Fl. Sun et al., Long-range nucleosome ordering is associated with gene silencing in Drosophila melanogaster pericentric heterochromatin, MOL CELL B, 21(8), 2001, pp. 2867-2879
We have used line HS-2 of Drosophila melanogaster, carrying a silenced tran
sgene in the pericentric heterochromatin, to investigate in detail the chro
matin structure imposed by this environment. Digestion of the chromatin wit
h micrococcal nuclease (MNase) shows a nucleosome array with extensive long
-range order, indicating regular spacing, and with well-defined MNase cleav
age fragments, indicating a smaller MNase target in the linker region. The
repeating unit is ca. 10 bp larger than that observed for bulk Drosophila c
hromatin, The silenced transgene shows both a loss of DNase I-hypersensitiv
e sites and decreased sensitivity to DNase I digestion within an array of n
ucleosomes lacking such sites; within such an array, sensitivity to digesti
on by MNase is unchanged. The ordered nucleosome array extends across the r
egulatory region of the transgene, a shift that could explain the loss of t
ransgene expression in heterochromatin, Highly regular nucleosome arrays ar
e observed over several endogenous heterochromatic sequences, indicating th
at this is a general feature of heterochromatin, However, genes normally ac
tive within heterochromatin (rolled and light) do not show this pattern, su
ggesting that the altered chromatin structure observed is associated with r
egions that are silent, rather than being a property of the domain as a who
le. The results indicate that long-range nucleosomal ordering is linked wit
h the heterochromatic packaging that imposes gene silencing.