Role of oxyR from Sinorhizobium meliloti in Regulating the Expression of Catalases

The process of symbiotic nitrogen fixation results in the generation of reactive oxygen speciessuch as the superoxide anion(O~-)and hydrogen peroxide(HO).The response of rhizobia to these toxicoxygen species is an important factor in nodulation and nitrogen fixation.In Sinorhizobium meliloti,oneoxyR homologue and three catalase genes,katA,katB,and katC were detected by sequence analysis.ThisoxyR gene is located next to and divergently from katA on the chromosome.To investigate the possible rolesof oxyR in regulating the expression of catalases at the transcriptional level in S.meliloti,an insertion mutantof this gene was constructed.The mutant was more sensitive and less adaptive to HOthan the wild typestrain,and total catalase/peroxidase activity was reduced approximately fourfold with the OxyR mutationrelative to controls.The activities of KatA and KatB and the expression of katA::lacZ and katB::lacZ promoterfusions were increased in the mutant strain compared with the parental strain grown in the absence of HO,indicating that katA and katB are repressed by OxyR.However,when exposed to HO,katA expressionwas also increased in both S.meliloti and Escherichia coli.When exposed to HO,OxyR is convertedfrom a reduced to an oxidized form in E.coli.We concluded that the reduced form of OxyR functions as arepressor of katA and katB expression.Thus,in the presence of HO,reduced OxyR is converted to theoxidized form of OxyR that then results in increased katA expression.We further showed that oxyR expressionis autoregulated via negative feedback.