Complex expression of murine heat shock transcription factors

A central step in the transcriptional activation of heat shock genes is the binding of the heat shock factor (HSF) to upstream heat shock elements (HSEs). In vertebrates, HSF1 mediates the ubiquitous response to stress stimuli, while the role of a second HSE-binding factor, HSF2, is still unclear. In this work we show that both factors are expressed in a wide range of murine tissues and each exists as two splicing isoforms. Although HSFs are virtually ubiquitous proteins, their abundance is predominant in testis and variable among other tissues, indicating specific regulations of their expression. A low level of DNA-binding activity of HSF1, detected in many tissues, is probably physiological and is not explained by an anomalous regulation of one of the two isoforms. Our observations suggest that these regulatory proteins may all have roles in fully developed tissues. This possibility is not mutually exclusive of a role of HSF2 during cellular differentiation and tissue development [L. Sistonen, K. D. Sarge and R. I. Morimoto (1994), Mol. Cell. Biol., 14, 2087-2099].