Essential regulator gene tox R for toxoflavin biosynthesis of Burkholderia glumae

Abstract

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.