ill-timed condensin turnover on chromatin, and cell death. Importantly, we demonstrate that the condensin IDR can impart cell cycle regulatory functions when transferred to other subunits within the complex, indicating its autonomous nature. Collectively, our study unveils the molecular basis for the initiation of chromosome condensation in early mitosis and how this process ultimately promotes genomic stability and faultless cell division.

The p53 protein induces stable miRNAs that have the potential to modify subsequent p53 responses.

The p53 tumour suppressor is a transcription factor that can increase the expression of mRNAs and microRNAs (miRNAs). HT29-tsp53 cells expressing a temperature sensitive variant of p53 have provided a useful model to rapidly and reversibly control p53 activity. In this model, the majority of p53-responsive mRNAs were upregulated rapidly but they were short-lived leading to rapid decay of the p53 response at the restrictive temperature. Here we used oligonucleotide microarrays and reverse transcriptase PCR to show that p53-induced miRNAs exhibited a distinct temporal pattern of expression. Whereas p53-induced miRNAs like miR-143-3p, miR-145-5p, miR-34a-5p and miR-139-5p increased as fast as mRNAs, they were extremely stable persisting long after p53 induced mRNAs and even their corresponding primary miRNAs had decayed to baseline levels. Three p53-induced mRNAs (MDM2, BTG2 and CDKN1A) are experimentally verified targets of one or more of these specific miRNAs so we hypothesized that the sustained expression of p53-induced miRNAs could be explained by a post-transcriptional feedback loop. Activation of consecutive p53 responses separated by a period of recovery led to the selective attenuation of a subset of p53 regulated mRNAs corresponding to those targeted by one or more of the p53-responsive miRNAs. Our results indicate that the long term expression of p53 responsive miRNAs leads to an excess of miRNAs during the second response and this likely prevents the induction of MDM2, BTG2 and CDKN1A mRNA and/or protein. These observations are likely to have important implications for daily cancer therapies that activate p53 in normal tissues and/or tumour cells.

A Temperature Sensitive Variant of p53 Drives p53-Dependent MicroRNA Expression without Evidence of Widespread Post-Transcriptional Gene Silencing.

The p53 tumour suppressor is a transcription factor that can regulate the expression of numerous genes including many encoding proteins and microRNAs (miRNAs). The predominant outcomes of a typical p53 response are the initiation of apoptotic cascades and the activation of cell cycle checkpoints. HT29-tsp53 cells express a temperature sensitive variant of p53 and in the absence of exogenous DNA damage, these cells preferentially undergo G1 phase cell cycle arrest at the permissive temperature that correlates with increased expression of the cyclin-dependent kinase inhibitor p21WAF1. Recent evidence also suggests that a variety of miRNAs can induce G1 arrest by inhibiting the expression of proteins like CDK4 and CDK6. Here we used oligonucleotide microarrays to identify p53-regulated miRNAs that are induced in these cells undergoing G1 arrest. At the permissive temperature, the expression of several miRNAs was increased through a combination of either transcriptional or post-transcriptional regulation. In particular, miR-34a-5p, miR-143-3p and miR-145-5p were strongly induced and they reached levels comparable to that of reference miRNAs (miR-191 and miR-103). Importantly, miR-34a-5p and miR-145-5p are known to silence the Cdk4 and/or Cdk6 G1 cyclin-dependent kinases (cdks). Surprisingly, there was no p53-dependent decrease in the expression of either of these G1 cdks. To search for other potential targets of p53-regulated miRNAs, p53-downregulated mRNAs were identified through parallel microarray analysis of mRNA expression. Once again, there was no clear effect of p53 on the repression of mRNAs under these conditions despite a remarkable increase in p53-induced mRNA expression. Therefore, despite a strong p53 transcriptional response, there was no clear evidence that p53-responsive miRNA contributed to gene silencing. Taken together, the changes in cell cycle distribution in this cell line at the permissive temperature is likely attributable to transcriptional upregulation of the CDKN1A mRNA and p21WAF1 protein and not to the down regulation of CDK4 or CDK6 by p53-regulated miRNAs.