REGULATION OF SULFUR ASSIMILATION IN HIGHER-PLANTS - A SULFATE TRANSPORTER INDUCED IN SULFATE-STARVED ROOTS PLAYS A CENTRAL ROLE IN ARABIDOPSIS-THALIANA

Proton/sulfate cotransporters in the plasma membranes are responsible for uptake of the environmental sulfate used in the sulfate assimilati on pathway in plants. Here we report the cloning and characterization of an Arabidopsis thaliana gene, AST68, a new member of the sulfate tr ansporter gene family in higher plants, Sequence analysis of cDNA and genomic clones of AST68 revealed that the AST68 gene is composed of 10 exons encoding a 677-aa polypeptide (74.1 kDa) that is able to functi onally complement a Saccharomyces cerevisiae mutant lacking a sulfate transporter gene. Southern hybridization and restriction fragment leng th polymorphism mapping confirmed that AST68 is a single-copy gene tha t maps to the top arm of chromosome 5. Northern hybridization analysis of sulfate-starved plants indicated that the steady-state mRNA abunda nce of AST68 increased specifically in roots up to 9-fold by sulfate s tarvation. In situ hybridization experiments revealed that AST68 trans cripts were accumulated in the central cylinder of sulfate-starved roo ts, but not in the xylem, endodermis, cortex, and epidermis. Among all the structural genes for sulfate assimilation, sulfate transporter (A ST68), APS reductase (APR1), and serine acetyltransferase (SATI) were inducible by sulfate starvation in A. thaliana. The sulfate transporte r (AST68) exhibited the most intensive and specific response in roots, indicating that AST68 plays a central role in the regulation of sulfa te assimilation in plants.