The High Mobility Group (HMG)-box is found in a variety of eukaryotic chromosomal proteins and transcription factors. HMGs bind to the minor groove of DNA and have been classified by DNA binding preferences. Two phylogenetically distinct groups of Class I proteins bind DNA in a sequence specific fashion and contain a single HMG box. One group (SOX-TCF) includes transcription factors, TCF-1, -3, -4, and also SRY and LEF-1, which bind four-way DNA junctions and duplex DNA targets. The second group (MATA) includes fungal mating type gene products MC, MATA1 and Ste11. Class II and III proteins (HMGB-UBF) bind DNA in a non-sequence specific fashion and contain two or more tandem HMG boxes. Class II members include non-histone chromosomal proteins, HMG1 and HMG2, which bind to bent or distorted DNA such as four-way DNA junctions, synthetic DNA cruciforms, kinked cisplatin-modified DNA, DNA bulges, cross-overs in supercoiled DNA, and can cause looping of linear DNA. Class III members include nucleolar and mitochondrial transcription factors, UBF and mtTF1, which bind four-way DNA junctions.
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