ABSTRACT
Proton/sulfate cotransporters in the plasma membranes are responsible for uptake of the environmental sulfate used in the sulfate assimilation pathway in plants. Here we report the cloning and characterization of an Arabidopsis thaliana gene, AST68, a new member of the sulfate transporter 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 functionally complement a Saccharomyces cerevisiae mutant lacking a sulfate transporter gene. Southern hybridization and restriction fragment length polymorphism mapping confirmed that AST68 is a single-copy gene that maps to the top arm of chromosome 5. Northern hybridization analysis of sulfate-starved plants indicated that the steady-state mRNA abundance of AST68 increased specifically in roots up to 9-fold by sulfate starvation. In situ hybridization experiments revealed that AST68 transcripts were accumulated in the central cylinder of sulfate-starved roots, but not in the xylem, endodermis, cortex, and epidermis. Among all the structural genes for sulfate assimilation, sulfate transporter (AST68), APS reductase (APR1), and serine acetyltransferase (SAT1) were inducible by sulfate starvation in A. thaliana. The sulfate transporter (AST68) exhibited the most intensive and specific response in roots, indicating that AST68 plays a central role in the regulation of sulfate assimilation in plants.
Subject(s)
Anion Transport Proteins , Arabidopsis Proteins , Arabidopsis/metabolism , Carrier Proteins/metabolism , Membrane Proteins , Plant Roots/metabolism , Sulfates/metabolism , Amino Acid Sequence , Arabidopsis/genetics , Carrier Proteins/genetics , Chromosome Mapping , Cloning, Molecular , Gene Expression Regulation, Plant , In Situ Hybridization , Molecular Sequence Data , Phylogeny , RNA, Messenger/genetics , RNA, Messenger/metabolism , Saccharomyces cerevisiae/genetics , Sulfate TransportersABSTRACT
A cDNA encoding a sulfate transporter was isolated from Arabidopsis thaliana. The isolated clone contained an open reading frame encoding a polypeptide of 658 amino acids, exhibiting the highest similarity (62%) with the sequence of the low-affinity sulfate transporter of a tropical legume Stylosanthes hamata. Northern blot analysis indicated the constitutive accumulation of a 2.6 kb length transcript in leaves and roots of seedlings. We also propose that A. thaliana contains three sulfate transporter genes which are expressed as 3.0, 2.7 and 2.6 kb length transcripts, respectively, in an organ-specific manner.
