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Published online 28 December 2004. doi:10.1083/jcb.200407182
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 168, Number 1, 41-54
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Article

 Condensed mitotic chromatin is accessible to transcription factors and chromatin structural proteins

Danyang Chen1, Miroslav Dundr2, Chen Wang1, Anthony Leung3, Angus Lamond3, Tom Misteli2, and Sui Huang1

1 Department of Cell and Molecular Biology, The Feinberg School of Medicine, Northwestern University, Chicago, IL 60611
2 National Cancer Institute, National Institutes of Health, Bethesda, MD 20892
3 Division of Gene Regulation and Expression, School of Life Sciences, Wellcome Trust Biocentre, University of Dundee, Dundee DD1 5EH, Scotland, UK

Correspondence to Sui Huang: s-huang2{at}northwestern.edu

During mitosis, chromosomes are highly condensed and transcription is silenced globally. One explanation for transcriptional repression is the reduced accessibility of transcription factors. To directly test this hypothesis and to investigate the dynamics of mitotic chromatin, we evaluate the exchange kinetics of several RNA polymerase I transcription factors and nucleosome components on mitotic chromatin in living cells. We demonstrate that these factors rapidly exchange on and off ribosomal DNA clusters and that the kinetics of exchange varies at different phases of mitosis. In addition, the nucleosome component H1c-GFP also shows phase-specific exchange rates with mitotic chromatin. Furthermore, core histone components exchange at detectable levels that are elevated during anaphase and telophase, temporally correlating with H3-K9 acetylation and recruitment of RNA polymerase II before the onset of bulk RNA synthesis at mitotic exit. Our findings indicate that mitotic chromosomes in general and ribosomal genes in particular, although highly condensed, are accessible to transcription factors and chromatin proteins. The phase-specific exchanges of nucleosome components during late mitotic phases are consistent with an emerging model of replication independent core histone replacement.

Abbreviations used in this paper: Br-U, bromo-uridine; FACT, facilitates chromatin transcription; FLIP, fluorescence loss in photobleaching; HMG, high mobility group; NOR, nucleolar organizing region; RNA pol I, RNA polymerase I; rDNA, ribosomal DNA; RFI, relative fluorescence intensity; RPA39/40, 39/40-kD subunit of RNA pol I; RPA43, 43-kD subunit of RNA pol I; t50, time of 50% fluorescence recovery; TBP, TATA-binding protein; UBF, upstream-binding factor.


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