Mechanism of resistance to the antibiotic trichothecin in the producing fungi

Suppressed recombination and unique candidate genes in the divergent haplotype encoding Fhb1, a major Fusarium head blight resistance locus in wheat

Fine mapping and sequencing revealed 28 genes in the non-recombining haplotype containing Fhb1 . Of these, only a GDSL lipase gene shows a pathogen-dependent expression pattern. Fhb1 is a prominent Fusarium head blight resistance locus of wheat, which has been successfully introgressed in adapted breeding material, where it confers a significant increase in overall resistance to the causal pathogen Fusarium graminearum and the fungal virulence factor and mycotoxin deoxynivalenol. The Fhb1 region has been resolved for the susceptible wheat reference genotype Chinese Spring, yet the causal gene itself has not been identified in resistant cultivars. Here, we report the establishment of a 1 Mb contig embracing Fhb1 in the donor line CM-82036. Sequencing revealed that the region of Fhb1 deviates from the Chinese Spring reference in DNA size and gene content, which explains the repressed recombination at the locus in the performed fine mapping. Differences in genes expression between near-isogenic lines segregating for Fhb1 challenged with F. graminearum or treated with mock were investigated in a time-course experiment by RNA sequencing. Several candidate genes were identified, including a pathogen-responsive GDSL lipase absent in susceptible lines. The sequence of the Fhb1 region, the resulting list of candidate genes, and near-diagnostic KASP markers for Fhb1 constitute a valuable resource for breeding and further studies aiming to identify the gene(s) responsible for F. graminearum and deoxynivalenol resistance.

Avoidance of suicide in antibiotic-producing microbes

Many microbes synthesize potentially autotoxic antibiotics, mainly as secondary metabolites, against which they need to protect themselves. This is done in various ways, ranging from target-based strategies (i.e. modification of normal drug receptors or de novo synthesis of the latter in drug-resistant form) to the adoption of metabolic shielding and/or efflux strategies that prevent drug-target interactions. These self-defence mechanisms have been studied most intensively in antibiotic-producing prokaryotes, of which the most prolific are the actinomycetes. Only a few documented examples pertain to lower eukaryotes while higher organisms have hardly been addressed in this context. Thus, many plant alkaloids, variously described as herbivore repellents or nitrogen excretion devices, are truly antibiotics-even if toxic to humans. As just one example, bulbs of Narcissus spp. (including the King Alfred daffodil) accumulate narciclasine that binds to the larger subunit of the eukaryotic ribosome and inhibits peptide bond formation. However, ribosomes in the Amaryllidaceae have not been tested for possible resistance to narciclasine and other alkaloids. Clearly, the prevalence of suicide avoidance is likely to extend well beyond the remit of the present article.

Release and regeneration of protoplasts from the fungus Trichothecium roseum

A protocol for isolating and regenerating protoplasts from Trichothecium roseum has been described. Protoplasts from T. roseum were isolated using (i) a lytic enzyme combination composed of Novozym 234, chitinase, cellulase, and pectinase at a 5-mg/mL concentration and (ii) 0.6 M KCl as an osmotic stabilizer. A maximum number of 28 x 10(4) protoplasts/mL were obtained at pH 5.5. Experiments on the regeneration and reversion of protoplasts revealed a maximum regeneration (60.8%) in complete medium (potato dextrose--yeast extract agar) amended with 0.6 M KCl. The regenerated protoplasts were similar to the original parent strain in morphology, pigmentation, growth, and sporulation.

The SCR1 gene from Schwanniomyces occidentalis encodes a highly hydrophobic polypeptide, which confers ribosomal resistance to cycloheximide

In Saccharomyces cerevisiae, the SCR1 gene from Schwanniomyces occidentalis is known to induce ribosomal resistance to cycloheximide (cyh). A 2.8 kb DNA fragment encoding this gene was sequenced. Its EMBL Accession No. is AJ419770. It disclosed a putative tRNA(Asn) (GUU) sequence located downstream of an open reading frame (ORF) of 1641 nucleotides. This ORF was shown to correspond to SCR1. It would encode a highly hydrophobic polypeptide (SCR1) with 12 transmembrane domains. SCR1 is highly similar to a variety of yeast proteins of the multidrug-resistance (MDR) family. However, SCR1 only conferred resistance to cyh but not to benomyl or methotrexate. The cyh-resistance phenotype induced by SCR1 was confirmed in several S. cerevisiae strains that expressed this gene to reside at the ribosomal level. In contrast, a beta-galacosidase-tagged SCR1 was found to be integrated in the endoplasmic reticulum (ER). It is proposed that the ribosomes of yeast cells expressing SCR1 undergo a conformational change during their interaction with the ER, which lowers their affinity for cyh-binding. If so, these findings would disclose a novel ribosomal resistance mechanism.

The mystery of the trichothecene 3-O-acetyltransferase gene. Analysis of the region around Tri101 and characterization of its homologue from Fusarium sporotrichioides

The trichothecene 3-O-acetyltransferase gene, Tri101, plays a pivotal role for the well-being of the type B trichothecene producer Fusarium graminearum. We have analyzed the cosmids containing Tri101 and found that this resistance gene is not in the biosynthetic gene cluster reported so far. It was located between the UTP-ammonia ligase gene and the phosphate permease gene which are not related to trichothecene biosynthesis. These two 'house-keeping' genes were also linked in Fusarium species that do not produce trichothecenes. The result suggests that the isolated occurrence of Tri101 is attributed to horizontal gene transfer and not to the reciprocal translocation of the chromosome containing the gene cluster. Interestingly, 3-O-acetylation was not always a primary self-defensive strategy for all the t-type trichothecene producers; i.e. the type A trichothecene producer Fusarium sporotrichioides did not acetylate T-2 toxin in vivo although the fungus possessed a functional 3-O-acetyltransferase gene. Thus Tri101 appears to be a defense option which the producers have independently acquired in addition to their original resistance mechanisms.

Trichothecene 3-O-acetyltransferase protects both the producing organism and transformed yeast from related mycotoxins. Cloning and characterization of Tri101

Trichothecene mycotoxins such as deoxynivalenol, 4,15-diacetoxyscirpenol, and T-2 toxin, are potent protein synthesis inhibitors for eukaryotic organisms. The 3-O-acetyl derivatives of these toxins were shown to reduce their in vitro activity significantly as assessed by assays using a rabbit reticulocyte translation system. The results suggested that the introduction of an O-acetyl group at the C-3 position in the biosynthetic pathway works as a resistance mechanism for Fusarium species that produce t-type trichothecenes (trichothecenes synthesized via the precursor trichotriol). A gene responsible for the 3-O-acetylation reaction, Tri101, has been successfully cloned from a Fusarium graminearum cDNA library that was designed to be expressed in Schizosaccharomyces pombe. Fission yeast transformants were selected for their ability to grow in the presence of T-2 toxin, and this strategy allowed isolation of 25 resistant clones, all of which contained a cDNA for Tri101. This is the first drug-inactivating O-acetyltransferase gene derived from antibiotic-producing organisms. The open reading frame of Tri101 codes for a polypeptide of 451 amino acid residues, which shows no similarity to any other proteins reported so far. TRI101 from recombinant Escherichia coli catalyzes O-acetylation of the trichothecene ring specifically at the C-3 position in an acetyl-CoA-dependent manner. By using the Tri101 cDNA as a probe, two least overlapping cosmid clones that cover a region of 70 kilobase pairs have been isolated from the genome of F. graminearum. Other trichothecene biosynthetic genes, Tri4, Tri5, and Tri6, were not clustered in the region covered by these cosmid clones. These new cosmid clones are considered to be located in other parts of the large biosynthetic gene cluster and might be useful for the study of trichothecene biosynthesis.

The TRI11 gene of Fusarium sporotrichioides encodes a cytochrome P-450 monooxygenase required for C-15 hydroxylation in trichothecene biosynthesis

Several genes in the trichothecene biosynthetic pathway of Fusarium sporotrichioides have been shown to reside in a gene cluster. Sequence analysis of a cloned DNA fragment located 3.8 kb downstream from TRI5 has led to the identification of the TRI11 gene. The nucleotide sequence of TRI11 predicts a polypeptide of 492 residues (Mr = 55,579) with significant similarity to members of the cytochrome P-450 superfamily. TRI11 is most similar to several fungal cytochromes P-450 (23 to 27% identity) but is sufficiently distinct to define a new cytochrome P-450 gene family, designated CYP65A1. Disruption of TRI11 results in an altered trichothecene production phenotype characterized by the accumulation of isotrichodermin, a trichothecene pathway intermediate. The evidence suggests that TRI11 encodes a C-15 hydroxylase involved in trichothecene biosynthesis.

Brazilian Baccharis toxins: livestock poisoning and the isolation of macrocyclic trichothecene glucosides

Samples of the toxic Brazilian plant, Baccharis coridifolia, which is responsible for numerous cases of livestock poisoning in southern Brazil and Argentina, were collected during the growing season, and the toxicities in calves of the plant materials were correlated with the levels of macrocyclic trichothecenes present. Female plants in flower were considerably more toxic than male plants or plants not in flower. Plants not in flower were of intermediate toxicity. The female plants in flower typically contained 5-10 times the levels of toxins as were found in the male plants. In addition, six new glucosides of the macrocyclic trichothecenes were isolated and characterized. The most prominent glucosides, those of roridins A and E, were found in high levels in the female plants.