Regulation of sulfate uptake in carrot cells: Properties of a hypercontrolled variant.

Sulfate uptake in haploid carrot cultures can be experimentally controlled by the sulfur source provided for growth. The rate of sulfate uptake is low in cells grown on cystine or sulfate and high in sulfur-starved cells. A selenate-resistant variant cell line has been isolated from a haploid carrot line. The variant shows hypersuppression of sulfate uptake by cystine and essentially normal control by the other treatments. While both lines efflux intracellular sulfate in the presence of external sulfate, the rate of efflux from the variant is 4-6 times higher at comparable levels of initial intracellular sulfate. Further, properties of the efflux and uptake in both lines suggest that they are mediated by the same system. We propose that the variant possesses an altered uptake-efflux system that is more readily reversed and more subject to control by some metabolite derived from cystine.