Adenosine-5′-Triphosphate-Sulfurylase fromArabidopsis thalianaandEscherichia coliAre Functionally Equivalent but Structurally and Kinetically Divergent: Nucleotide Sequence of Two Adenosine-5′-Triphosphate-Sulfurylase cDNAs fromArabidopsis thalianaand Ana

ATP-sulfurylase, the first enzyme of sulfate assimilation, catalyzes the formation of adenosine-5′-phosphosulfate from ATP and sulfate. Here we report that the higher plant,Arabidopsis thaliana,contains a three-member, expressed gene family encoding plastid localized forms of ATP sulfurylase. Three cDNAs fromA. thaliana,designatedAPS1, APS2,andAPS3,were isolated by their ability to functionally complement amet3(ATP sulfurylase) mutant strain ofSaccharomyces cerevisiae(yeast). The nucleotide sequence ofAPS1was reported previously (1).APS2andAPS3,reported here, have 476- and 465-codon open-reading frames encoding 53.6- and 52.0-kDa polypeptides, respectively. The translation products of both clones are highly homologous toAPS1(66 and 86% identity, respectively) over their entire lengths, including amino terminal sequences resembling transit peptides for plastid localization. Both clones are less homologous toMET3(25 and 30% identity, respectively). Genomic blot analysis ofA. thalianarevealed only three genes with homology to the APS cDNAs and RNA blot analysis showed thatAPS1is the most highly expressed member of this gene family. The APS polypeptides share homology with ATP-sulfurylases from fungi, a marine worm and a chemoautotrophic bacterium, but, not fromEscherichia coliorRhizobium meliloti.Analysis of recombinantAPS3showed that the protein is structurally and kinetically similar to fungal ATP-sulfurylase, but very different from theE. colienzyme. TheAPS3polypeptide is a homotetramer with specific activities (μmol primary product × mg protein−1at pH 8.0, 25°C) of 2.9 for APS synthesis, 30.1 for molybdolysis, and 48.7 for ATP synthesis. Despite the sequence, structural, and kinetic differences between higher plant andE. coliATP-sulfurylases,APS2andAPS3are able to functionally complementE. coli cysDandcysN(ATP-sulfurylase) mutant strains.