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Cahoon, E.B. et al.

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

We demonstrate that naturally occurring C(14) and C(16)-specific acyl-acyl 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, C(18)-specific delta(9)-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 C(14) and C(16) acyl-ACPs but a relative lack of C(18) 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 delta(9)-18:0- ACP desaturase that display enhanced specificity for C(14) and C(16) 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 delta(9)-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.[1]

References

Substrate-dependent mutant complementation to select fatty acid desaturase variants for metabolic engineering of plant seed oils. Cahoon, E.B., Shanklin, J. Proc. Natl. Acad. Sci. U.S.A. (2000) [Pubmed]

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