The Naturally Occurring High Oleate Oil Character in Some Peanut Varieties Results from Reduced Oleoyl-PC Desaturase Activity from Mutation of Aspartate 150 to Asparagine
2001
Commercially important high oleate seed oils, meaning that they are low in polyunsaturated fatty acid content, are resistant to developing rancidity. A peanut cultivar (Arachis hypogaea L.) derived from a naturally occurring peanut has low oleoyl-PC desaturase activity, the key enzyme in the production of linoleate, normally an abundant polyunsaturated fatty acid. Two genes for this enzyme are expressed in peanut seeds, and when they were separately expressed in yeast (Saccharomyces cerevisiae), one produced less linoleate than the other. Although the two cDNA encode similar sequence proteins, they differ by four amino acids. Aspartate at position 150 was asparagine in the low activity copy. An aspartate was present in many membrane desaturases and other related membrane enzymes at an equivalent location suggesting that the mutation at 150 was key to the low activity. In this work, site-specific mutagenesis was used to change the aspartate in the high activity enzyme to asparagine and to change asparagine in the lower activity enzyme to aspartate. These changes, upon expression in yeast, resulted in nearly complete loss of activity of the previously more active desaturase and restored activity to the previously less active desaturase. This decrease in activity of the one gene, a consequence of mutation from aspartate 150 to asparagine, together with reduction in transcript level of the high activity gene in the mutant variety, suggest that these alterations are the molecular basis of the high oleate phenotype in some commercial varieties of peanut.
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