Red blood cell differentiation requires a stickier histone H1, which closes chromatin to transcription, based on findings from Dhananjay Yellajoshyula and David Brown (University of Mississippi, Jackson, MS). The less sticky form, however, might favor transcription not by falling off faster but by relaxing chromatin.
Undifferentiated cells need relaxed, accessible chromatin to keep their transcriptional options open. At their most pluripotent, stem cells contain a pool of highly mobile H1 that was thought to fall off easily and give way to transcription factors.
This H1 mobility, Brown finds, must be reined in during red blood cell maturation, to “shut down genes you don't need and leave on only the ones you do.” The crucial loss of mobility is gained by dephosphorylating H1. A mutant H1 that mimics the phosphorylated form blocked differentiation. It also boosted the mobility of its unphosphorylated counterpart—an effect that cannot easily...
