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¹ Division of Biological Sciences, Department of Medicine
² Division of Physical Sciences, Enrico Fermi Institute and Department of Physics, University of Chicago, Chicago, IL 60637

Address correspondence to Reiner Strick, Dept. of Medicine, University of Chicago, 5841 S. Maryland Ave., MC2115, Chicago, IL 60637-1470. Tel.: (773) 834-1539. Fax: (773) 702-3002. E-mail: rstrick{at}medicine.bsd.uchicago.edu

Mammalian interphase and mitotic cells were analyzed for their cation composition using a three-dimensional high resolution scanning ion microprobe. This instrument maps the distribution of bound and unbound cations by secondary ion mass spectrometry (SIMS). SIMS analysis of cryofractured interphase and mitotic cells revealed a cell cycle dynamics of Ca²+, Mg²+, Na⁺, and K⁺. Direct analytical images showed that all four, but no other cations, were detected on mitotic chromosomes. SIMS measurements of the total cation content for diploid chromosomes imply that one Ca²+ binds to every 12.5–20 nucleotides and one Mg²+ to every 20–30 nucleotides. Only Ca²+ was enriched at the chromosomal DNA axis and colocalized with topoisomerase II (Topo II) and scaffold protein II (ScII). Cells depleted of Ca²+ and Mg²+ showed partially decondensed chromosomes and a loss of Topo II and ScII, but not hCAP-C and histones. The Ca²+-induced inhibition of Topo II catalytic activity and direct binding of Ca²+ to Topo II by a fluorescent filter-binding assay supports a regulatory role of Ca²+ during mitosis in promoting solely the structural function of Topo II. Our study directly implicates Ca²+, Mg²+, Na⁺, and K⁺ in higher order chromosome structure through electrostatic neutralization and a functional interaction with nonhistone proteins.

Key Words: cations; chromosome structure; condensation; ion microscopy; topoisomerase II

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