It is well-known that H1-H1 interactions are very important for the inducti
on of 30 nm chromatin fiber and that, among all posttranslational modificat
ions, poly(ADP-ribosyl)ation is one of those capable of modifying chromatin
structure, mainly through H1 histone. As this protein can undergo both cov
alent and noncovalent modifications by poly(ADP-ribosyl)ation, our aim was
to investigate whether and how ADP-ribose polymers, by themselves, are able
to affect the formation of H1-H1 oligomers, which are normally present in
a condensed chromatin structure. The results obtained in our in vitro exper
imental system indicate that ADP-ribose polymers are involved in chromatin
decondensation. This conclusion was reached as the result of two different
observations: (a) H1 histone molecules can be hosted in clusters on ADP-rib
ose polymers, as shown by their ability to be chemically cross-linked, and
(b) H1 histone has a higher affinity for ADP-ribose polymers than for DNA;
ADP-ribose polymers compete, in fact, with DNA for HI histone binding.