Characterization of a palmitoyl-acyl carrier protein thioesterase (FatB1) in cotton

The relatively high level of palmitic acid (22 mol%) in cotton seeds may be due in part to a palmitoyl-acyl carrier protein (ACP) thioesterase (PATE), which prefers C16:0-ACP as its substrate. In embryo extracts, FATE activit y was highest at the maximum rate of reserve accumulation (oil and protein) , occurring about 30-35 d post anthesis, Thioesterase activity toward oleoy l-ACP was relatively similar at all developmental stages examined, but was considerably lower than the FATE activity. In developing seeds and in cotyl edons and hypocotyls of seedlings, the FATE activity predominated. A cotton FATE cDNA done isolated by screening a cDNA library with a heterologous Ar abidopsis FatB1 probe has a 1.7-kb insert sequence with an open reading fra me of 410 amino acids, lacking codons for the three N-terminal amino acids. The predicted amino acid sequence of the cotton RATE preprotein has a char acteristic stromal-targeting domain and a 63% identity to the Arabidopsis l ong-chain acyl ACP-thioesterase FatB1 sequence. Alkaline blot hybridization of cotton genomic DNA with the Arabidopsis FatB1 probe suggested the prese nce of at least two FatB1 thioesterase genes in cotton, Relative cotton Fat B1 transcript abundance was compared by RT-PCR and slot blot analysis in to tal RNA extracts from embryos, seedlings and leaves of mature plants. The c otton FafB1 mRNA apparently was expressed in all tissues but paralleled the profiles of FATE enzyme activity and seed oil accumulation in embryos.