The Drosophila Df31 Protein Interacts with Histone H3 Tails and Promotes Chromatin Bridging In vitro

Df31 is a small hydrophilic protein from Drosophila melanogaster that can act as a histone chaperone in vitro. The protein is also detected as an integral component of chromatin, present at approximately the same level as histone H1. We have developed a simple assay to measure protein binding to oligonucleosomes and used it to characterise the DF31–oligonucleosome interaction. DF31 bound to chromatin in vitro at a level comparable to that observed in vivo. The DF31–chromatin interaction required the presence of core histone tails but binding was independent of the presence of H1 in the chromatin. Multiple regions of DF31 contributed to the interaction. Df31–chromatin binding still occurred on chromatin containing only H3/4, and cross-linking experiments showed that Df31 made intimate contact with H3, suggesting that this might be the primary contact site. Finally, using immobilised chromatin templates, we showed that DF31 promoted interstrand bridging between two independent oligonucleosome chains. These results provide strong evidence for a structural role of DF31 in chromatin folding and give an indication of the mechanism involved.