Summarized loline-alkaloid biosynthetic pathway.

<p>Labeled arrows are for steps that contribute to diversity of the lolines. Presence or absence of functional copies of <i>lolO, lolN, lolM</i>, or <i>lolP</i>, or a plant acetyltransferase activity, determine which alkaloids accumulate in the symbiotic plant as the pathway end-products.</p

Replacement of <i>lolN</i> and <i>lolM</i> with <i>hph</i> maker gene.

<p>(A) Schematic representation of <i>lolN</i>-<i>lolM</i> replacement by the <i>hph</i> marker gene via homologous recombination. Shown are maps of the wild-type <i>lolN</i> and <i>lolM</i> in <i>Epichloë festucae</i> E2368 (WT), targeting vector (pKAES323), and the locus after homologous recombination (KO). Black bars represent DNA sequence, and filled arrows represent genes. Bent blue lines on the bars represent <i>Hin</i>dIII digestion sites. Colored arrowheads represent primers used to generate pKAES323 and to screen the transformants. (B) Southern-blot analysis of <i>E. festucae</i> strains. Wild-type E2368 and transformants were probed with a <i>lolN</i> fragment or <i>lolM</i> gene amplified from E2368 (old probe was stripped off the membrane before new hybridization). Lanes contained <i>Hin</i>dIII-digested genomic DNA from E2368 (WT), <i>lolN</i>-<i>lolM</i> knockout transformant (KO), ectopic transformant of E2368 with pKAES323 (Ect), and E2368 transformed with the empty vector pKAES173 (WT+vec).</p

Fungal isolates in this study.

<p>Fungal isolates in this study.</p

Partial LolN amino-acid sequence alignment of <i>Epichloë coenophiala</i> e4309 and <i>N</i>-formylloline (NFL) producers.

<p>Red-framed sequences are three different <i>E. coenophiala</i> isolates. <i>Ecoe</i>  =  <i>Epichloë coenophiala</i>, <i>Eaot</i>  =  <i>Epichloë aotearoae</i>, <i>Efes</i>  =  <i>Epichloë festucae</i>, <i>Echis</i>  =  <i>Epichloë chisosa</i>, <i>Esig</i>  =  <i>Epichloë siegelii</i>, <i>Eunci</i>  =  <i>Epichloë uncinata</i>.</p

Loline alkaloid profiles and <i>LOL-</i>gene screening results for endophyte isolates. ^a

a<p>Abbreviations are: tr  =  trace amount; +  =  alkaloid detected or full gene present; −  =  alkaloid not detected or gene not present; Ψ  =  pseudogene; nd  =  gene not detected in PCR screen.</p>b<p>Two nonsynonymous mutations found in otherwise conserved sites G495D and E551K.</p><p>Loline alkaloid profiles and <i>LOL-</i>gene screening results for endophyte isolates. ^{<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115590#nt101" target="_blank">a</a>}</p

GC-MS traces showing loline-alkaloid profiles of meadow fescue symbiotic with different <i>E. festucae</i> strains.

<p>(A) The <i>lolN</i>-<i>lolM</i> knockout (KO), (B) an empty-vector control transformant (WT+vec), and (C and D) complementation strains (KO+<i>lolN</i>+<i>lolM</i>). The numbers after complementation strains represent different meadow fescue plants inoculated with independent transformants. (E) Proposed roles of LolN and LolM (this work), and reported role of LolP <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115590#pone.0115590-Spiering2" target="_blank">[14]</a>, in the biosynthetic pathway from <i>N</i>-acetylnorloline (NANL) to the final product, <i>N</i>-formylloline (NFL).</p

Enzymes from Fungal and Plant Origin Required for Chemical Diversification of Insecticidal Loline Alkaloids in Grass<i>-Epichloë</i> Symbiota

<div><p>The lolines are a class of bioprotective alkaloids that are produced by <i>Epichloë</i> species, fungal endophytes of grasses. These alkaloids are saturated 1-aminopyrrolizidines with a C2 to C7 ether bridge, and are structurally differentiated by the various modifications of the 1-amino group: -NH₂ (norloline), -NHCH₃ (loline), -N(CH₃)₂ (<i>N</i>-methylloline), -N(CH₃)Ac (<i>N</i>-acetylloline), -NHAc (<i>N</i>-acetylnorloline), and -N(CH₃)CHO (<i>N</i>-formylloline). Other than the LolP cytochrome P450, which is required for conversion of <i>N</i>-methylloline to <i>N</i>-formylloline, the enzymatic steps for loline diversification have not yet been established. Through isotopic labeling, we determined that <i>N</i>-acetylnorloline is the first fully cyclized loline alkaloid, implying that deacetylation, methylation, and acetylation steps are all involved in loline alkaloid diversification. Two genes of the loline alkaloid biosynthesis (<i>LOL</i>) gene cluster, <i>lolN</i> and <i>lolM</i>, were predicted to encode an <i>N-</i>acetamidase (deacetylase) and a methyltransferase, respectively. A knockout strain lacking both <i>lolN</i> and <i>lolM</i> stopped the biosynthesis at <i>N</i>-acetylnorloline, and complementation with the two wild-type genes restored production of <i>N</i>-formylloline and <i>N</i>-acetylloline. These results indicated that <i>lolN</i> and <i>lolM</i> are required in the steps from <i>N</i>-acetylnorloline to other lolines. The function of LolM as an <i>N</i>-methyltransferase was confirmed by its heterologous expression in yeast resulting in conversion of norloline to loline, and of loline to <i>N</i>-methylloline. One of the more abundant lolines, <i>N</i>-acetylloline, was observed in some but not all plants with symbiotic <i>Epichloë siegelii</i>, and when provided with exogenous loline, asymbiotic meadow fescue (<i>Lolium pratense</i>) plants produced <i>N</i>-acetylloline, suggesting that a plant acetyltransferase catalyzes <i>N</i>-acetylloline formation. We conclude that although most loline alkaloid biosynthesis reactions are catalyzed by fungal enzymes, both fungal and plant enzymes are responsible for the chemical diversification steps <i>in symbio</i>.</p></div

Assay of LolM methyltransferase activity.

<p>(A) Chromatogram of loline alkaloids from incubation of norloline and AdoMet with protein extract of yeast transformed with empty vector. (B) Chromatogram of loline alkaloids from incubation of norloline and AdoMet with crude protein extract from yeast that expresses LolM. (C) Chromatogram of loline alkaloids from incubation of loline and AdoMet with protein extract of yeast transformed with empty vector. (D) Chromatogram of loline alkaloids from incubation of loline and AdoMet with crude protein extract from yeast that expresses LolM. (E) Proposed scheme of loline and <i>N</i>-methylloline (NML) formation from norloline. AdoHcy  =  <i>S</i>-adenosyl homocysteine.</p

Enrichment of deuterated <i>N</i>-acetylnorloline (NANL) from application of tetradeuterated <i>exo</i>-1-acetemidopyrrolizidine ([²H₄]AcAP) to loline alkaloid producing culture.

<p>Shown are (A) GC-MS total ion chromatogram, (B) mass spectrum at retention time 13.948 and (C) 13.921 min, and (D) proposed scheme of [²H₄]NANL formation from [²H₄]AcAP.</p

Structures of common loline alkaloids.

<p>Substitutions on the nitrogen at C1 differentiate the lolines.</p