The FATTY ACID ELONGATION1 (FAE1) gene of Arabidopsis is required for the synthesis of very long chain fatty acids in the seed. The product of the FAE1 gene is presumed to be a condensing enzyme that extends the chain length of fatty acids from C18 to C20 and C22. We report here the cloning of FAE1 by directed transposon tagging with the maize element Activator (Ac). An unstable fae1 mutant was isolated in a line carrying Ac linked to the FAE1 locus on chromosome 4. Cosegregation and reversion analyses established that the new mutant was tagged by Ac. A DNA fragment flanking Ac was cloned by inverse polymerase chain reaction and used to isolate FAE1 genomic clones and a cDNA clone from a library made from immature siliques. The predicted amino acid sequence of the FAE1 protein shares homology with those of other condensing enzymes (chalcone synthase, stilbene synthases, and beta-ketoacyl-acyl carrier protein synthase III), supporting the notion that FAE1 is the structural gene for a synthase or condensing enzyme. FAE1 is expressed in developing seed, but not in leaves, as expected from the effect of the fae1 mutation on the fatty acid compositions of those tissues.

We have investigated the pattern of transposition of an intact, 4.6-kbAc element inArabidopsis thaliana. Because the trans-acting transposition function (transposase) ofAc is not fully penetrant in Arabidopsis, it is not possible to use it as a diagnostic feature to scoreAc genetically, as has been done in maize and tobacco. Instead, the presence or absence of a transposedAc (trAc) was monitored by Southern blots. Germinal transpositions from the marker SPT::Ac were selected using a streptomycin germination assay and scored for the presence of atrAc. Segregation of thetrAc element and the SPT donor locus was scored in the F2 progeny of the germinal revertants, and the recombination fraction between thetrAc element and SPT was estimated by the method of maximum likelihood. We have found that, as in maize and tobacco, receptor sites fortrAcs in Arabidopsis tend to be linked to theAc donor locus.