Negative supercoiling in nucleosome is introduced due to:
Topological analysis demonstrates wrapping DNA in left-handed superhelix around histone octamer introduces negative supercoiling, equivalent to underwinding helix and decreasing linking number. When plasmid DNA assembles into nucleosomes in extracts depleted of topoisomerases, agarose gel electrophoresis with intercalators shows more negative writhe, allowing eukaryotes to store negative supercoils without bacterial gyrase. Right-handed wrapping would create positive supercoiling contrary to observations. H1 binding stabilizes fiber but does not generate writhe, methylation adds epigenetic mark without topology change. Negative supercoiling facilitates DNA strand separation during transcription.
Ref: Germond et al., PNAS 1975; Lewin, Genes XII, Negative Supercoiling due to Left-handed Wrap