REGULATION OF EXPRESSION OF A CDNA FROM BARLEY ROOTS ENCODING A HIGH-AFFINITY SULFATE TRANSPORTER

A cDNA encoding a high-affinity sulphate transporter has been isolated from barley by complementation of a yeast mutant. The cDNA, designate d HVST1, encodes a polypeptide of 660 amino acids (M-r = 72 550), whic h is predicted to have 12 membrane-spanning domains and has extensive sequence homology with other identified eukaryotic sulphate transporte rs. The K-m for sulphate was 6.9 mu M when the HVST1 cDNA was expresse d in a yeast mutant deficient in the gene encoding for the yeast SUL1 sulphate transporter. The strong pH-dependency of sulphate uptake when HVST1 was expressed heterologously in yeast suggests that the HVST1 p olypeptide is a proton/sulphate co-transporter. The gene encoding HVST 1 is expressed specifically in root tissues and the abundance of the m RNA is strongly influenced by sulphur nutrition. During sulphur-starva tion of barley, the abundance of mRNA corresponding to HVST1, and the capacity of the roots to take up sulphate, both increase. Upon re-supp ly of sulphate, the abundance of the mRNA corresponding to HVST1, and the capacity of the roots to take up sulphate, decrease rapidly, conco mitant with rises in tissue sulphate, cysteine and glutathione content s. Addition of the cysteine precursor, O-acetylserine, to plants grown with adequate sulphur supply, leads to increases in sulphate transpor ter mRNA, sulphate uptake rates and tissue contents of glutathione and cysteine. It is suggested, that whilst sulphate, cysteine and glutath ione may be candidates for negative metabolic regulators of sulphate t ransporter gene expression, this regulation may be overridden by O-ace tylserine acting as a positive regulator.