This article is available online at http://www.jlr.org However, due to the presence of a highly potent toxin (ricin), this native oilseed plant is not considered an ideal source for hydroxy fatty acid production. Therefore, tremendous effort has recently been made in identifying genes involved in the biosynthesis of this fatty acid and using the genes to engineer oilseed crops for producing ricinoleic acid ( 3-7 ). However, to date these attempts have met with only partial success. When a hydroxylase gene from a native plant species was introduced into oilseed crops, the hydroxy fatty acid content in transgenic seeds rarely exceeded 20% of the total fatty acids ( 8-10 ). In 2008, a new hydroxylase for the biosynthesis of ricinoleic acid was identifi ed from a nonplant origin, and expression of this gene in Arabidopsis resulted in accumulation of a slightly higher level of hydroxy fatty acids in transgenic seeds ( 11 ). Very recently, coexpression of genes encoding factors involved in the networks of the hydroxylation or acyl traffi cking process along with a hydroxylase has also been attempted ( 4-7 ). Although the improved production of hydroxy fatty acids is observed, the amount in transgenic seeds still rarely exceeds 25% of the total fatty acids. It appears that selection of genes from diverse sources and addition of cofactors can make a difference for transgenic production of hydroxyl fatty acids, but production of a single hydroxy fatty acid at the commercially viable level in plants still remains as a challenging task.Microorganisms have recently emerged as promising systems for producing biofuel for transportation and platform chemicals for biopolymers because some microbes offer high output of biomass with good oil content in a short period of time and are easily used for genetic manipulation to implement a metabolic engineering strategy. Exploitation of metabolic engineering of lipid pathways in Abstract Ricinoleic acid (12-hydroxyoctadec-cis -9-enoic acid) has many specialized uses in bioproduct industries, while castor bean is currently the only commercial source for the fatty acid. This report describes metabolic engineering of a microbial system ( Pichia pastoris ) to produce ricinoleic acid using a "push" (synthesis) and "pull" (assembly) strategy. CpFAH, a fatty acid hydroxylase from Claviceps purpurea , was used for synthesis of ricinoleic acid, and CpDGAT1, a diacylglycerol acyl transferase for the triacylglycerol synthesis from the same species, was used for assembly of the fatty acid. Coexpression of CpFAH and CpDGAT1 produced higher lipid contents and ricinoleic acid levels than expression of CpFAH alone. Coexpression in a mutant haploid strain defective in the ⌬ 12 desaturase activity resulted in a higher level of ricinoleic acid than that in the diploid strain. Intriguingly, the ricinoleic acid produced was mainly distributed in the neutral lipid fractions, particularly the free fatty acid form, but with little in the polar lipids. This work demonstrates the effectiveness of the metabolic ...