Characterization of the iron superoxide dismutase gene of Azotobacter vinelandii: sodB may be essential for viability

Azotobacter vinelandii contains two superoxide dismutases (SODs), a cytopla smic iron-containing enzyme (FeSOD), and a periplasmic copper/zinc-containi ng enzyme (CuZnSOD). In this study, the FeSOD was found to be constitutive, while the activity of CuZnSOD increased as the culture entered the station ary phase. Total SOD (units/mg protein) in stationary phase cells grown und er nitrogen-fixing conditions was not significantly different from those gr own under non-nitrogen-fixing conditions. The gene encoding FeSOD (sodB) wa s isolated from an A. vinelandii cosmid library. A 1-kb fragment containing the coding region and 400 base pairs of upstream sequence was cloned and s equenced. The nucleotide sequence and the deduced amino acid sequence had a high degree of homology with other bacterial FeSODs, particularly with P. aeruginosa. Attempts to construct a sodB mutant by recombination of a sodB: :kan insertion mutation into the multicopy chromosome of A. vinelandii were unsuccessful even in the presence of SOD mimics or nutritional supplements . These results suggest that FeSOD may be essential for the growth and surv ival of A. vinelandii, and that the periplasmic CuZnSOD cannot replace the function of FeSOD.