The key enzyme of sulfate assimilation, adenosine 5'-phosphosulfate reductase, is regulated by HY5 in Arabidopsis.

Plant sulfate assimilation is regulated by demand for reduced sulfur, as is its key enzyme, adenosine 5'-phosphosulfate reductase (APR). In a genetic screen for mutants lacking this regulation, we identified the bZIP transcription factor LONG HYPOCOTYL 5 (HY5) as a necessary component of the regulatory circuit. Regulation of APR activity by the inhibitor of glutathione synthesis, buthionine sulfoximine, or by the precursor of cysteine, O-acetylserine, was disrupted in the hy5 mutant. When dark-adapted plants were re-illuminated, the rapid induction of APR1 and APR2 mRNA levels was attenuated in hy5 seedlings, but APR3 regulation was not affected. Chromatin immunoprecipitation revealed that HY5 binds directly to the APR1 and APR2 promoters but not to the APR3 promoter. Accordingly, the regulation of APR1 and APR2 by O-acetylserine was disturbed in hy5 roots. HY5 is also important for the coordination of nitrogen and sulfur assimilation, as, unlike the wild-type, hy5 mutants do not undergo a reduction in sulfate uptake and APR activity during nitrogen starvation. Altogether, these data show that HY5 plays an important role in regulation of APR gene expression and plant sulfate assimilation.