Sinapic acid esters are final products of the phenylpropanoid pathway in Arabidopis and some other members of the Brassicaceae (crucifers). These soluble secondary metabolites have been shown in Arabidopsis to play an important role as protectants against damaging UV light. By their visible and typical fluorescence under UV light, they are relatively easy to detect both in vivo and in vitro, for which reason they are suitable as indicators for the mutagenesis of the phenylpropanoid biosynthesis. In the Arabidopsis wild-type, 1-O-sinapoyl-ß-D-glucose is the precursor of 2-O-sinapoyl-L-malate, which accumulates especially in the upper leave epidermis. The interconversion of these esters is catalyzed by the sinapoylglucose:L-malate O-sinapoyltransferase (SMT, EC 18.104.22.168), an enzyme that is located in the vacuole. Such transacylations, which are based on the high group transfer potential of an 1-O-ß-acylglucoseester, are an important component of the secondary metabolism in higher plants. Nevertheless, the genes and proteins of the involved acyltransferases were so far unknown on the molecular level. The Arabidopsis mutant sinapoylglucose accumulator 1 (sng1) contains no SMT activity and accumulates sinapoylglucose instead of sinapoylmalate, indicating a defect in the SMT gene. To clone it, a T-DNA tagged sng1 mutant was identified, and the inverse polymerase chain reaction was used to isolate genomic DNA adjacent to the insertion element. The obtained DNA was part of the Arabidopsis gene No. At2g22990 (GenBank No. AF275313), which complemented the sng1 mutant after Agrobacterium tumefaciens transformation. The SNG1 gene is located in the section 131 of the Arabidopsis chromosome 2 (GenBank No. AC004401), what corresponds to a position at 38,06 cM (marker pCC300) on the Lister and Dean recombinant inbred lines map. The encoded protein (Gen-Bank No. AAF78760) consists of 433 amino acids, has a molecular mass of ~ 50 kDa and possesses SMT activity when expressed in Escherichia coli. The sequence exhibits high similarity to serine carboxypeptidases (EC 3.4.16, family of peptidases S10), what is shown in particular by the presence of a potential catalytic triade (Ser-173, His-411 and Asp-358). The activity of the natural SMT from Arabidopsis was inhibited partially by phenylmethylsulfonylfluorid (PMSF). Thus, the SMT has developed from a serine carboxypeptidase. Accordingly, the acyltransfer by this enzyme is perhaps based on a mechanism analogous to the serine-catalyzed peptide hydrolysis. Simultaneously with the SMT, the same was found for an acylglucose-dependent transferase from Lycopersicon pennellii (wild tomato; GenBank No. AAF64227) and may apply also to other enzymes of that type. In favor of that argues, that numerous expressed genes have been identified in Arabidopsis by the genome project, which encode proteins with at present unknown functions belonging to the family of serine carboxypeptidases. Similar is possibly the case in other higher plants as well.