HSF1 transcription factor concentrates in nuclear foci during heat shock: relationship with transcription sites

In this paper, we show that upon heat shock, HSF1 concentrates in the nucleus of diploid human fibroblasts in two large foci. The relative distribution of HSF1 nuclear foci and active heat shock protein (hsp) genes was investigated by combining fluorescence in situ hybridization (FISH) for the detection of hsp nuclear transcripts and immunofluorescence for the detection of HSF1. We show that the HSF1 foci are distinct from the sites of hsp70 and hsp90 genes transcription. This is the second report of ploidy-dependent foci of transcription factors that are independent of their specific transcription sites. However, the correlation between the number of HSF1 foci and the ploidy of the cells strongly supports the existence of a specific chromosomal target for HSF1 foci.