These inactive DNA stretches are binding sites for HSF1, a transcription factor that activates individual heat shock genes and accumulates in nuclear granules after heat shock. These granules were, like HSF1, expected to activate heat shock genes. So their recent localization to heterochromatic satellite III regions, which do not contain the major heat shock genes, was surprising. Jolly now shows that the stress granules activate transcription even in these repeat sequences.
The stress granules on satellite III repeats were found to contain RNA polymerase II and acetylated histones, which together produced large transcripts in response to heat shock. Heat stress has also been shown to induce RNA pol III–mediated transcription of repeat sequences in both human and mice. Together, the results suggest that transcription of many heterochromatic regions might be revealed given the right stress conditions.
The function of the human satellite III transcripts remains an unanswered question. Most of the transcripts remained associated with the DNA long after their transcription, so the authors hypothesize that they might maintain the architecture of the chromosome at high temperatures. ▪