Chromatin structure of active and inactive human X chromosomes.

Nuclei from a variety of human cell lines and tissues were digested with gradually increasing levels of DNase I. The DNA was then purified, treated with restriction enzymes and subjected to Southern blot hybridization using a cloned cDNA probe to 3-phosphoglycerate kinase (PGK) a housekeeping enzyme. At relatively high levels of DNase I, a specific, slightly sensitive site in chromatin sequences encoding PGK was observed in all of the cell types examined. This slightly sensitive site resides on the active X-chromosome since cell lines with increased numbers of inactive X-chromosomes do not show an increase in the region of chromatin which is sensitive. Except for this restricted region of enhanced sensitivity on the active X-chromosome, the data suggest that, for PGK encoding sequences, chromatin configurations on the active and inactive X-chromosomes are similar.