Fluctuation of chromatin unfolding associated with variation in the level of gene expression

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We examined whether spontaneous alteration of chromatin structure, if any, correlates with variation in gene expression. Gene activation is associated with changes in chromatin structure at different levels. Large-scale chromatin unfolding is one such change detectable under the light microscope. We established cell clones carrying tandem repeats (more than 50 copies spanning several hundred kb) of the GFP (green fluorescent protein)-ASK reporter genes driven by a tetracycline responsive promoter. These clones constitutively express the transcriptional transactivator. Flow cytometry and live-recording fluorescence microscopy revealed that, although fully activated by a saturating amount of doxycycline, GFP-ASK expression fluctuated in individual cell clones, regardless of the cell cycle stage. The GFP-ASK expression changed from lower to higher levels and vice versa within a few cell cycles. Furthermore, the levels of GFP-ASK expression were correlated with the degrees of chromatin unfolding of the integrated array as detected by FISH (fluorescent in situ hybridization). The chromatin unfolding was not coupled to a mitotic event; around one-third of the daughter-pairs exhibited dissimilar degrees of chromatin unfolding. We concluded that fluctuation of chromatin unfolding was likely to result in variation in gene expression, although the source of the fluctuation of chromatin unfolding remains to be studied.

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