Substrate-dependent mutant complementation to select fatty acid desaturase variants for metabolic engineering of plant seed oils

Abstract

We demonstrate that naturally occurring C14 and C16-specific acylacyl carrier protein (ACP) desaturases from plants can complement the unsaturated fatty acid (UFA) auxotrophy of an Escherichia coli fabA/fadR mutant. Under the same growth conditions, C18-specific Δ9-stearoyl (18:0)-ACP desaturases are unable to complement the UFA auxotrophy. This difference most likely results from the presence of sufficient substrate pools of C14 and C16 acyl-ACPs but a relative lack of C18 acyl-ACP pools in E. coli to support the activities of the plant fatty acid desaturase. Based on this, a substrate-dependent selection system was devised with the use of the E. coli UFA auxotroph to isolate mutants of the castor Δ9-18:0-ACP desaturase that display enhanced specificity for C14 and C16 acyl-ACPs. Using this selection system, a number of desaturase variants with altered substrate specificities were isolated from pools of randomized mutants. These included several G188L mutant isolates, which displayed a 15-fold increase in specific activity with 16:0-ACP relative to the wild-type castor Δ9-18:0-ACP desaturase. Expression of this mutant in Arabidopsis thaliana resulted in the accumulation of unusual monounsaturated fatty acids to amounts of >25% of the seed oil. The bacterial selection system described here thus provides a rapid means of isolating variant fatty acid desaturase activities for modification of seed oil composition.