LysR-type transcriptional regulator of high temperature inducible class A β-lactamase gene controls pathogenic R-body production genes in Azorhizobium caulinodans

Abstract

ABSTRACT Azorhizobium caulinodans ORS571 is a rhizobium of the tropical legume Sesbania rostrata. This bacterium possesses the reb operon, which is associated with pathogenic R-bodies. To establish symbiosis with host plants, the expression of reb operon is suppressed at a low level by multiple regulators, such as PraR transcription factor and Lon protease. The reb operon is highly expressed in the presence of 2-oxoglutarate at temperatures lower than the optimum growth temperatures (37–38°C). 2-oxoglutarate has been found to inhibit the binding of PraR to the promoter region of the reb operon, but it is still unclear how temperature effects reb operon expression. Therefore, the purpose of our research is to clarify the temperature-depending expression mechanism. In this study, we found that PenR, which is a LysR-type transcriptional regulator of a class A β-lactamase gene (penA), is involved in reb operon expression. The reb operon of the praR mutant was highly expressed at 37°C, but not at 38°C, while the reb operon of the praR penR mutant was highly expressed even at 38°C. We also found that the expression of penA is temperature-responsive and under the control of Lon protease. penA of the wild type (WT) strain was more highly expressed at higher growth temperatures in the range of 25°C to 41°C, and the expression levels of penA in the lon mutant were lower than those in the WT strain at any growth temperatures. The penA of the penR mutant was hardly expressed irrespective of growth temperatures. Electrophoretic mobility shift assay, using His6-tagged PenR, revealed PenR bound to the promoter region of penA, but not to that of the reb operon. From these results, it was concluded that, in response to temperature, PenR represses the expression of the reb operon in an indirect manner, and induces penA in a direct manner, respectively.