Random Mutagenesis of Filamentous Fungi Strains for High-Yield Production of Secondary Metabolites: The Role of Polyamines

Abstract

A filamentous fungus (also called molds or moldy fungus) is a taxonomically diverse organism from phylum Zygomycota and Ascomycota with filamentous hyphae and has the ability to produce airborne spores or conidia. Currently, more than 70,000 molds are known, and some of them contain unique and unusual biochemical pathways. A number of products from such pathways, especially, the secondary metabolite (SM) pathways are used as important pharmaceuticals, including antibiotics, statins, and immunodepresants. Under different conditions, the individual species can produce more than 100 SM. The strain improvement programs lead to high yielding in target SM and significant reduction of spin-off products. The main tool for the strain improvement of filamentous fungi is random mutagenesis and screening. The majority of industrial overproducing SM strains were developed with the help of such technique over the past 50–70 years; the yield of the target SM increased by 100- to 1000-fold or more. Moreover, most of the strains have reached their technological limit of improvement. A new round of mutagenesis has not increased overproduction. Recently, it was shown that that the addition of exogenous polyamines may increase the production of such improved strains of filamentous fungi. The possible molecular mechanism of this phenomenon and its biotechnological applications are discussed.

High-Yielding Lovastatin Producer Aspergillus terreus Shows Increased Resistance to Inhibitors of Polyamine Biosynthesis

“…The shift in PAs content in filamentous fungi strains obtained as a result of CSI programs, has recently been proposed [16,21]. As a result of multi-round random mutagenesis and screening, useful mutations are selected, leading to an increase in the yield of the target SM [16].…”

“…The inhibition was assessed quantitatively by comparing the colony growth. ODC is the first and rate-limiting enzyme of PAs de novo synthesis in filamentous fungi; in this regard, inhibition of this enzyme on a synthetic medium without PAs can lead to a significant or complete loss of growth of fungal cells [16,21]. Inhibitory analysis was performed using DFMO, an irreversible suicide inhibitor of ODC, recently studied versus ODC from A. terreus [27], and APA, isosteric hydroxylamine-containing analogue of Put, showing more potent ODC inhibition than DFMO in some filamentous fungi [21,28].…”

“…This may be due to the fact that CSI programs operate with random mutations and, during strain improvement, concomitant changes can be accumulated that negatively affect strain viability [13][14][15]. As a result, in the process of multi-round mutagenesis, a stage begins when the next mutagenic effect no longer leads to a further increase in the production of the target SM [16]. This final stage usually occurs after 20-50 rounds of mutagenesis, and determines the technological limit of the method, in relation to a particular strain [9,[16][17][18].…”

“…As a result, in the process of multi-round mutagenesis, a stage begins when the next mutagenic effect no longer leads to a further increase in the production of the target SM [16]. This final stage usually occurs after 20-50 rounds of mutagenesis, and determines the technological limit of the method, in relation to a particular strain [9,[16][17][18].…”

“…The improved strain A. chrysogenum strain also demonstrated the increased resistance to inhibitors of ornithine decarboxylase (ODC; EC 4.1.1.17), an enzyme of the key stage of PAs biosynthesis [21]. The question of whether this effect is unique or reflects universal trend arising from CSI of filamentous fungi has recently been discussed [16].…”