Regulation of domains and whole chromosomes
Clusters of genes that encode similar products, such as the β-globin, the ribosomal RNA (rRNA) and the histone genes, are regulated in a coordinated fashion. An extreme case of coordinate regulation—dosage compensation—involves the genes present on the sex chromosomes. In Drosophila males, a complex (MSL) associates with the X chromosome where it enhances the activity of most X-linked genes. In Caenorhabditis, a complex (DCC) decreases the level of transcription of both X chromosomes in the XX hermaphrodite. In mammals, dosage compensation is achieved by the inactivation, early during development, of most X-linked genes on one of the two X chromosomes in females. In the mammalian embryo, X inactivation of either X chromosome is random and clonally inherited. The mechanism involves the synthesis of an RNA (Tsix) that protects one of the two Xs from inactivation, and of another RNA (Xist) that coats the other X chromosome and recruits histone- and DNA-modifying enzymes.