Cloning, expression, and characterization of two manganese superoxide dismutases from Caenorhabditis elegans

Two genes encoding manganese superoxide dismutase (sod-2 and sod-3) have been identified in the nematode Caenorhabditis elegans. Each gene is composed of five exons, and intron positions are identical; however, intron sizes and sequences are not the same. The predicted protein sequences are 86.3% homologous (91.8% conservative), and the cDNAs are only 75.2% homologous. Both deduced protein sequences contain the expected N-terminal mitochondrial transit peptides. Reverse transcriptase polymerase chain reaction analysis shows that both genes are expressed under normal growth conditions and that their RNA transcripts are trans-spliced to the SL-1 leader sequence. The latter result together with Northern blot analysis indicate that both genes have mono-cistronic transcripts. The sod-3 gene was mapped to chromosome X, and the location of sod-2 was confirmed to be chromosome I. Polymerase chain reaction was used to amplify the cDNA regions encoding the predicted mature manganese superoxide dismutase proteins and each was cloned and expressed to high levels in Escherichia coli cells deficient in cytosolic superoxide dismutases. Both proteins were shown to be active in E. coli, providing similar protection against methyl viologen-induced oxidative stress. The expressed enzymes, which were not inhibited by hydrogen peroxide or cyanide, are dimeric, show quite different electrophoretic mobilities and isoelectric points, but exhibit comparable specific activities.