39-Phosphoadenosine 59-phosphosulfate (PAPS) is the high-energy sulfate donor for sulfation reactions. Plants produce some PAPS in the cytosol, but it is predominantly produced in plastids. Accordingly, PAPS has to be provided by plastids to serve as a substrate for sulfotransferase reactions in the cytosol and the Golgi apparatus. We present several lines of evidence that the recently described Arabidopsis thaliana thylakoid ADP/ATP carrier TAAC transports PAPS across the plastid envelope and thus fulfills an additional function of high physiological relevance. Transport studies using the recombinant protein revealed that it favors PAPS, 39-phosphoadenosine 59-phosphate, and ATP as substrates; thus, we named it PAPST1. The protein could be detected both in the plastid envelope membrane and in thylakoids, and it is present in plastids of autotrophic and heterotrophic tissues. TAAC/PAPST1 belongs to the mitochondrial carrier family in contrast with the known animal PAPS transporters, which are members of the nucleotide-sugar transporter family. The expression of the PAPST1 gene is regulated by the same MYB transcription factors also regulating the biosynthesis of sulfated secondary metabolites, glucosinolates. Molecular and physiological analyses of papst1 mutant plants indicate that PAPST1 is involved in several aspects of sulfur metabolism, including the biosynthesis of thiols, glucosinolates, and phytosulfokines.

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cAll organisms require S-adenosylmethionine (SAM) as a methyl group donor and cofactor for various biologically important processes. However, certain obligate intracellular parasitic bacteria and also the amoeba symbiont Amoebophilus asiaticus have lost the capacity to synthesize this cofactor and hence rely on its uptake from host cells. Genome analyses revealed that A. asiaticus encodes a putative SAM transporter. The corresponding protein was functionally characterized in Escherichia coli: import studies demonstrated that it is specific for SAM and S-adenosylhomocysteine (SAH), the end product of methylation. SAM transport activity was shown to be highly dependent on the presence of a membrane potential, and by targeted analyses, we obtained direct evidence for a proton-driven SAM/SAH antiport mechanism. Sequence analyses suggest that SAM carriers from Rickettsiales might operate in a similar way, in contrast to chlamydial SAM transporters. SAM/SAH antiport is of high physiological importance, as it allows for compensation for the missing methylation cycle. The identification of a SAM transporter in A. asiaticus belonging to the Bacteroidetes phylum demonstrates that SAM transport is more widely spread than previously assumed and occurs in bacteria belonging to three different phyla (Proteobacteria, Chlamydiae, and Bacteroidetes).