HISTONE H1 PREFERENTIALLY BINDS TO SUPERHELICAL DNA-MOLECULES OF HIGHER COMPACTION

In chromatin, the physiological amount of H1 is one molecule per nucle osome or, roughly, one molecule per 200 bp of DNA. We observed that al such a stoichiometry, H1 selectively binds to supercoiled DNA with \s igma\ greater than or equal to 0.012 (both negative and positive), lea ving relaxed, linear, or nicked DNA molecules unbound. When negative a nd positive DNA topoisomers of varying superhelicity are simultaneousl y present in the binding mixture, H1 selectively binds to the molecule s with highest superhelicity; less supercoiled forms are gradually inv olved in binding upon increasing the amount of input protein, We expla in this topological preference of H1 as the consequence of an increase d probability for more than one H1-DNA contact provided by the superco iling. The existence of simultaneous contacts of H1 with both intertwi ned DNA strands in the supercoiled DNA molecules is also inferred by t opoisomerase relaxation of H1-DNA complexes that had been prefixed wit h glutaraldehyde.