The four SOD isozymes of maize are coded by the four nonallelic nuclear genes: Sod1, Sod2, Sod3, and Sod4 (Baum and Scandalios, J. Hered. 73:95-100, 1982). The cytosolic isozymes, SOD-2 and SOD-4, and the chloroplast isozyme, SOD-1, are copper and zinc-containing homodimeric enzymes. The mitochondrial isozyme, SOD-3, is a manganese-containing homotetrameric enzyme (Baum and Scandalios, Arch. Biochem. Biophys. 206:249-264, 1981, Baum and Scandalios, Plant Physiol. 73:31-35, 1983).
To investigate these genes we have constructed a cDNA library from W64A scutellar mRNA. The cDNA library was screened with synthetic oligonucleotide probes complementary to the mRNA coding for the N-terminal of SOD-1, SOD-2, SOD-3 and SOD-4 proteins. Positive clones from this work have been characterized by restriction endonuclease analysis, partial DNA sequence analysis, Sod2 has been completely sequenced (Cannon, White and Scandalios, PNAS in press), and hybrid select translation. Results indicate that Sod2 and Sod4 are similar in sequence, especially within the coding region. Most differences in DNA sequence between Sod2 and Sod4 occur in the third base of the codon and result in conservative amino acid replacement. The 5' and 3' noncoding regions of Sod2 and Sod4 cDNAs contain noticeable regions of similarity with shifts and minor changes of the DNA sequence. These results suggest that both Sod2 and Sod4 originated from a single progenitor Sod gene.
The mitochondrial associated SOD-3 (Mn) of maize has been restriction mapped and partially sequenced. The restriction profile is different from the SOD-2 and SOD-4 cDNAs. Preliminary DNA sequence results for the 5' end indicate that the protein contains a highly charged signal peptide of at least 25 amino acids.
Sod1 positive clones have been isolated and are in the preliminary stages of characterization. Additional collaborative work with Dr. T. Helentjaris, NPI, has shown the genomic location of the Sod2 and Sod4 genes to be on the short arm of chromosome 1 and the long arm of chromosome 7 respectively.
Ronald E. Cannon and John G. Scandalios
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