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Mol. Cell. Biol., Oct 1997, 5897-5904, Vol 17, No. 10
CL Hsieh
CpG DNA methylation has previously been correlated with the suppression of
transcription. The mechanism of this suppression is not understood, and
many aspects of the temporal and positional relationships between the
region of methylation and transcription have not yet been defined. Here,
12-kb stable replicating episomes that can be maintained in human somatic
cells for weeks to months were used. Such a system allows more direct
manipulation and is free from the positional effects attendant with the
analysis of endogenous loci or integrated transgenes. By using these
circular minichromosomes, patches of CpG methylation were created to
include or exclude the regions of transcriptional initiation and
elongation. I found that a 0.5-kb patch of methylation that covered the
promoter suppressed expression only 2-fold and that a 1.9-kb patch of
methylation that covered the coding portion of the gene (but not the
promoter) suppressed expression about 10-fold. In contrast, methylation of
the entire minichromosome except for the promoter or the coding portion
suppressed transcription about 50- to 200-fold. I infer the following.
Methylation of the 0.5-kb promoter fragment does not significantly affect
transcription at the level of transcription factor binding or local
chromatin structure. The dominant effect on transcription occurs when the
length of methylated DNA is long, with little disproportionate effect of
methylation of specific regions, such as that of initiation or elongation.
I also found that the boundaries between these methylated and unmethylated
regions remained stable for the many weeks that I monitored them.
Copyright © 1997, American Society for Microbiology
Stability of patch methylation and its impact in regions of transcriptional initiation and elongation
Norris Cancer Center, Department of Urology, University of Southern California, Los Angeles 90033, USA. hsieh_c@froggy.hsu.usc.edu
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