Xavier Darzacq | bioRxiv Science

bioRxiv2016

A Dynamic Mode of Mitotic Bookmarking by Transcription Factors

During mitosis, transcription is shut off, chromatin condenses, and most transcription factors (TFs) are reported to be excluded from chromosomes. How do daughter cells re-establish the original transcription program? Recent discoveries that a select set of TFs remain bound on mitotic chromosomes suggest a potential mechanism for maintaining transcriptional programs through the cell cycle termed mitotic bookmarking. Here we report instead that many TFs remain associated with chromosomes, and that the exclusion previously described is largely a fixation artifact. In particular, most TFs we tested are significantly enriched on mitotic chromosomes. Studies with Sox2 reveal that this mitotic interaction is more dynamic than in interphase and requires both DNA binding and nuclear import. Furthermore, this dynamic mode results from lack of transcriptional activation rather than decreased accessibility of underlying DNA sequences in mitosis. The nature of the cross-linking artifact prompts careful re-examination of the role of TFs in mitotic bookmarking.\n\nHighlightsO_LIMany transcription factors bind to mitotic chromosomes\nC_LIO_LISox2 mitotic interaction is dynamic and requires DNA binding and nuclear import\nC_LIO_LIDNA remains highly accessible in mitotic chromosomes\nC_LIO_LIFormaldehyde-based cross-linking leads to mis-localization of TFs\nC_LI

Molecular Biology↗

bioRxiv2016

Fast imaging of DNA motion reveals distinct sub-diffusion regimes at the site of DNA damage

The dynamic organization of genes inside the nucleus is an important determinant for their function. Using ultra-fast microscopy in S. cerevisiae cells and improved analysis of mean square displacements, we quantified DNA motion at time scales ranging from 10 milliseconds to minutes and found that following DNA damage, DNA exhibits distinct sub-diffusive regimes. In response to double-strand breaks, chromatin is more mobile at large time scales but, surprisingly, its mobility is dramatically reduced at short time scales. This effect is even more pronounced at the break. Such pattern of dynamics is consistent with a global increase in chromatin persistence length following DNA damage. Scale-dependent nuclear exploration is regulated by the Rad51 repair protein, both at the break and throughout the genome. We propose a model in which stiffening of the damaged ends by the repair complex, combined with global increased stiffness, act like a \"needle in a decompacted ball of yarn\", enhancing the ability of the break to traverse the chromatin meshwork.

Biophysics↗