Linker histones inhibit T4 and Escherichia coli DNA ligases

Based on some preliminary observations that linker histones strongly inhibit the activity of prokaryotic DNA ligases, we studied the effect of these histones on the ligation of short restriction DNA fragments by either T4 or E. coli DNA ligases. The inhibitory effect was strong, but it appeared only after two molecules of H1 bound to a approximately 200 bp-long DNA fragment. A similar pattern of inhibition (but at much higher concentration) was observed with the isolated globular domain of histone H5. That the inhibition was specific to the linker histones became clear when other basic proteins, such as the core histone octamer or cytochrome C, were tested. They did not inhibit the ligases but rather significantly stimulated them. The other major linker DNA-binding protein in chromatin, the non-histone protein HMG1, showed no significant effect on the ligase activity.