Different strategies of X-inactivation in germinal and somatic cells: histone H4 underacetylation does not mark the inactive X chromosome in the mouse male germline.

It has previously been shown by immunocytochemistry that the inactive X chromosome (Xi) in somatic cells of human and mouse females is marked by underacetylation of histone H4. It has been suggested that this may be important for transcriptional silencing of genes on Xi. We have now investigated X-inactivation in meiotic cells of the male germline. In these cells the single X chromosome is transcriptionally inactive and expresses XIST, a gene that in somatic cells is transcribed only from Xi. By immunostaining with antibodies to H4 acetylated at lysines 5, 8, 12, or 16, we demonstrate that histone H4 on the male X is not underacetylated. We conclude that there is a differential germline strategy for maintenance of X-inactivation and that H4 underacetylation, though associated with the long-term marking of inactive X chromosomes in the female soma, is not always essential for the transcriptional down-regulation of X-linked genes.