Isolation, sequencing and expression of the superoxide dismutase-encoding gene ( sod) of Nocardia asteroides strain GUH-2

Nocardia asteroides (Na) superoxide dismutase (SOD) has been implicated as a virulence factor that allows the organism to survive intracellular killing by phagocytic cells. A full-length Na sod gene from a pathogenic strain of Na (strain GUH-2) was cloned from a recombinant phage library using the Mycobacterium tuberculosis (Mt) sod gene ( Mt sod) as a probe. The promoter region and structural gene (624 bp) of Na sod was sequenced and nucleotide sequence comparisons reveal 77% homology with Mt sod. The Na sod gene also shares considerable sequence homology with sod of other mycobacterial species. In addition, conserved amino acid (aa) sequences important for metal binding indicate that Mn 2+ is the preferred metal ion ligand for Na SOD. An Na sod expression plasmid, pYEX1, under transcriptional control of the Mt hsp70 promoter (pY6013), produced a 25-kDa protein product which showed SOD activity when stained in a native polyacrylamide gel and reacted with rabbit polyclonal antibody specific for Na SOD by Western blot. pYEX1, via transformation, was able to complement an Escherichia coli double sodAB mutant deficient in SOD production in the presence of paraquat (methyl viologen) which stimulates the production of superoxide radicals.