Yoshinori Shingu

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.

Essential regulator gene toxR for toxoflavin biosynthesis of Burkholderia glumae

Burkholderia glumae (synonym: Pseudomonas glumae) is the causal agent of rice grain rot and seedling rot. This bacterium produces toxoflavin as a virulence factor for disease elicitation. Toxoflavin biosynthesis is completed by the transfer of methyl groups with catalytic action of a methyltransferase that is encoded by the toxA gene. In this study, we identified a 900-bp nucleotide sequence as a candidate gene to regulate the toxA gene. It was located upstream of the toxA gene. This novel regulatory element was named the toxR gene. When the toxR gene of B. glumae was disrupted by homologous recombination, one mutant (MY411) lost the ability to produce toxoflavin and to elicit the disease in rice seedlings. In addition, the expression of toxA mRNA was not detected by the reverse transcription-polymerase chain reaction, suggesting that the toxR gene is responsible for transcription of the toxA gene. The amino acid sequence deduced from the toxR gene was highly homologous to the LysR family transcriptional activators in some prokaryotes. Its amino-terminal had a helix-turn-helix DNA-binding motif to bind to a T-N11-A sequence motif of the toxA promoter. These results indicated that the toxR gene encoded the activator protein to promote transcription of the toxA gene and conceivably of downstream toxoflavin biosynthesis genes.