Freedom in condensation

Abstract

Even highly condensed mitotic chromatin allows for the comings and goings of transcription factors, according to Chen et al. on page 41. Their measurements of the dynamics of RNA polymerase I (Pol I) transcription factors show that mitotic transcriptional silencing is not due to inaccessible chromatin. RNA polymerases II and III stay off chromatin during cell division, but RNA pol I is found at rDNA sites throughout mitosis. The new fluorescence recovery experiments show that pol I subunits and the transcription initiation factor UBF1 are not trapped within the condensed DNA during this time, but instead come and go. By keeping RNA pol I at rDNA, transcription activation may begin as soon as sister chromatids separate, thus maximizing ribosome synthesis, which is so fundamental to survival. Indeed, the earliest detectable RNA synthesis was that of rRNA at late anaphase. Dynamic exchange was also seen for histone H1 throughout mitosis and for core histones at late anaphase and telophase. Core histone exchanges coincided with H3 lysine 9 acetylation (a hallmark of transcriptionally active chromatin), pol II association, and the first hints of transcription. Chromatin remodeling therefore takes place before widespread DNA decondensation and the bulk of transcription activation at mitotic exit.