The quorum sensing-dependent gene katG of Burkholderia glumae is important for protection from visible light

Abstract

Quorum sensing (QS) plays important roles in the pathogenicity of Burkholderia glumae, the causative agent of bacterial rice grain rot. We determined how QS is involved in catalase expression in B. glumae. The QS-defective mutant of B. glumae exhibited less catalase activity than wild-type B. glumae. A beta-glucuronidase assay of a katG::Tn3-gusA78 reporter fusion protein revealed that katG expression is under the control of QS. Furthermore, katG expression was upregulated by QsmR, a transcriptional activator for flagellar-gene expression that is regulated by QS. A gel mobility shift assay confirmed that QsmR directly activates katG expression. The katG mutant produced toxoflavin but exhibited less severe disease than BGR1 on rice panicles. Under visible light conditions and a photon flux density of 61.6 micromol(-1) m(-2), the survival rate of the katG mutant was 10(5)-fold lower than that of BGR1. This suggests that KatG is a major catalase that protects bacterial cells from visible light, which probably results in less severe disease caused by the katG mutant.

FIG. 1.

(A) Catalase assay based on the production of air bubbles from BGR1, BGS2 (tofI::Ω), and BGS9 (qsmR::Ω) cells with hydrogen peroxide. (B) Catalase activity staining. The arrowhead indicates a band that disappeared from the katG mutant. Lanes: 1, BGR1; 2, BGS2; 3, BGS2 with 1 μM C₈-HSL; 4, BGS9; 5, BGC2 (katG::Tn3-gusA); 6, BGC2(pHJ4). (C) Western blot analysis. Lanes: 1, BGR1; 2, BGS2; 3, BGS9; 4, BGC2; 5, BGC2(pHJ4).

FIG. 2.

QsmR activates katG expression. Expression of katG was reduced in the tofI mutant strain BGS2 and the qsmR mutant strain BGS9, but it recovered to wild-type levels after the addition of 1 μM C₈-HSL to the BGS2 culture. Values are means and standard deviations from triplicate experiments.

FIG. 3.

Gel mobility shift assays using purified QsmR-His and a DNA fragment containing the katG promoter region.

FIG. 4.

Survival rates of BGR1, BGS2 (tofI::Ω), BGS9 (qsmR::Ω), BGS6 (toxR′ toxA′::Ω), BGC2 (katG::Tn3-gusA78), S6C2 (toxR′ toxA′::Ω and katG::Tn3-gusA78), and BGC2(pHJ4) to visible light. Values are means and standard deviations from triplicate experiments.

FIG. 5.

(A) Pathogenicity assay of BGR1, BGC2 (katG::Tn3-gusA78), BGC2(pHJ4), and BGS2 (tofI::Ω). The photographs were taken 7 days after inoculation. (B) Distribution patterns of disease severity for each treatment.