Promoter trapping of a novel medium-chain acyl-CoA oxidase, which is induced transcriptionally during Arabidopsis seed germination

The first step of peroxisomal fatty acid beta -oxidation is catalyzed by a family of acyl-CoA oxidase isozymes with distinct fatty acyl-CoA chain-leng th specificities. Here we identify a new acyl-CoA oxidase gene from Arabido psis (AtACX3) following the isolation of a promoter-trapped mutant in which beta -glucuronidase expression was initially detected in the root meristem . In acx3 mutant seedlings medium-chain acyl-CoA oxidase activity was reduc ed by 95%, whereas long- and short-chain activities were unchanged. Despite this reduction in activity lipid catabolism and seedling development were not perturbed. AtACX3 was cloned and expressed in Escherichia coli. The rec ombinant enzyme displayed medium-chain acyl-CoA substrate specificity. Anal ysis of beta -glucuronidase activity in acx3 revealed that, in addition to constitutive expression in the root axis, AtACX3 is also up-regulated stron gly in the hypocotyl and cotyledons of germinating seedlings. This suggests that beta -oxidation is regulated predominantly at the level of transcript ion in germinating oilseeds. After the discovery of AtACX3, the Arabidopsis acyl-CoA oxidase gene family now comprises four isozymes with substrate sp ecificities that encompass the full range of acyl-CoA chain lengths that ex ist in vivo.