A HISTONE OCTAMER CAN STEP AROUND A TRANSCRIBING POLYMERASE WITHOUT LEAVING THE TEMPLATE

The mechanism by which nucleosome cores are displaced and re-formed during transcription in vitro has been investigated. A nucleosome core was assembled on a short linear DNA template (227 bp) containing an SP6 RNA polymerase promoter and a nucleosome-positioning sequence. Transcription induced the translocation of the nucleosome core over 75 or 80 bp to two positions at the other end of the template, blocking the promoter. At low rNTP concentrations, transfer occurred only on the same template molecule, even in the presence of large excesses of competitor DNA. On a longer template (262 bp), nucleosome core position after transcription depended on its position before transcription. The data suggest that the octamer transfers without dissociation from DNA and provide strong evidence for a translocation mechanism in which DNA ahead of the polymerase uncoils from the octamer as the DNA behind coils around it. In this way, the octamer steps around the transcribing polymerase.