We have identified two families of acyl-CoA thioesterase (ACHs) in Arabidopsis thaliana. One family, consisting of AtACH1 and AtACH2, appears to be peroxisomal, as they have type-1 peroxisomal targeting sequences. The other family, consisting of AtACH4 and AtACH5, resides in the endoplasmic reticulum, as shown by green fluorescent protein studies. AtACH2 has been overexpressed in Escherichia coli and shows high levels of acyl-CoA thioesterase activity against both 16:0-CoA and 18:1-CoA. AtACH5 has also been overexpressed in E. coli and shows thioesterase activity as well. ACHs have been characterized in other many other organisms and in various subcellular locations, but their true physiological role is not yet understood. Indeed, atach5 gene knockout mutants have no observable phenotype.

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