Growth-Phase Sterigmatocystin Formation on Lactose Is Mediated via Low Specific Growth Rates in Aspergillus nidulans

Abstract

Seed contamination with polyketide mycotoxins such as sterigmatocystin (ST) produced by Aspergilli is a worldwide issue. The ST biosynthetic pathway is well-characterized in A. nidulans, but regulatory aspects related to the carbon source are still enigmatic. This is particularly true for lactose, inasmuch as some ST production mutant strains still synthesize ST on lactose but not on other carbon substrates. Here, kinetic data revealed that on d-glucose, ST forms only after the sugar is depleted from the medium, while on lactose, ST appears when most of the carbon source is still available. Biomass-specified ST production on lactose was significantly higher than on d-glucose, suggesting that ST formation may either be mediated by a carbon catabolite regulatory mechanism, or induced by low specific growth rates attainable on lactose. These hypotheses were tested by d-glucose limited chemostat-type continuous fermentations. No ST formed at a high growth rate, while a low growth rate led to the formation of 0.4 mg·L−1 ST. Similar results were obtained with a CreA mutant strain. We concluded that low specific growth rates may be the primary cause of mid-growth ST formation on lactose in A. nidulans, and that carbon utilization rates likely play a general regulatory role during biosynthesis.

Analysis of the Relationship between Alternative Respiration and Sterigmatocystin Formation in Aspergillus nidulans

“…In a previous publication [ 21 ], we demonstrated that ST formation only occurs at low specific growth rates. Consequently, in a medium with d -glucose as the growth substrate, ST production occurs only after carbon source exhaustion, during the late stationary phase of growth.…”

“…The velvet protein was present in the nuclei both in the light and in the darkness at the tested glucose concentrations between 1 and 2%. In our experience, using the medium and growth conditions set out in Németh et al [ 21 ], a velvet wild-type strain produced over twice as much ST when grown in the dark, regardless of whether the cultivation was on agar plates or in fermenters. Our results with the aodA mutant backgrounds imply that a basal level production under illumination (“stage 1”) is essentially unchanged regardless of the absence or presence of AOX, or its exact amount; that is, it seems irresponsive to the ratio of the alternative/total respiration.…”

“…However, in A. nidulans , ST is the stable end product of the corresponding secondary metabolism pathway [ 17 ], as it lacks the genes encoding the enzymes for the final two conversions ( aflP and aflQ ) from the AT gene cluster in A. flavus [ 18 , 19 ]. The biosynthesis of ST and AT shares similar regulatory mechanisms, including the two pathway-specific transcription factors AflR and AflJ (AflS), thus allowing the study of AT biosynthesis in A. nidulans without producing the potent mycotoxin itself [ 20 , 21 ]. The AT/ST biosynthetic pathway is well-characterized in A. nidulans [ 17 , 22 ], but many of the mechanisms by which environmental factors influence mycotoxin formation remain enigmatic [ 23 , 24 ].…”

Ethanol Production from Cheese Whey and Expired Milk by the Brown Rot Fungus Neolentinus lepideus

“…This indicated that among naturally occurring microorganisms, strain RS1911 had a relatively high ability to ferment lactose. Although lactase (β-galactosidase) from ascomycete fungi, such as the genus Aspergillus or Penicillium, are well known [15,[34][35][36][37], little information is available for basidiomycete fungi. Lactose does not exist in regular mushroom habitats in the first place, so it is presumed that lactase is not particularly necessary for growth.…”

Citric acid and itaconic acid accumulation: variations of the same story?

“…fumigatus (Fréalle et al 2013 ). A role of LaeA in oxidative stress response would be in agreement with its essential role in fungal sporulation (Bayram and Braus 2012 ), its necessity for extracellular enzyme production (Karimi-Aghcheh et al 2014 ), and requirement for secondary metabolite production (Németh et al 2016 ). Since both citric and itaconic acid require high oxygen tensions for their accumulation, they are very likely under oxidative stress, although this has never been experimentally investigated so far.…”