F. Ye et Er. Signer, RIGS (REPEAT-INDUCED GENE SILENCING) IN ARABIDOPSIS IS TRANSCRIPTIONAL AND ALTERS CHROMATIN CONFIGURATION, Proceedings of the National Academy of Sciences of the United Statesof America, 93(20), 1996, pp. 10881-10886
We have previously reported repeat-induced gene silencing (RIGS) in Ar
abinopsis, in which transgene expression may be silenced epigeneticall
y when repeated sequences are present, Among an allelic series of line
s comprising a primary transformant and various recombinant progeny ca
rrying different numbers of drug resistance gene copies at the same lo
cus, silencing was found to depend strictly on repeated sequences and
to correlate with an absence of steady-state mRNA, We now report chara
cterization, in nuclei isolated from the same transgenic lines, of gen
e expression by nuclear run-on assay and of chromatin structure by nuc
lease protection assay. We find that silencing is correlated with abse
nce of run-on transcripts, indicating that expression is silenced at t
he level of transcription, We find further that silencing is also corr
elated with increased resistance to both DNase I and micrococcal nucle
ase, indicating that the silenced state reflects a change in chromatin
configuration. We propose that silencing results when a locally paire
d region of homologous repeated nucleotide sequences is flanked by unp
aired heterologous DNA, which leads chromatin to adopt a local configu
ration that is difficult to transcribe, and possibly akin to heterochr
omatin.