Lauroyl-ACP thioesterase

Using the probe YAP140 [3], a cDNA (TE 3-2) from an A. thaliana library was purified. The deduced amino acid was 51 % identical to the lauroyl-ACP thioesterase from California Bay [3] and 39 % identical to the safflower oleoyl-ACP thioesterase [2]. The 1814 bp TE 3-2 cDNA contained an open reading frame from bp 226 to 1551 the first in frame ATG at bp 316 (Figure 1). Tins allowed the translation of a 412 amino add polypeptide. By screening the same Hbrary with an oleoyl-ACP thioesterase cDNA probe from coriander [5] a cDNA was found (IE 1-7) that was 66 % identical to the safflower oleoyl-ACP thioesterase and therefore most likely encodes the corresponding enzyme from A. thaliana. 

In most plants the major product of fatty acid biosynthesis in plastids is oleic acid and a highly active oleoyl-ACP thioesterase has been described [1, 2]. In the seeds of California Bay Umbellularia californicd) however, a lauroyl-ACP oesterase has been found that is involved in the production of 12 0 [3]. Recently, a partial cDNA clone (YAP140) fi om Arabidopsis thaliana siliques was isolated [4]. This clone showed a high sequence identity with the California Bay thioesterase cDNA but lower similarity with the safflower oleoyl-ACP thioesterase cDNA [2]. A. thaliana seeds show a "normal" fatty acid pattern (6.9 mol % 16 0 3.1 % 18 0 14.4% 18 1 26.9% 18 2,21.2% 18 3, 2.2% 20 0, 20.6% 20 1) and they contain only trace amounts of 12 0 [3]. In addition, the thioesterase from extracts oiA. thaliana seeds is specific for oleoyl-ACP [3]. fri this study we address the question of whether the novels, thaliana cDNA encodes a fimctional thioesterase and we examine its substrate specificity and expression in different, thaliana tissues and in developing Brassica napus seeds. 

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