The repression of transcription by histones. An in vitro transcription extract is used to monitor transcription from a promoter in the presence or absence of histone proteins and the transcription activator Sp1. The first two lanes are controls in the absence of histones: transcription is optimal, and Sp1 stimulates. Lanes 3 and 4 are also controls, showing results with core histones present but no H1. In lanes 5 to 10, histone H1 was added in concentrations of 0.5, 1.0, or 1.5 H1 molecules per nucleosome. Sp1 prevents gene repression by binding specific sequences in the promoter.