A gene controlling variation in arabidopsis glucosinolate composition is part of the methionine chain elongation pathway

Arabidopsis and other Brassicaceae produce an enormous diversity of aliphat ic glucosinolates, a group of methionine (Met)-derived plant secondary comp ounds containing a P-thio-glucose moiety, a sulfonated oxime, and a variabl e side chain. We fine-scale mapped GSL-ELONG, a locus controlling variation in the side-chain length of aliphatic glucosinolates. Within this locus, a polymorphic gene was identified that determines whether Met is extended pr edominantly by either one or by two methylene groups to produce aliphatic g lucosinolates with either three- or four-carbon side chains. Two allelic mu tants deficient in four-carbon side-chain glucosinolates were shown to cont ain independent missense mutations within this gene. In cell-free enzyme as says, a heterologously expressed cDNA from this locus was capable of conden sing 2-oxo-4-methylthiobutanoic acid with acetyl-coenzyme A, the initial re action in Met chain elongation. The gene methylthioalkylmalate synthase1 (M AM1) is a member of a gene family sharing approximately 60% amino acid sequ ence similarity with 2-isopropylmalate synthase, an enzyme of leucine biosy nthesis that condenses 2-oxo-3-methylbutanoate with acetylcoenzyme A.