Molecular Characteristics of the Conserved Aspergillus nidulans Transcription Factor Mac1 and Its Functions in Response to Copper Starvation

MAC Family Transcription Factors Enhance the Tolerance of Mycelia to Heat Stress and Promote the Primordial Formation Rate of Pleurotus ostreatus

Pleurotus ostreatus is a typical tetrapolar heterologous edible mushroom, and its growth and development regulatory mechanism has become a research hotspot in recent years. The MAC1 protein is a transcription factor that perceives copper and can regulate the expression of multiple genes, thereby affecting the growth and development of organisms. However, its function in edible mushrooms is still unknown. In this study, two transcription factor genes, PoMCA1a and PoMAC1b, were identified. Afterwards, PoMAC1 overexpression (OE) and RNA interference (RNAi) strains were constructed to further explore gene function. The results showed that the PoMAC1 mutation had no significant effect on the growth rate of mycelia. Further research has shown that OE-PoMAC1a strains and RNAi-PoMAC1b strains exhibit strong tolerance under 32 °C heat stress. However, under 40 °C heat stress, the OE of PoMAC1a and PoMAC1b promoted the recovery of mycelial growth after heat stress. Second, the OE of PoMAC1a can promote the rapid formation of primordia and shorten the cultivation cycle. In summary, this study indicated that there are functional differences between PoMAC1a and PoMAC1b under different heat stresses during the vegetative growth stage, and PoMAC1a has a positive regulatory effect on the formation of primordia during the reproductive growth stage.

Identification and Characterization of the Determinants of Copper Resistance in the Acidophilic Fungus Acidomyces richmondensis MEY-1 Using the CRISPR/Cas9 System

Copper (Cu) homeostasis has not been well documented in filamentous fungi, especially extremophiles. One of the main obstacles impeding their characterization is the lack of a powerful genome-editing tool. In this study, we applied a CRISPR/Cas9 system for efficient targeted gene disruption in the acidophilic fungus Acidomyces richmondensis MEY-1, formerly known as Bispora sp. strain MEY-1. Using this system, we investigated the basis of Cu tolerance in strain MEY-1. This strain has extremely high Cu tolerance among filamentous fungi, and the transcription factor ArAceA (A. richmondensis AceA) has been shown to be involved in this process. The ArAceA deletion mutant (ΔArAceA) exhibits specific growth defects at Cu concentrations of ≥10 mM and is transcriptionally more sensitive to Cu than the wild-type strain. In addition, the putative metallothionein ArCrdA was involved in Cu tolerance only under high Cu concentrations. MEY-1 has no Aspergillus nidulans CrpA homologs, which are targets of AceA-like transcription factors and play a role in Cu tolerance. Instead, we identified the Cu-transporting P-type ATPase ArYgA, homologous to A. nidulans YgA, which was involved in pigmentation rather than Cu tolerance. When the ΔArYgA mutant was grown on medium supplemented with Cu ions, the black color was completely restored. The lack of CrpA homologs in A. richmondensis MEY-1 and its high tolerance to Cu suggest that a novel Cu detoxification mechanism differing from the AceA-CrpA axis exists. IMPORTANCE Filamentous fungi are widely distributed worldwide and play an important ecological role as decomposers. However, the mechanisms of their adaptability to various environments are not fully understood. Various extremely acidophilic filamentous fungi have been isolated from acidic mine drainage (AMD) with extremely low pH and high heavy metal and sulfate concentrations, including A. richmondensis. The lack of genetic engineering tools, particularly genome-editing tools, hinders the study of these acidophilic and heavy metal-resistant fungi at the molecular level. Here, we first applied a CRISPR/Cas9-mediated gene-editing system to A. richmondensis MEY-1. Using this system, we identified and characterized the determinants of Cu resistance in A. richmondensis MEY-1. The conserved roles of the Cu-binding transcription factor ArAceA in Cu tolerance and the Cu-transporting P-type ATPase ArYgA in the Cu-dependent production of pigment were confirmed. Our findings provide insights into the molecular basis of Cu tolerance in the acidophilic fungus A. richmondensis MEY-1. Furthermore, the CRISPR/Cas9 system used here would be a powerful tool for studies of the mechanisms of adaptability of acidophilic fungi to extreme environments.

Copper Homeostasis in Aspergillus nidulans Involves Coordinated Transporter Function, Expression and Cellular Dynamics

Copper ion homeostasis involves a finely tuned and complex multi-level response system. This study expands on various aspects of the system in the model filamentous fungus Aspergillus nidulans. An RNA-seq screen in standard growth and copper toxicity conditions revealed expression changes in key copper response elements, providing an insight into their coordinated functions. The same study allowed for the deeper characterization of the two high-affinity copper transporters: AnCtrA and AnCtrC. In mild copper deficiency conditions, the null mutant of AnctrC resulted in secondary level copper limitation effects, while deletion of AnctrA resulted in primary level copper limitation effects under extreme copper scarcity conditions. Each transporter followed a characteristic expression and cellular localization pattern. Although both proteins partially localized at the plasma membrane, AnCtrC was visible at membranes that resembled the ER, whilst a substantial pool of AnCtrA accumulated in vesicular structures resembling endosomes. Altogether, our results support the view that AnCtrC plays a major role in covering the nutritional copper requirements and AnCtrA acts as a specific transporter for extreme copper deficiency scenarios.

Expanded role of the Cu-sensing transcription factor Mac1p in Candida albicans

As part of the innate immune response, the host withholds metal micronutrients such as Cu from invading pathogens, and microbes respond through metal starvation stress responses. With the opportunistic fungal pathogen Candida albicans, the Cu-sensing transcription factor Mac1p governs the cellular response to Cu starvation by controlling Cu import. Mac1p additionally controls reactive oxygen species (ROS) homeostasis by repressing a Cu-containing superoxide dismutase (SOD1) and inducing Mn-containing SOD3 as a non-Cu alternative. We show here that C. albicans Mac1p is essential for virulence in a mouse model for disseminated candidiasis and that the cellular functions of Mac1p extend beyond Cu uptake and ROS homeostasis. Specifically, mac1∆/∆ mutants are profoundly deficient in mitochondrial respiration and Fe accumulation, both Cu-dependent processes. Surprisingly, these deficiencies are not simply the product of impaired Cu uptake; rather mac1∆/∆ mutants appear defective in Cu allocation. The respiratory defect of mac1∆/∆ mutants was greatly improved by a sod1∆/∆ mutation, demonstrating a role for SOD1 repression by Mac1p in preserving respiration. Mac1p downregulates the major Cu consumer SOD1 to spare Cu for respiration that is essential for virulence of this fungal pathogen. The implications for such Cu homeostasis control in other pathogenic fungi are discussed.

New insights into copper homeostasis in filamentous fungi

Copper is a metal ion that is required as a micronutrient for growth and proliferation. However, copper accumulation generates toxicity by multiple mechanisms, potentially leading to cell death. Due to its toxic nature at high concentrations, different chemical variants of copper have been extensively used as antifungal agents in agriculture and medicine. Most studies on copper homeostasis have been carried out in bacteria, yeast, and mammalian organisms. However, knowledge on filamentous fungi is less well documented. This review summarizes the knowledge gathered in the last few years about copper homeostasis in the filamentous fungi Aspergillus fumigatus and Aspergillus nidulans: The mechanism of action of copper, the uptake and detoxification systems, their regulation at the transcriptional level, and the role of copper homeostasis in fungal pathogenicity are presented.