DEPOSITION OF HISTONE H1 ONTO RECONSTITUTED NUCLEOSOME ARRAYS INHIBITS BOTH INITIATION AND ELONGATION OF TRANSCRIPTS BY T7 RNA-POLYMERASE

The effect of histone H1 on transcription by bacteriophage T7 RNA poly merase was examined using reconstituted chromatin templates. A 3.8 kb linear DNA template consisting of a specific transcription promoter fo r T7 RNA polymerase placed upstream of 18 tandem repeats of a 207 bp n ucleosome positioning sequence derived from the 5S rRNA gene of Lytech inus variegatus was used as a template for chromatin reconstitution. R egularly spaced arrays of nucleosome cores were assembled onto this DN A template from donor histone octamers by salt step dialysis. Histone H1 was incorporated onto free DNA or reconstituted chromatin templates and double label transcription assays were performed. The experiments indicated that histone H1 has a strong inhibitory effect on both tran scription initiation and elongation. These effects are especially pron ounced on chromatin templates, where both transcription initiation and elongation are virtually halted. The inhibition of transcription elon gation appears to result from a dramatic increase in premature termina tion of transcripts. These experiments indicate that assembly of histo ne H1 into chromatin can result in structures which are completely rep ressed with respect to transcription.