Preferential binding of histone H1 to four-way helical junction DNA.

Histone H1 is a major chromatin protein, which stabilizes the nucleosome, has an essential role in organizing nucleosomes into higher order structures, and may have a role as a repressor of transcription (van Holde, K. E. (1989) Chromatin, Springer Publishing Co., New York). Here we show that H1 forms a defined complex with a synthetic four-way junction of DNA strands even in the presence of an excess of linear nonspecific competitor DNA. The four-way junction also competes efficiently against two duplex DNA molecules, which together have the same sequence information as the four-way junction molecule. Another major chromatin protein, high mobility group protein 1, also binds four-way junction structures specifically (Bianchi, M. E., Beltrame, M., and Paonessa, G. (1989) Science 243, 1056-1059), and this similar behavior may indicate a related function of these proteins. Our finding may suggest that four-way DNA junction is a structural equivalent to the main H1 binding site in the nucleosome: a crossover of double helical DNA at the point where the DNA enters and exits the nucleosome (Allan, J., Hartman, P. G., Crane-Robinson, C., and Aviles, F. X. (1980) Nature 288, 675-679).