| Research Abstract |
The vertebrate genome encodes a family of heat shock factors (HSFs 1-4) of which the DNA binding and transcriptional activities of HSF1 and HSF3 are activated upon heat shock. HSF1 has the properties of the classical heat shock factor and exhibits rapid activation of DNA binding and transcriptional activity upon exposure to conditions of heat shock and other stresses, whereas HSF3 is typically activated at higher temperatures and with distinct delayed kinetics. While these data suggest that HSF3 functions as a redundant stress activator, its role relative to HSF1, in the regulation of the heat shock response, was uncertain. To address the role of HSF3 in the heat shock response, null cells lacking the HSF3 gene were constructed by disruption of the resident gene by somatic recombination in an avian lymphoid cell line. Null cells lacking HSF3, yet expressing normal levels of HSF1, exhibited a severe reduction in the heat shock response as measured by inducible expression of heat shock g
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enes. Consequently, cells lacking HSF3 did not exhibit thermotolerance. These results reveal that HSF3 has a dominant role in the regulation of the heat shock response.
HSF4 is a mammalian factor characterized by its lack of a suppression domain that modulates formation of DNA-binding homotrimer. We have determined the exon structure of the human HSF4 gene and identified a major new isoform, HSF4b, derived by alternative RNA splicing, events, in addition to a previously reported HSF4a isoform. In mouse tissues HSF4b mRNA was more abundant than HSF4a as examined by RT-PCR, and its protein was detected in the brain and lung. Although both mouse HSF4a and HSF4b form trimers in the absence of stress, these two isoforms exhibit different transcriptional activity, HSF4a acts as an inhibitor of the constitutive expression of heat shock genes, and hHSF4b as a transcriptional activator. Furthermore HSF4b, but not HSF4a complements the viability defect of yeast cells lacking HSF.Moreover heat shock and other stresses stimulate transcription of target genes by HSF4b in both yeast and mammalian cells. These results suggest that differential splicing of HSF4 mRNA gives rise to both an inhibitor and activator of tissue-specific heat shock gene expression. Less
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