Structure and expression of a gene encoding heat-shock protein Hsp70 from the Oomycete fungus Bremia lactucae.

A gene encoding a protein homologous to a 70-kDa heat-shock protein (Hsp70) was isolated from Bremia lactucae and its structure and pattern of expression were determined. This is the first report on the structure of a protein-coding gene from an Oomycete fungus. The cloned gene is a member of a small multigene family. The level of hsp70 mRNA in germlings increases from a low constitutive level in response to heat or cold treatment. A high level of the mRNA is also detected in spores. The hsp70 gene is expressed as a primary transcript of 2241 nucleotides (nt) and contains a continuous open reading frame of 2025 nt. Near the C terminus of the coding sequence is an unusual region that contains repeated enhancer-like sequences. This insert has not been described in other hsp70 genes and is not an intron. Upstream from the 5' terminus of the mRNA are multiple CCAAT motifs, a sequence similar to a consensus heat-shock regulatory element, and an A + T-rich putative 'TATA' box. A canonical polyadenylation recognition sequence is present downstream from the coding sequence. The deduced amino acid sequence is equally similar to yeast and maize Hsp70, providing further evidence of the dissimilarity between Oomycetes and true fungi. The cloning of this gene is part of our strategy to develop a transformation system for B. lactucae.