Differences in evolutionary stability among mammalian H1 subtypes. Implications for the roles of H1 subtypes in chromatin.

The metabolic and developmental properties that differentiate five H1 subtypes of the mouse, H1a-H1e, are shown to have been conserved in the course of mammalian evolution. The structures of the subtypes, however, as judged by their mobilities in two-dimensional gel electrophoresis, have been conserved to varying extents. H1a and H1c have undergone the most structural variation, each having a different electrophoretic mobility in every species examined. H1b is much less varied, while H1d and H1e are invariant by these electrophoretic criteria. These results suggest that H1a and -c differ fundamentally from H1b, -d, and -e in their interactions with other chromatin components. It is suggested that this difference is in the ability to promote the coiling of 10-nm chromatin fibers into higher order chromatin structures.