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ISSN
1672-9145
Acta Biochim Biophys Sin
2005, 37(6): 421–428
CN 31-1940/Q
Role of oxyR from Sinorhizobium meliloti in Regulating
the Expression of Catalases
Li LUO1,3, Ming-Sheng QI1,3, Shi-Yi YAO2,
Hai-Ping CHENG2, Jia-Bi ZHU1, and Guan-Qiao YU1*
1 National Laboratory of
Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai
Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai
200032, China;
2 Biological Sciences Department,
Lehman College, the City University of New York, Bronx, NY 10468, USA;
3 Graduate School of the
Chinese Academy of Sciences, Shanghai 200032, China
Abstract The process of symbiotic
nitrogen fixation results in the generation of reactive oxygen species such as
the superoxide anion (O2–) and hydrogen peroxide (H2O2).
The response of rhizobia to these toxic oxygen species is an important factor
in nodulation and nitrogen fixation. In Sinorhizobium meliloti, one oxyR
homologue and three catalase genes, katA, katB, and katC
were detected by sequence analysis. This oxyR gene is located next to
and divergently from katA on the chromosome. To investigate the possible
roles of oxyR in regulating the expression of catalases at the
transcriptional level in S. meliloti, an insertion mutant of this gene
was constructed. The mutant was more sensitive and less adaptive to H2O2
than the wild type strain, and total catalase/peroxidase activity was reduced
approximately fourfold with the OxyR mutation relative to controls. The
activities of KatA and KatB and the expression of katA::lacZ and katB::lacZ
promoter fusions were increased in the mutant strain compared with the parental
strain grown in the absence of H2O2, indicating that katA
and katB are repressed by OxyR. However, when exposed to H2O2,
katA expression was also increased in both S. meliloti and Escherichia
coli. When exposed to H2O2,
OxyR is converted from a reduced to an oxidized form in E. coli. We
concluded that the reduced form of OxyR functions as a repressor of katA
and katB expression. Thus, in the presence of H2O2,
reduced OxyR is converted to the oxidized form of OxyR that then results in
increased katA expression. We further showed that oxyR expression
is autoregulated via negative feedback.
Key words Sinorhizobium
meliloti; OxyR; catalase; katA; H2O2
—————–
Accepted: February 25, 2005
Received: April 1, 2005
This work was supported by the grants from
the National Key Program for Basic Research of China (2001CB108901), the NIH
(5S06GM8225) and PSC-CUNY (617320030, 632140032)
*Corresponding author: Tel, 86-21-54924165;
Fax, 86-21-54924015; E-mail, [email protected]