Stress-related Transcription Factor AtfB Integrates Secondary Metabolism with Oxidative Stress Response in Aspergilli

Abstract

In filamentous fungi, several lines of experimental evidence indicate that secondary metabolism is triggered by oxidative stress; however, the functional and molecular mechanisms that mediate this association are unclear. The basic leucine zipper (bZIP) transcription factor AtfB, a member of the bZIP/CREB family, helps regulate conidial tolerance to oxidative stress. In this work, we investigated the role of AtfB in the connection between oxidative stress response and secondary metabolism in the filamentous fungus Aspergillus parasiticus. This well characterized model organism synthesizes the secondary metabolite and carcinogen aflatoxin. Cellular response to oxidative stress in vertebrates, plants, and fungi is of fundamental importance; it enables the cell to survive a variety of extra-and intracellular oxidative stressors. An uncontrolled increase in reactive oxygen species (ROS) 4 in mammalian cells is associated with various pathological conditions such as inflammation and cardiovascular and neurodegenerative disorders, including hypertension, atherosclerosis, Parkinson disease, and Alzheimer disease; oxidative stress is also linked to premature aging and cancer (1-10). A detailed understanding of the regulatory network that coordinates the cellular response to oxidative stress will enable better control over its detrimental impacts on humans.As a part of the response to oxidative stress, transcription factors activated directly or indirectly by ROS bind to the promoters of specific genes that trigger defense and signaling related activities. In mammalian cells, Drosophila, Caenorhabditis elegans, and yeast, response to oxidative stress is mediated by an evolutionarily conserved bZIP transcription factor Nrf2 that binds as a heterodimer with Maf or ATF4 to antioxidantresponse elements in the promoters of more than 200 mammalian genes (11-16). Signaling pathways that involve PKC, PI3K, and MAPK participate in Nrf2 activation under ROS exposure. In Arabidopsis, 175 genes were demonstrated to be regulated by hydrogen peroxide, including genes with MYB and AP-1-response elements (17). 140 core stress-related genes were identified in Schizosaccharomyces pombe (18,19).Filamentous fungi in the genus Aspergillus must cope with ROS during their growth and development. Genetic and biochemical studies shed light on the role of ROS in fungal defense, pathogenicity, and development and suggest that fungi use similar stress response pathways as mammalian and plant cells (20 -24). The transcription factors AP-1, AtfA, and AtfB (all members of the basic leucine zipper (bZIP) transcription factor family) have been identified as major players in providing conidia with resistance to oxidative stress in aspergilli. Saccharomyces cerevisiae yap-1 is an ortholog of mammalian AP-1 (25-27). A yap-1 ortholog in Aspergillus parasiticus (ApyapA) is reported to regulate the timing of ROS accumulation, conidiospore development, and stress tolerance in conidiospores (26,27). AtfA, an ortholog of S. pombe Atf1, controls conidial respon...

VeA Is Associated with the Response to Oxidative Stress in the Aflatoxin Producer Aspergillus flavus

“…These results were also verified by semiquantitative PCR (data not shown). Analysis of the effect of veA on the expression of atfB, encoding a transcription factor known to bind to the promoter regions of oxidative stress response genes (29,33), was done separately. Our results indicated that veA is also necessary for the increase in the expression of atfB in the presence of hydrogen peroxide over time (see Fig.…”

“…Polyclonal anti-AtfB antibodies (YSR) were used to block formation of DNA-protein complexes (shift inhibition EMSA) as described previously (29). Five micrograms of nuclear protein extracts was incubated in a DNA binding buffer for 15 min at 30°C with 5 l of anti-AtfB antibody (5.5 g/l), or preimmune serum obtained from the same rabbit as the antibody was generated.…”

bZIP transcription factors affecting secondary metabolism, sexual development and stress responses in Aspergillus nidulans

“…bZIP-type transcription factors may regulate secondary metabolite production, either directly by binding at promoters of genes encoding key elements of the biosynthetic machinery, as shown for RsmA and other bZIPs (Roze et al, 2011;Yin et al, 2012), or via modulating intracellular ROS levels through fine-tuning the activity of the antioxidative defence system, as demonstrated for NapA orthologues in A. ochraceus and A. parasiticus (Reverberi et al, 2007(Reverberi et al, , 2008(Reverberi et al, , 2012. This theory has also been supported by observations that the inhibition of oxidation processes including lipid peroxidation hinders biosynthesis of mycotoxins, including aflatoxin (Jayashree & Subramanyam, 1999, 2000, and the addition of lipid peroxidationstimulating agents to fungal cultures increases the yields of certain metabolites (e.g.…”

Aspergillus parasiticus SU-1 Genome Sequence, Predicted Chromosome Structure, and Comparative Gene Expression under Aflatoxin-Inducing Conditions: Evidence that Differential Expression Contributes to Species Phenotype

“…YES liquid medium (2% yeast extract, 6% sucrose [pH 5.8]) (6) was used as an aflatoxin-inducing rich growth medium, and YEP liquid medium (2% yeast extract, 6% peptone [pH 5.8]) (7) was used as an aflatoxin-noninducing medium. An aliquot of frozen conidiospores from each strain was used to inoculate 100 ml of these growth media (250-ml Erlenmeyer flask carrying 5-or 6-mm glass beads) at 10 4 spores per ml, and these cultures were incubated for 40 h in the dark at 30°C with shaking (150 rpm) as described previously (standard growth conditions) (7).…”