Novel isoforms of heat shock transcription factor 1 are induced by hypoxia in the Pacific oyster Crassostrea gigas.

The Pacific oyster Crassostrea gigas inhabits the intertidal zone and shows tolerance to various stress conditions such as hypoxia and heat shock. However, little is known about the cellular mechanism of responses to these stresses. Heat shock transcription factor 1 (HSF1) regulates the transcription of several genes, including heat shock proteins (HSPs). In this study, we cloned HSF1 from the oyster and investigated its response to air-exposure. The cDNA of oyster Hsf1 contains 2,931 bp, of which 1,389 bp encode a protein of 463 amino acid residues. Moreover, we found that the oyster has seven novel alternatively spliced isoforms, Hsf1b-h, consisting of insertion A with 48 bp, insertion B with 42 bp and/or insertion C with 42 bp. We determined the sequences of oyster genomic DNA containing Hsf1 insertions A, B and C. The results indicated that eight isoforms of Hsf1 are generated from a single Hsf1 gene by alternative splicing without frameshifting. Real-time PCR analysis showed that Hsf1a is expressed constitutively, and the expression of Hsf1b-h and Hsp70 mRNA is induced by air exposure and/or hypoxia. In addition, we found that 11 putative hypoxia response elements, which are hypoxia-inducible factor 1 (HIF-1) binding sequences, are located in the Hsf1 promoter region. These data suggest that the oyster has an HIF-HSF pathway in which HSPs are induced in an HIF-dependent manner, and that it also has a novel mechanism of alternative splicing of Hsf1 in response to hypoxia.