Differences in the DNA‐Binding Properties of the Hmg‐Box Domains of HMG1 and the Sex‐Determining Factor SRY

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Abstract
High‐mobility‐group protein 1 (HMG1) is an abundant, non‐sequence‐specific, chromosomal protein with two homologous, HMG‐box, DNA‐binding domains, A and B, and an acidic tail. The HMG‐box motif also occurs, as a single copy, in some sequence‐specific transcription factors, e.g. the sex‐determining factor, SRY. We have investigated whether or not there are differences in the DNA‐binding properties of the isolated A and B HMG‐box domains of HMG1 and SRY and whether, in the case of A and B, there might also be differences due to different sequence contexts within the native protein. The basic regions that flank the HMG1 B box, giving B', enhance its DNA‐binding, supercoiling and DNA‐bending activities, and promote the self‐association of the DNA‐bound B‐box. All the HMG‐box domains bind with structure specificity to four‐way junctions, but the structure selectivity is significantly greater for A and the SRY box than for the HMG1 B or B' domains, as judged by competition with excess plasmid DNA. The domains self‐associate to different extents on supercoiled DNA and this may explain differences in the ability to discriminate between four‐way junctions and supercoiled DNA. The HMG1 A, B and B' domains constrain negative superhelical turns in DNA, but the SRY HMG box does not. Only the full B domain (B') bends DNA in a ligase‐mediated circularisation assay; the minimal B box, the A domain and the SRY box do not. Thus, despite a common global fold, the HMG box appears to have been adapted to various functions in different protein contexts.