Oleosins are a class of seed proteins which can be found associated with the oil-body membrane in the developing and mature embryos of monocotyledons and dicotyledons [4-6, 8-11]. They are characterized by three distinct domains; a hydrophilic N-terminal, a hydrophobic central and an amphipathic C-terminal domain. The function of these proteins is still unclear but it has been suggested that they have a role in stabilizing the oilbody or function as a lipase attachment site [6, 9, 11 ]. Oleosins are seed-specific and their expression is regulated by abscisic acid (ABA) [4, 5, 11, 12]. We report here the isolation and nucleic acid sequence of a genomic clone containing an Arabidopsis thaliana cv. Columbia oleosin gene. A Brassica napus oleosin gene [6] was used to screen a genomic library of A. thaliana cv. C o lumbia in EMBL3A (Stratagene). Presented in Fig. 1 are 1800 nucleotides of a Nco I/Kpn I genomic fragment which include 867 nucleotides of the 5' untranslated upstream region, 522 bp corresponding to the coding region for the oleosin polypeptide, 240 nucleotides corresponding to the intron in the oleosin gene (lower case) and 171 nucleotides of the 3' untranslated region. In the B. napus oleosin gene the coding region has been interrupted by an intron at exactly the same position [ 7 ], indicating the high degree of evolutionary conservation between Arabidopsis and B. napus. In Fig. 1, putative regulatory sequences are indicated. Underlined are direct repeats (R1 and R2), and inverted repeats (I); a TAACACA sequence, which matches the so-called CACA sequence which has been found in many legume seed protein genes [2]; TATA and CAAT box and polyadenylation signals. A putative abscisic acid responsive element (ARE), C A C G T G G is indicated in bold type. This sequences matches the ARE from the 5' regulatory region of the wheat Em gene [3] and the putative ARE found in the B. napus oleosin [7]. The deduced amino acid sequence of this Arabidopsis oleosin gene was lined up with the reported oleosin sequences of B. napus [7], a 16kDa Zea mays oleosin [11], an 18kDa Z. mays oleosin [9] and a 19 kDa Daucus carota oleosin [4], using the 'BESTFIT' program [1] of the University of Wisconsin Genetics Computer Group (gap weight) 3.00, length weight 0.10). This Arabidopsis oleosin is 94, 67, 53 and 53 ~o similar and 91, 53, 31 and 31~o identical to the B. napus [7], 16kDa Z. mays oleosin [11], 18kDa Z. mays oleosin [9], and 19 kDa D. carota oleosin [4], respectively. Earlier reports suggest that oleosin isoforms of different molecular weight occur in a wide variety of seed plants [ 10]. In Arabidopsis at least three other oleosins of apparent molecular mass of 19, 20 and 21 .5kDa can be found (results not shown). We are currently identifying the genes encoding these isoforms in Arabidopsis,