The intron-containing hsp82 gene of the dimorphic pathogenic fungus Histoplasma capsulatum is properly spliced in severe heat shock conditions.

Abstract

We have isolated and characterized a heat-inducible gene, hsp82, from the dimorphic pathogenic fungus Histoplasma capsulatum, which is a filamentous mold at 25°C and a unicellular yeast at 37°C. This gene, which has a high degree of homology with other members of the hsp82 gene family, is split into three exons and two introns of 122 and 86 nucleotides, respectively. Contrary to what has been demonstrated in Drosophila melanogaster, Saccharomyces cerevisiae, and other organisms, hsp82 mRNA in H. capsulatum is properly spliced during the severe heat conditions of 37 to 40°C in the temperature-sensitive Downs strain. Splicing accuracy was also observed at 42°C in the temperature-tolerant G222B strain, which showed no evidence of accumulation of primary transcripts. Furthermore, the intron containing the ,-tubulin gene is also properly spliced at the upper temperature range, suggesting that the lack of a block in splicing may be a general phenomenon in this organism.Histoplasma capsulatum, a common respiratory pathogen of humans, is a dimorphic fungus that exists in a multicellular filamentous state in nature and as unicellular budding yeasts in tissue (1). The transition between the multicellular and unicellular phases can be reversibly induced by shifting the temperature from 25°C (mycelial phase) to 37°C (yeast phase). The mycelial-to-yeast conversion is of particular interest since it is triggered by an increase in the temperature of incubation and conversion to the yeast morphology is necessary for virulence. In fact, an important pathological feature of disease is that only yeasts are found in infected tissues (25,34). Therefore, the unique ability of dimorphic fungal pathogens to change shape in order to colonize, adapt to, and survive in host tissues is a process that parallels and may be intimately involved with this developmentally regulated morphological process (24).Since the mycelial (saprobic)-to-yeast (parasitic) transition is induced by heat, it was suggested that the early events of the phase transition were part of a heat shock response which is followed by cell adaptation to higher temperatures (19). This notion was supported by the report of Caruso et al. (7), who cloned the hsp70 gene from H. capsulatum and studied its expression in strains that differed in virulence and sensitivity to heat, and it was confirmed by the report of Shearer et al. (35), who observed the synthesis of new proteins soon after the temperature shift from 25°C to between 34 and 41°C.To understand the effects of severe heat shock on mRNA precursor processing during the mycelium-to-yeast transition in this fungus, we cloned a heat shock 82 (hsp82) gene and investigated its expression in two strains that differ in sensitivity to temperature and virulence.In Drosophila melanogaster cells as well as in Saccharomyces cerevisiae, hsp83 mRNA processing is inhibited at the upper temperature range of the heat shock response (38, 40). * Corresponding author.In Drosophila cells, the block in RNA processing was not specific to hsp83 ...

Biology of the Heat Shock Response and Protein Chaperones: Budding Yeast (Saccharomyces cerevisiae) as a Model System

“…Interestingly, the heat shock proteins (HSPs) have been frequently shown to be immunodominant in infection by diverse pathogenic fungi, which is against the common rule that the immune response is generally targeted against microbe-specific antigens (55,102,219,236,294,295,364,418,442,457). Candida albicans is a significant human fungal pathogen capable of disseminating through blood and colonizing almost all organs (514).…”

Fungal Hsp90: a biological transistor that tunes cellular outputs to thermal inputs

“…Hsp90 is naturally abundant in fungal cells and is induced to even greater levels by heat shock and other proteotoxic stresses 10,15,[32][33][34][35] . Hsp90 protein levels are regulated at transcriptional and post-transcriptional levels ( Figure 2).…”

The relevance of heat shock regulation in fungal pathogens of humans

“…Molecular biologists regularly apply acute insults to provoke dramatic molecular responses that are easy to assay 19 Hsp90 negatively regulates hyphal development in C. albicans (ca). 24 temperature upshifts also induce HSP90 expression in C. neoformans (cn), H. capsulatum (Hc) and P. brasiliensis (Pb) [31][32][33] and promote yeast growth rather than hyphal development in these fungi. this raises the possibility that Hsp90 might also regulate morphogenesis in these pathogenic fungi (see text).…”

“…In H. capsulatum, C. neoformans and P. brasiliensis, Hsp90 expression levels have been shown to increase in response to the elevated temperatures that induce their yeast growth form. [31][32][33] Hence the Hsf1-Hsp90 module might play a role in the regulation of morphogenesis in these fungi (Fig. 1).…”