A characteristic of seed-borne clavicipitaceous symbionts (endophytes) that mediates their mutualisms with host grasses is production of anti-herbivore metabolites. Ergot alkaloids and indolediterpenes are potent neurotoxins in vertebrates ; saturated 1-aminopyrrolizidines (lolines) and the pyrrolopyrazine alkaloid, peramine, are active against insects. All except lolines are reported to be produced in cultures of fungal endophytes free of plant material.We identified lolines in defined-medium cultures of Neotyphodium uncinatum.We have also developed Epichloe festucae and Epichloe typhina as Mendelian genetic models to test the effects on aphids of lolines and peramine, respectively. In each case, the phenotypic difference of expression or non-expression was apparently governed by a single locus. Genotypes of E. typhina expressing peramine caused killing of greenbug aphid (Schizaphis graminum) on the host plants.Lolines were associated with killing of both greenbug and bird-cherry oat aphid (Rhopalosiphum padi). Statistically, the anti-aphid activities of the endophytes were entirely attributable to their alkaloids.Recent progress on genetic control of ergot alkaloid and indolediterpene expression holds promise for analogous tests for roles of these alkaloids in host benefits. Mendelian segregation and molecular knockouts can be used eventually to test the ecological importance of all known endophyte alkaloids in the many established endophyte effects, including increased drought tolerance, competitiveness, resistance to nematodes, and resistance to vertebrate and insect herbivores.Genetic knockouts in endophytes of genes for anti-vertebrate alkaloids will likely become an integral part of forage cultivar development.
GlnR activates nitrogen metabolism genes under nitrogen-limited conditions whereas MtrA represses these genes under nutrient-rich conditions in Streptomyces. In this study, we compared the transcription patterns of nitrogen metabolism genes in a double deletion mutant (ΔmtrA-glnR) lacking both mtrA and glnR and in mutants lacking either mtrA (ΔmtrA) or glnR (ΔglnR). The nitrogen metabolism genes were expressed similarly in ΔmtrA-glnR and ΔglnR under both nitrogen-limited and nutrient-rich conditions, with patterns distinctly different from that of ΔmtrA, suggesting a decisive role for GlnR in the control of nitrogen metabolism genes and further suggesting that regulation of these genes by MtrA is GlnR-dependent. MtrA and GlnR utilize the same binding sites upstream of nitrogen metabolism genes, and we showed stronger in vivo binding of MtrA to these sites under nutrient-rich conditions and of GlnR under nitrogen-limited conditions, consistent with the higher levels of MtrA or GlnR under those respective conditions. In addition, we showed that both mtrA and glnR are auto-regulatory. Our study provides new insights into the regulation of nitrogen metabolism genes in Streptomyces.
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