After absorption of exogenous fatty acids from the environment, the enzymes, Δ9-, Δ12- and Δ15-desaturases catalyze the conversion of fatty acids (stearic acid) into α-linolenic acid, ALA (Qiu, 2002). Because E. coli originally lacks the genes encoding the desaturases, we have to construct and transform the recombinant DNA plasmid containing them into E. coli