Genomic and functional characterization of the oas gene family encoding O-acetylserine (thiol) lyases, enzymes catalyzing the final step in cysteine biosynthesis in Arabidopsis thaliana

The final step of cysteine biosynthesis in plants is catalyzed by O-acetyls erine (thiol) lyase (OAS-TL), which occurs as several isoforms found in the cytosol, the plastids and the mitochondria. Genomic DNA blot hybridization and isolation of genomic clones indicate single copy genes (oasA1, oasA2, oasB and oasC) that encode the activities of OAS-TL A, B and C found in sep arate subcellular compartments in the model plant Arabidopsis thaliana. Seq uence analysis reveals that the newly discovered oasA2 gene represents a ps eudogene that is still transcribed, but is not functionally translated. The comparison of gene structures suggests that oasA1/oasA2 and oasB/oasC are closely related and may be derived from a common ancestor by subsequent dup lications. OAS-TL A, B and C were overexpressed in an Escherichia coli muta nt lacking cysteine synthesis and exhibited bifunctional OAS-TL and beta-cy anoalanine synthase (CAS) activities. However, all three proteins represent true OAS-TLs according to kinetic analysis and are unlikely to function in cyanide detoxification or secondary metabolism. In addition. it was demons trated that the mitochondrial OAS-TL C exhibits in vivo protein-protein int eraction capabilities with respect to cysteine synthase complex formation s imilar to cytosolic OAS-TL A and plastid OAS-TL B. Multiple database access ions for each of the A. thaliana OAS-TL isoforms can thus be attributed to a specified number of oas genes to which functionally defined gene products are assigned, and which art responsible for compartment-specific cysteine synthesis. (C) 2000 Elsevier Science B.V. All rights reserved.