Identification and Characterization of an Autophagy-Related Gene Acatg12 in Acremonium chrysogenum

The Autophagy-Related Gene CpAtg4 Is Required for Fungal Phenotypic Traits, Stress Tolerance, and Virulence in Cryphonectria parasitica

Autophagy is an evolutionarily ancient process wherein cells are able to break down intracellular contents to support normal physiology and development. Autophagosome formation is regulated by several different proteins, including the key cysteine protease Atg4. The contribution of Atg4 protein in the pathogenic fungus Cryphonectria parasitica, which causes blight in chestnut plants, has not been completely understood. In this context, we aimed to investigate the role of Atg4 during autophagy formation and their contribution to nonautophagic events in C. parasitica. By complementation assay, we determined that the CpAtg4 gene from C. parasitica was able to functionally complement the deletion of yeast Atg4. Using a yeast two-hybrid assay system, we confirmed that CpAtg4 and CpAtg8 directly interact with one another, and amino acids 377 to 409 of CpAtg4 were identified as being responsible for its binding with CpAtg8. The deletion mutant of CpAtg4 did not demonstrate positive monodansylcadaverine staining, which indicated that CpAtg4 is required for autophagy in C. parasitica. Moreover, the ΔCpAtg4 strain exhibited a decrease in aerial hyphae formation and sporulation, and reduction in virulence on apple and chestnut stem. The ΔCpAtg4 strains were also more sensitive to H₂O₂ and Congo red-induced stress. We further determined that amino acids 377 to 409 of CpAtg4 were essential for the function of CpAtg4 in vivo. Together, our findings indicated that CpAtg4 is required for the autophagy formation, fungal phenotypic traits, stress tolerance, and virulence in C. parasitica.

Roles of autophagy-related genes in conidiogenesis and blastospore formation, virulence, and stress response of Beauveria bassiana

Autophagy is a conserved intracellular recycling mechanism, in which autophagy-related genes 12 and 16 (ATG12 and ATG16) function in a complex controlling the ubiquitin-like conjugation system. In the insect-pathogenic fungus Beauveria bassiana, ATG12 and ATG16 were functionally characterized. Disruption of BbATG12 or BbATG16 resulted in the absence of autophagic bodies under starvation stress. ΔBbATG12 and ΔBbATG16 mutant strains displayed similar defects in asexual development (conidiation and blastospore formation) and tolerance to oxidative stress. ΔBbATG16 strain exhibited the impaired growth on the media with gelatin or chitin as a single nitrogen source, and ΔBbATG12 displayed decreased growth on the media with sucrose, fructose or maltose as a single carbon source. Both BbATG12 and BbATG16 were required for fungal virulence. BbATG16 mutation had more effects on fungal virulence than BbATG12 in topical infection assay, although both genes had similar contributions to fungal virulence in intrahemocoel injection assay. This study indicates that BbATG12 and BbATG16 mediate diverse biological functions in addition to their convergent roles in autophagy.

The critical role of plasma membrane H+-ATPase activity in cephalosporin C biosynthesis of Acremonium chrysogenum

The filamentous fungus Acremonium chrysogenum is the main industrial producer of cephalosporin C (CPC), one of the major precursors for manufacturing of cephalosporin antibiotics. The plasma membrane H⁺-ATPase (PMA) plays a key role in numerous fungal physiological processes. Previously we observed a decrease of PMA activity in A. chrysogenum overproducing strain RNCM 408D (HY) as compared to the level the wild-type strain A. chrysogenum ATCC 11550. Here we report the relationship between PMA activity and CPC biosynthesis in A. chrysogenum strains. The elevation of PMA activity in HY strain through overexpression of PMA1 from Saccharomyces cerevisiae, under the control of the constitutive gpdA promoter from Aspergillus nidulans, results in a 1.2 to 10-fold decrease in CPC production, shift in beta-lactam intermediates content, and is accompanied by the decrease in cef genes expression in the fermentation process; the characteristic colony morphology on agar media is also changed. The level of PMA activity in A. chrysogenum HY OE::PMA1 strains has been increased by 50–100%, up to the level observed in WT strain, and was interrelated with ATP consumption; the more PMA activity is elevated, the more ATP level is depleted. The reduced PMA activity in A. chrysogenum HY strain may be one of the selected events during classical strain improvement, aimed at elevating the ATP content available for CPC production.

Transport systems, intracellular traffic of intermediates and secretion of β-lactam antibiotics in fungi

Fungal secondary metabolites are synthesized by complex biosynthetic pathways catalized by enzymes located in different subcellular compartments, thus requiring traffic of precursors and intermediates between them. The β-lactam antibiotics penicillin and cephalosporin C serve as an excellent model to understand the molecular mechanisms that control the subcellular localization of secondary metabolites biosynthetic enzymes. Optimal functioning of the β-lactam biosynthetic enzymes relies on a sophisticated temporal and spatial organization of the enzymes, the intermediates and the final products. The first and second enzymes of the penicillin pathway, ACV synthetase and IPN synthase, in Penicillium chrysogenum and Aspergillus nidulans are cytosolic. In contrast, the last two enzymes of the penicillin pathway, phenylacetyl-CoA ligase and isopenicillin N acyltransferase, are located in peroxisomes working as a tandem at their optimal pH that coincides with the peroxisomes pH. Two MFS transporters, PenM and PaaT have been found to be involved in the import of the intermediates isopenicillin N and phenylacetic acid, respectively, into peroxisomes. Similar compartmentalization of intermediates occurs in Acremonium chrysogenum; two enzymes isopenicillin N-CoA ligase and isopenicillin N-CoA epimerase, that catalyse the conversion of isopenicillin N in penicillin N, are located in peroxisomes. Two genes encoding MFS transporters, cefP and cefM, are located in the early cephalosporin gene cluster. These transporters have been localized in peroxisomes by confocal fluorescence microscopy. A third gene of A. chrysogenum, cefT, encodes an MFS protein, located in the cell membrane involved in the secretion of cephalosporin C, although cefT-disrupted mutants are still able to export cephalosporin by redundant transporters. The secretion of penicillin from peroxisomes to the extracellular medium is still unclear. Attempts have been made to identify a gene encoding the penicillin secretion protein among the 48 ABC-transporters of P. chrysogenum. The highly efficient secretion system that exports penicillin against a concentration gradient may involve active penicillin extrusion systems mediated by vesicles that fuse to the cell membrane. However, there is no correlation of pexophagy with penicillin or cephalosporin formation since inactivation of pexophagy leads to increased penicillin or cephalosporin biosynthesis due to preservation of peroxisomes. The penicillin biosynthesis finding shows that in order to increase biosynthesis of novel secondary metabolites it is essential to adequately target enzymes to organelles.