Genes transcribed at diverse rates have a similar conformation in chromatin

Abstract

The DNA generated upon treatment of hen oviduct nuclei with pancreatic DNase I (deoxyribonucleate 3''-oligonucleotidohydrolase; EC 3.1.4.6), with c[complementary]DNA copies of specific mRNA sequences were analyzed to study the structure and organization of transcriptionally active genes in chromatin. The kinetics of digestion of 3 classes of genes in the oviduct which are transcribed at significantly different rates were examined. The ovalbumin genes appear to be organized by chromatin proteins in such a way that they are rendered exceedingly sensitive to digestion by DNase I. This sensitivity is not observed in the liver, a tissue in which these genes are transcriptionally inert. The transcriptionally inactive globin genes in the oviduct are not selectively sensitive to nuclease attack and are digested 5 times more slowly in the ovalbumin genes in this tissue. The accessibility of a complex subset of genes that are rarely represented in the mRNA and are likely to be transcribed at a frequency orders of magnitude below that of the ovalbumin gene were also examined. Comparison of the accessibility of these sequences with that of the ovalbumin gene indicates that these 2 subsets of genes are recognized and cleaved by DNase I at similar rates. The maintenance of an active conformation about specific genes does not reflect the polymerase distribution about these genes. This active conformation is therefore not confined to sequences actively engaged in the transcription of process and may reflect the structure about a subpopulation of the genome which represents the transcriptional potential of a given cell type.