INFLUENCE OF CHROMATIN-STRUCTURE ON ACCESSIBILITY AND TRANSCRIPTIONALREGULATION OF THE DUCK HISTONE H5-GENE

The structure in which genes are packaged in chromatin is presumably o f paramount importance for the regulation of their expression in vivo and much current research is centred on unravelling the underlying mec hanisms. Here we show, that a nucleosomal core particle can be efficie ntly reconstituted in vitro over the transcription initiation site of the duck histone H5-gene specifically expressed in avian and amphibian erythrocytes. Under these conditions binding of transcription activat or eUSF is still possible although the specific binding sire for this factor is incorporated into the nucleosome. In contrast we found that corresponding promoter regions of the duck histone H5-gene are nucleas e-sensitive in differentiated erythrocytes and are hence free of nucle osomes. In vitro reconstitution of nucleosomes on plasmids harboring t he H5-gene leads to complete repression of H5-transcription. can be sp ecifically prevented by preincubating the template DNA with a particul ar protein fraction from HeLa cells, whereas addition of the same frac tion after nucleosome assembly can not relieve repression.