Facilitated transcription through the nucleosome at high ionic strength occurs via a histone octamer transfer mechanism

The rate of transcription through the nucleosome, the fine structure of the nucleosomal barrier, and the fate of the nucleosome during transcription a t different salt concentrations were analyzed using linear 227-base pair mo nonucleosomal templates containing a uniquely positioned nucleosome core. A t lower ionic strength (30 mm NaCl), the nucleosome constitutes a strong ba rrier for SP6 RNA polymerase. At higher ionic strength (330 mm NaCl), the r ates of transcription on nucleosomal and histone-free DNA templates are ver y similar. At both higher and lower ionic strengths, the complete histone o ctamer is transferred over the same distance by fundamentally similar mecha nisms. The data indicate that even at the rate of transcription characteris tic of histone-free DNA, the transfer intermediates can be formed quite eff iciently. This suggests possible mechanisms that could facilitate transcrip tion through the nucleosome at physiological ionic strength.