Calcium binding to HMG1 protein induces DNA looping by the HMG‐box domains

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Abstract
Electron microscopy has shown that non‐histone chromosomal HMG1 could induce DNA looping or compaction in the presence (but not in the absence) of Ca2+. The effect of calcium on DNA looping and compaction was interpreted as calcium binding to the acidic C‐domain of HMG1. Both individual DNA‐binding HMG1‐box domains A and B were found to be involved in DNA looping and compaction. Treatment of HMG1 with a thiol‐specific reagent, N‐ethylmaleimide, inhibited the ability of the protein to induce DNA looping and compaction but not the electrostatic interaction with DNA. These results indicated that cysteine‐sulfhydryl groups of the HMG1‐box domains A and B are specifically involved in DNA looping and compaction, and that in the absence of calcium the acidic C‐domain down‐regulates these effects by modulation of the DNA‐binding properties of the HMG1‐box domains.