Cytochrome b₅ and fatty acid desaturation

Abstract

Cytochrome b₅ is a small haem-protein which participates in a number of oxidation/reduction reactions in the microsomal membranes of plants, animals and fungi. It has recently been shown to be involved in the microsomal Δ12 desaturation of oleate to form linoleate in Safflower (Carthamus tinctorius) (1,2), and the Δ12 hydroxylation of oleate to form ricinoleate in Castor bean (Ricinus communis) (3). Cytochrome b₅ appears to be an essential component of the acyl-complex lipid desaturase systems of the plant ER, and in the developing oil-seed may be almost totally dedicated to polyunsaturated fatty acid biosynthesis. The manipulation in vivo of cytochrome b₅ in oil-seed species could thus provide a means of regulating the fatty acid quality of the storage triacylglycerols. We have isolated a cDNA encoding cytochrome b₅ from a tobacco (Nicotiana tabacum) leaf cDNA library with the aim of altering cytochrome b₅ levels by anti-sense techniques and by over-expression of the protein in developing seeds. We have chosen tobacco for our studies as the plant is easily transformable and produces seeds rich in C18:2 fatty acids. The deduced amino acid sequence of the tobacco cytochrome shows considerable homology to mammalian cytochrome b₅. Here we describe the production of the plant cytochrome b₅ in E. coli, and the expression of the cytochrome b₅ message in the tobacco plant.