The Sinorhizobium meliloti LysR family transcriptional factor LsrB is involved in regulation of glutathione biosynthesis

Glutathione, a key antioxidant in Sinorhizobium meliloli, is required for the development of alfalfa （Medicago sativa） nitro- gen-fixing nodules. This tripeptide can be synthesized by both γ-glutamyl cysteine synthetase （GshA） and glutathione synthe- tase （GshB） in Escherichia coil and S. meliloti. Genetic evidence has indicated that the null mutant of S. meliloti gshA or gshB1 does not establish efficient symbiosis on alfalfa. However, the transcriptional regulation ofgshA and gshB has not been well understood. Here, S. meliloti LsrB, a member of LysR family transcriptional factors, was found to positively regulate glutathione biosynthesis by activating the transcrip- tion of gshA and gshB1 under both free-living and symbiotic conditions. The decrease in glutathione production in the lsrB in-frame deletion mutant （lsrB1-2） was determined by using quadrupole time-of-flight liquid chromatography-mass spec- trometry. The expression ofgshA and gshB1 was correspond- ingly reduced in the mutant under free-living and symbiotic conditions by analyses of real-time quantitative reverse transcription-polymerase chain reaction and promoter-GUS fusions. Interestingly, LsrB positively regulated the transcrip- tion of oxyR, which encodes another member of LysR family regulators and responds to oxidative stresses in S. meliloti. The oxyR null mutant produced less glutathione, in which the transcription of gshA was consistently down-regulated. These findings demonstrate that glutathione biosynthesis is positively regulated by both LsrB and OxyR in S. meliloti.