Fig. 4.

Intranucleosomal DNA Ø-loop containing transcribing Pol II. Small intranucleosomal DNA loops containing transcribing enzyme likely form several times during transcription through a nucleosome [31,35]. Formation of the such loops results in disruption of some DNA-histone interactions, formation of new histone-polymerase interactions, steric interference with the next superhelical coil of nucleosomal DNA (as in Ø-loop [35]) and displacement of ~50-bp DNA region (shown by dashed lines). Formation of small intranucleosomal, topologically closed DNA loops constrains rotation of Pol II around DNA and therefore could result in accumulation of unconstrained DNA supercoiling (positive in front and negative behind the enzyme, respectively [105]). Positive unconstrained DNA supercoiling, in turn, could induce reversible unfolding of nucleosomal DNA that remains associated with core histones [44]. Note: Pol II is not drawn to scale.