[Structure of the transcription-active chromatin]

A method of attaching proteins to specific sites of DNA has been developed. The proteins are fixed on DNA directly in the cell nucleus by covalent bonding. DNA's associated with different proteins are divided into several diagonals by the two-dimensional gel-electrophoresis. The nucleotide sequences linked to proteins are then identified by hybridizing them with cloned probes of DNA. This method was used for Drosophila cells to determine the presence of histones in the transcribed site of the hsp-22 gene, activated by heat shock, and also in the neighboring active gene and non-transcribed insertion in ribosomal RNA genes. It was found that moderate transcription removed H1 and part of core histones, whereas intensive transcription removed all histones from the structural region of the genes. The substitution of core histones by RNA-polymerase within the transcribed region seems to cause the removal of first H1 and then core histones (depending on the intensity of the process) from prolonged regions of chromatin, which leads to a 25 nm fibrilla being decondensed to a 10 nm thread and, finally, to a completely linearized histone-free DNA.