EFFICIENT TRANSCRIPTION OF A DNA-TEMPLATE ASSOCIATED WITH HISTONE (H3-CENTER-DOT-H4)(2) TETRAMERS

A histone-DNA transcription template has been assembled, by dialysis a gainst decreasing salt concentrations, from pGEMEX-1 (4 kilobases), a plasmid containing a promoter for bacteriophage T7 RNA polymerase, and from isolated histone (H3.H4)2 tetramers. Electron microscopy after p soralen cross-linking shows that each histone tetramer protects approx imately 80 base pairs of DNA from psoralen action and that, under the employed conditions, an average of 15 tetramer particles are assembled per DNA molecule. This (H3.H4)2-DNA template is efficiently transcrib ed in vitro by T7 RNA polymerase as compared to naked DNA. The presenc e of (H3-H4)2 tetramers does not affect initiation, in contrast with t he complete histone octamer, (H2A.H2B.H3.H4)2, assembled with the comp lementary addition of H2A-H2B dimers, which causes transcriptional inh ibition mainly by blocking initiation.