The tandemly organised ribosomal DNA (rDNA) repeats are transcribed by a
dedicated RNA polymerase in a specialised nuclear compartment, the
nucleolus. There appears to be an intimate link between the maintenance of
nucleolar structure and the presence of heterochromatic chromatin domains.
This is particularly evident in many large neurons, where a single
nucleolus is present, which is separated from the remainder of the nucleus
by a characteristic shell of heterochromatin. Using a combined
fluorescence in situ hybridisation and immunocytochemistry approach, we
have analysed the molecular composition of this highly organised neuronal
chromatin, to investigate its functional significance. We find that
clusters of inactive, methylated rDNA repeats are present inside large
neuronal nucleoli, which are often attached to the shell of
heterochromatic DNA. Surprisingly, the methylated DNA-binding protein
MeCP2, which is abundantly present in the centromeric and perinucleolar
heterochromatin, does not associate significantly with the methylated rDNA
repeats, whereas histone H1 does overlap partially with these clusters.
Histone H1 also defines other, centromere-associated chromatin subdomains,
together with the mammalian Polycomb group factor Eed. These dat
