SERINE ACETYLTRANSFERASE FROM ARABIDOPSIS-THALIANA CAN FUNCTIONALLY COMPLEMENT THE CYSTEINE REQUIREMENT OF A CYSE MUTANT STRAIN OF ESCHERICHIA-COLI

Serine acetyltransferase, a key enzyme in the L-cysteine biosynthetic pathway of sulfate assimilating organisms, catalyzes the formation of O-acetylserine, the immediate precursor of L-cysteine. In higher plant s, it is thought that sulfur assimilation occurs primarily in leaf chl oroplasts; however, serine acetyltransferase is not localized exclusiv ely in this tissue and organelle. At least three genes for serine acet yltransferase have been identified in the higher plant Arabidopsis tha liana. Reported here is a cDNA corresponding to one of these genes, SA TI, a 1,079 bp clone with an open reading frame predicted to encode a 34-kDa protein that is able to functionally complement a serine acetyl transferase mutant strain of Escherichia coli. The predicted amino aci d sequence of SATI shows significant homology with bacterial serine ac etyltransferases. SATI, expressed as a recombinant protein, shows seri ne acetyltransferase enzyme activity and cross-reacts with an antibody against the homologous E. coli enzyme. The first 40 amino acids of th e SATI polypeptide resembles a plastid transit peptide, but the polype ptide is probably not plastid localized. Genomic DNA blot analysis of A. thaliana showed that SA TI is a single copy gene and RNA blot analy sis revealed that SATI is expressed in both leaves and roots.