Cloning, expression, and characterization of a methionine sulfoxide reductase B gene from Nicotiana tabacum.

Reactive oxygen species (ROS) are generated during normal aerobic metabolism and in plants exposed to environmental stress. Methionine (Met) residues are particularly sensitive to ROS-mediated oxidation, leading to the formation of methionine sulfoxide (MetSO) under mild oxidative conditions. Methionine sulfoxide reductase (MSR) repairs oxidized Met and protects plants from oxidative damage. Herein, we identified a tobacco (Nicotiana tabacum) MSRB gene, referred to as NtMSRB3. Analysis of the sequence showed that the NtMSRB3 protein had four typical motifs in a SelR domain, which is known as the catalytic region of MSRBs. NtMSRB3 specifically reduced the R epimer of MetSO and converted either free MetSO or Dabsyl-MetSO in the presence of dithiothreitol. Escherichia coli cells harboring NtMSRB3 displayed relative high viability under H₂O₂ stress. Subcellular localization of NtMSRB3 revealed that it was a plastid-targeted protein. Furthermore, the semi-quantitative reverse transcription polymerase chain reaction assay showed that NtMSRB3 was upregulated apparently by abscisic acid, salt, cold, and methyl viologen stress within 24 h of treatment.