Abstract
Several fungi in two different families––the Clavicipitaceae and the Trichocomaceae––produce different profiles of ergot alkaloids, many of which are important in agriculture and medicine. All ergot alkaloid producers share early steps before their pathways diverge to produce different end products. EasA, an oxidoreductase of the old yellow enzyme class, has alternate activities in different fungi resulting in branching of the pathway. Enzymes beyond the branch point differ among lineages. In the Clavicipitaceae, diversity is generated by the presence or absence and activities of lysergyl peptide synthetases, which interact to make lysergic acid amides and ergopeptines. The range of ergopeptines in a fungus may be controlled by the presence of multiple peptide synthetases as well as by the specificity of individual peptide synthetase domains. In the Trichocomaceae, diversity is generated by the presence or absence of the prenyl transferase encoded by easL (also called fgaPT1). Moreover, relaxed specificity of EasL appears to contribute to ergot alkaloid diversification. The profile of ergot alkaloids observed within a fungus also is affected by a delayed flux of intermediates through the pathway, which results in an accumulation of intermediates or early pathway byproducts to concentrations comparable to that of the pathway end product.
Highlights
Ergot alkaloids are secondary metabolites synthesized by members of the Clavicipitaceae and Trichocomaceae and have been both harmful and beneficial to humans for thousands of years [1]
Based on research with purified EasA of N. fumigata (EasAreductase) and on a wealth of basic studies of other old yellow enzymes [24,25], we propose that the product of the easA allele found in N. fumigata reduces the 8,9-double bond in chanoclavine-I aldehyde in two steps
Claviceps gigantea terminates its pathway at dihydrolysergol, the first oxidation product of festuclavine [52], whereas C. africana completes the oxidation of festuclavine to dihydrolysergic acid and incorporates dihydrolysergic acid exclusively into the dihydroergopeptine dihydroergosine [53,54]
Summary
Ergot alkaloids are secondary metabolites synthesized by members of the Clavicipitaceae and Trichocomaceae and have been both harmful and beneficial to humans for thousands of years [1]. It is estimated that hundreds of millions of dollars of losses to the livestock industry are incurred annually by the accumulation of ergot alkaloids in forages [7]. Members of this diverse class of alkaloids share an ergoline nucleus, which is modified in lineage-specific alkaloid profiles. Despite the Clavicipitaceae and Trichocomaceae being distantly related families, early enzymatic steps of ergot alkaloid biosynthesis are conserved (Figure 1); after the pathways diverge, lineage-specific steps give rise to different profiles of ergot alkaloids in fungi from these two families. The focus of this review is to describe points of diversification within the pathway leading to distinct profiles of ergot alkaloids among and within species of fungi
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