Transcription of the Neurospora crassa 70-kDa class heat shock protein genes is modulated in response to extracellular pH changes

Abstract

Heat shock proteins belong to a conserved superfamily of molecular chaperones found in prokaryotes and eukaryotes. These proteins are linked to a myriad of physiological functions. In this study, we show that the N. crassa hsp70-1 (NCU09602.3) and hsp70-2 (NCU08693.3) genes are preferentially expressed in an acidic milieu after 15 h of cell growth in sufficient phosphate at 30°C. No significant accumulation of these transcripts was detected at alkaline pH values. Both genes accumulated to a high level in mycelia that were incubated for 1 h at 45°C, regardless of the phosphate concentration and extracellular pH changes. Transcription of the hsp70-1 and hsp70-2 genes was dependent on the pacC + background in mycelia cultured under optimal growth conditions or at 45°C. The pacC gene encodes a Zn-finger transcription factor that is involved in the regulation of gene expression by pH. Heat shock induction of these two hsp genes in mycelia incubated in low-phosphate medium was almost not altered in the nuc-1 − background under both acidic and alkaline pH conditions. The NUC-1 transcriptional regulator is involved in the derepression of nucleases, phosphatases, and transporters that are necessary for fulfilling the cell's phosphate requirements. Transcription of the hsp70-3 (NCU01499.3) gene followed a different pattern of induction-the gene was depressed under insufficient phosphate conditions but was apparently unaffected by alkalinization of the culture medium. Moreover, this gene was not induced by heat shock. These results reveal novel aspects of the heatsensing network of N. crassa.

Transcription of the Hsp30, Hsp70, and Hsp90 heat shock protein genes is modulated by the PalA protein in response to acid pH-sensing in the fungus Aspergillus nidulans

“…Thus, in spite of the different patterns of induction, accumulation of these three hsp transcripts upon heating for 2 h was almost unaffected by ambient pH changes in the palA − background. The PalA protein mediates the proteolytic activation of PacC, a regulator required for the activation/repression of acid or alkaline genes in A. nidulans and other filamentous fungi Silva et al 2008;Silveira et al 2010;Squina et al 2010;Tilburn et al 1995). PacC binds to 5′-GCCARG-3′ sequences upstream of pH-conditioned genes, and the presence of this hexanucleotide in the sequences upstream of the hsp30, hsp70, and hsp90 genes (Table 1) suggests that their expression in an acidic milieu and at an optimal temperature for fungal growth might be directly controlled by PacC, i.e., the transcription of these heat shock genes may be PalA-dependent.…”

“…Thus, the hsp30, hsp70, and hsp90 genes are preferentially expressed in the palA + strain incubated at an optimal temperature for fungal growth at acidic pH. Eukaryotic heat shock and stress-related (Sfl1) transcription factors regulate the constitutive and stress-inducible transcription of several genes (Galeote et al 2007;Hjorth-Sorensen et al 2001;Truman et al 2007), including the hsp genes, and thereby play a central role in the regulation of numerous cellular reprogramming events (Sakurai and Takemori 2007;Squina et al 2010). Therefore, interactions between the PacC/Pal conserved signaling cascade and these transcription factors, which include their competition for target genes, are highly complex.…”

“…Ambient pH signaling ensures that nutritional enzymes will be secreted at pH values at which they can function effectively; for instance, active acid and alkaline Pi-repressible phosphatases are secreted at acid and alkaline pH, respectively. This adaptive response involves the highly conserved PacC signal transduction pathway that mediates many metabolic events in model and pathogenic fungi, including the fungal virulence (Caddick et al 1986;Ferreira-Nozawa et al 2006;Leal et al 2010;Silveira et al 2010;Squina et al 2010;Tilburn et al 1995). The pacC gene encodes a Zn-finger transcription factor that is activated by a conserved signaling cascade composed of six pal genes (palA,B,C,F,H,I), and it has been reported that this activation would occur under exclusively alkaline growth conditions (Tilburn et al 1995).…”

“…Thus, irrespective of the ambient pH and assuming that PacC is inactive at acidic pH in the absence of pal signaling, loss-of-function mutations in any of the pal genes should lead to a wildtype acidic growth phenotype (Tilburn et al 1995). However, there is increasing evidence that pacC and pal genes are functional when cultivation is carried out at both acidic and alkaline pH Silva et al 2008;Silveira et al 2010;Squina et al 2010).…”

The Genomes of Three Uneven Siblings: Footprints of the Lifestyles of Three Trichoderma Species

“…In the published genome of T. reesei 10 different members of the HSP70 family can be identified, whereas the genome of T. atroviride comprises 13 genes of this family, and even in T. virens 15 homologs can be found (see Table S1 in the supplemental material). The gene products of hsp70-1 and hsp70-2 (the latter gene encodes a mitochondrial protein) of N. crassa exert their function preferentially in acidic milieus and accumulate after incubation at 45°C, whereas the gene product of hsp70-3 is induced under insufficient phosphate conditions and not by heat shock (909). Members of this family were also shown to be induced during developmental changes (910).…”