The plant-specific family of DNA-binding proteins containing three HMG-box domains interacts with mitotic and meiotic chromosomes.

• The high mobility group (HMG)-box represents a DNA-binding domain that is found in various eukaryotic DNA-interacting proteins. Proteins that contain three copies of the HMG-box domain, termed 3 × HMG-box proteins, appear to be specific to plants. The Arabidopsis genome encodes two 3 × HMG-box proteins that were studied here. • DNA interactions were examined using electrophoretic mobility shift assays, whereas expression, subcellular localization and chromosome association were mainly analysed by different types of fluorescence microscopy. • The 3 × HMG-box proteins bind structure specifically to DNA, display DNA bending activity and, in addition to the three HMG-box domains, the basic N-terminal domain contributes to DNA binding. The expression of the two Arabidopsis genes encoding 3 × HMG-box proteins is linked to cell proliferation. In synchronized cells, expression is cell cycle dependent and peaks in cells undergoing mitosis. 3 × HMG-box proteins are excluded from the nuclei of interphase cells and localize to the cytosol, but, during mitosis, they associate with condensed chromosomes. The 3 × HMG-box2 protein generally associates with mitotic chromosomes, while 3 × HMG-box1 is detected specifically at 45S rDNA loci. • In addition to mitotic chromosomes the 3 × HMG-box proteins associate with meiotic chromosomes, suggesting that they are involved in a general process of chromosome function related to cell division, such as chromosome condensation and/or segregation.

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