Molecular basis of a null mutation in soybean lipoxygenase 2: substitution of glutamine for an iron-ligand histidine.

We have investigated the nucleotide sequence and expression of lox2, a mutant form of the gene encoding lipoxygenase 2, an enzyme responsible for unpleasant flavors in soybean [Glycine max (L) Merr.] seeds. Although lox2 transcripts accumulate normally, there are no detectable transcripts for lipoxygenase 1 or 3 in mutant lines that display similar phenotypes characterized by the lack of corresponding lipoxygenase activity and protein in mature seeds. The enzymatically inactive lox2 gene product is readily detectable in mid-maturation-stage seeds but is apparently unstable, since it is absent from mature seed. The protein sequence deduced from the cDNA and genomic DNA sequences of lox2 differs from that of the wild-type gene, Lox2, in the substitution of glutamine for His-532. It is known that His-504 in soybean lipoxygenase 1, which corresponds to His-532 in lipoxygenase 2, is one of the iron-binding ligands essential for lipoxygenase activity. Here we present evidence that the missense mutation substituting Gln for His-532 results in the loss of lipoxygenase 2 from mature soybean seeds.