Enhancer-dependent transcriptional oscillations in mouse erythroleukemia cells

By using recombinase-mediated cassette exchange, a method that allows integ ration of single copies of different constructs at the same predetermined c hromosomal location, several expression cassettes have been integrated at a randomly chosen locus in the genome of mouse erythroleukemia cells. The ca ssettes studied contain the human beta-globin promoter fused to lacZ coding sequences either alone or linked to DNase I-hypersensitive site HS2, HS3, or HS234 (a large locus control region fragment containing HS2, HS3, and HS 4) of the human beta-globin locus control region. Analysis of expression of these cassettes revealed mosaic expression patterns reminiscent of, but cl early different from, position effect variegation. Further investigations d emonstrated that these mosaic expression patterns are caused by dynamic act ivation and inactivation of the transcription unit, resulting in oscillatio ns of expression. These oscillations occur once in every few cell cycles at a rate specific for the enhancer present at the locus. DNase I sensitivity studies revealed that the chromatin is accessible and that DNase-hypersens itive sites were present whether or not the transcription unit is active, s uggesting that the oscillations occur between transcriptionally competent a nd transcriptionally active chromatin conformations, rather than between op en and closed chromatin conformations. Treatment of oscillating cells with trichostatin A eliminates the oscillations only after the cells have passed through late G(1) or early S, suggesting that these oscillations might be caused by changes in histone acetylation patterns.