Intracellular bacteria live in an environment rich in most essential metabolites but need special mechanisms to access these substrates. Nucleotide transport proteins (NTTs) catalyze the import of ATP and other nucleotides from the eukaryotic host into the bacterial cell and render de novo synthesis of these compounds dispensable. The draft genome sequence of Simkania negevensis strain Z, a chlamydial organism considered a newly emerging pathogen, revealed four genes encoding putative nucleotide transport proteins (SnNTT1 to SnNTT4), all of which are transcribed during growth of S. negevensis in Acanthamoeba host cells, as confirmed by reverse transcription-PCR. Using heterologous expression in Escherichia coli, we could show that SnNTT1 functions as an ATP/ADP antiporter, SnNTT2 as a guanine nucleotide/ATP/H ؉ symporter driven by the membrane potential, and SnNTT3 as a nucleotide triphosphate antiporter. In addition, SnNTT3 is able to transport dCTP, which has not been shown for a prokaryotic transport protein before. No substrate could be identified for SnNTT4. Taking these data together, S. negevensis employs a set of nucleotide transport proteins to efficiently tap its host's energy and nucleotide pools. Although similar to other chlamydiae, these transporters show distinct and unique adaptations with respect to substrate specificities and mode of transport.Nucleotide transport proteins (NTTs) are commonly linked to the term "energy parasitism" (55) because they enable obligate intracellular bacteria to harvest ATP and other highenergy compounds from eukaryotic host cells. Among bacteria, NTT proteins catalyzing ATP/ADP exchange have been found in Chlamydiae and Rickettsiales, comprising major intracellular pathogens of humans (15,45,48,62,64,67), and recently also in the obligate intracellular veterinary pathogen Lawsonsia intracellularis belonging to the Deltaproteobacteria (63), in the plant pathogen "Candidatus Liberibacter asiaticus" belonging to the Rhizobiales (75), and in the obligate intracellular amoeba symbiont "Candidatus Amoebophilus asiaticus" belonging to the Bacteroidetes (65). In addition to ATP/ADP translocases, some obligate intracellular bacteria encode nucleotide transport isoforms for the import of nucleotides other than ATP or the cofactor NAD ϩ (7,22,26,27), thereby compensating for their inability to synthesize these metabolites de novo (22,33,66,76). Nucleotide transporters are thus important proteins for host cell interaction of obligate intracellular bacteria. Interestingly, nucleotide transport proteins were also found in eukaryotes: in plant and algal plastids (5,48,54,68,79) and in the microsporidian parasite Encephalitozoon cuniculi (73).In Chlamydiae, ATP uptake activity was first described for the avian pathogen Chlamydophila psittaci (28). Nearly 2 decades later, two nucleotide transport proteins of the human pathogen Chlamydia trachomatis were identified on the molecular level; one transporter catalyzes ATP/ADP exchange, and the second transporter mediates net uptake of RN...

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).