The considerable length of DNA in eukaryotic genomes requires packaging int
o chromatin to rt inside the small dimensions of the cell nucleus. Histone
H1 functions in the compaction of chromatin into higher order structures de
rived from the repeating 'beads on a string' nucleosome polymer. Modulation
of H1 binding activity is thought to be an important step in the potentiat
ion/depotentiation of chromatin structure for transcription(1-4). It is gen
erally accepted that H1 binds less tightly than other histones to DNA in ch
romatin and can readily exchange in living cells(5-8). Fusion proteins of H
istone H1 and green fluorescent protein (GFP) have been shown(9) to associa
te with chromatin in an apparently identical fashion to native histone H1.
This provides a means by which to study histone H1-chromatin interactions i
n living cells. Here we have used human cells with a stably integrated H1.1
-GFP fusion protein to monitor histone H1 movement directly by fluorescence
recovery after photobleaching in living cells. We rnd that exchange is rap
id in both condensed and decondensed chromatin, occurs throughout the cell
cycle, and does not require fibre-fibre interactions. Treatment with drugs
that alter protein phosphorylation significantly reduces exchange rates. Ou
r results show that histone H1 exchange in vivo is rapid, occurs through a
soluble intermediate, and is modulated by the phosphorylation of a protein
or proteins as yet to be determined.