Genomic Nucleotide Sequence of a Brassica napus 20-Kilodalton Oleosin Gene.

The storage triacylglycerols in seeds are confined to discrete spherical organelles called oil bodies (10). Each oil body of 1 ,um diameter contains a matrix of triacylglycerols surrounded by a layer of phospholipids embedded with abundant and unique proteins called oleosins (2, 6). Oleosins are hydrophobic proteins of low Mr ranging from 16 to 26 kD, depending on the isoforms and plant species. They are present in the seeds of both dicotyledons and monocotyledons. Their possible functions include stabilizing the oil bodies in an aqueous environment and providing signal recognition for the specific binding of lipase during germination. The partial amino acid sequences ofthe oleosins from maize (16 kD) (9) and Brassica (20 kD) (4) and the complete amino acid sequences of the oleosins from maize (18 kD) (5) and carrot (19 kD) (1) have been deduced from their cDNA and genomic sequences, respectively. Here we report the complete nucleotide sequence of a Brassica napus oleosin gene (Fig. 1; Table I). The Brassica oleosin gene possesses one intron, whereas all the previously reported oleosin genes (two from maize and one from carrot) do not contain introns. The intron in the Brassica oleosin gene occurs between the two exons encoding the central hydrophobic domain and the C-terminal domain, respectively. The deduced amino acid sequence of the Brassica oleosin shows the same three basic structural domains that are common to the four sequenced oleosins. These three domains are an N-terminal hydrophilic domain, a central hydrophobic domain, and a C-terminal amphipathic domain. The Brassica oleosin shares significant homology in amino acid sequences at the central hydrophobic domain, but not at the other two domains, with the other reported oleosins. The Brassica oleosin does not possess appreciable cleavable signal sequence at the N-terminus, and this is in agreement with its Mr of 20,000 estimated by SDS-PAGE being similar to its Mr of 20,682 derived from the amino acid sequence. The N-terminal amino acid sequence of the Brassica oleosin bears little similarity with those of maize 18-kD oleosin and carrot 1 9-kD oleosin. This is intriguing, because the maize 18-kD oleosin is correctly targeted to the seed oil bodies in Brassica transformed with the maize oleosin gene (3). Presumably, the intracellular targeting signal for oleosin resides in