Cloning and characterization of cDNAs encoding for long-chain saturated acyl-ACP thioesterases from the developing seeds of Brassica juncea.

Four types of cDNAs corresponding to the fatty acyl-acyl carrier protein (ACP) thioesterase (Fat) enzyme were isolated from the developing seeds of Brassica juncea, a widely cultivated species amongst the oil-seed crops. The mature polypeptides deduced from the cDNAs showed sequence identity with the FatB class of plant thioesterases. Southern hybridization revealed the presence of at least four copies of BjFatB gene in the genome of this amphidiploid species. Western blot and RT-PCR analyses showed that the BjFatB class thioesterase is expressed poorly in flowers and leaves, but significantly in seeds at the mid-maturation stage. The enzymatic activities of different BjFatB isoforms were established upon heterologous expression of the four BjFatB CDSs in Escherichia coli K27fadD88, a mutant strain of fatty acid beta-oxidation pathway. The substrate specificity of each BjFatB isoform was determined in vivo by fatty acid profile analyses of the culture supernatant and membrane lipid of the recombinant K27fadD88 and E. coli DH10B (fadD(+)) clones, respectively. The BjFatB1 and BjFatB3 were predominantly active on C18:0-ACP substrate, whereas BjFatB2 and BjFatB4 were specific towards C18:0-ACP as well as C16:0-ACP. These novel FatB genes may find potential application in metabolic engineering of crop plants through their over-expression in seed tissues to generate stearate-rich vegetable fats/oils of commercial importance.