Chlamydia trachomatis transports NAD via the Npt1 ATP/ADP translocase

Obligate intracellular bacteria comprising the order Chlamydiales lack the ability to synthesize nucleotides de novo and must acquire these essential compounds from the cytosol of the host cell. The environmental protozoan endosymbiont Protochlamydia amoebophila UWE25 encodes five nucleotide transporters with specificities for different nucleotide substrates, including ATP, GTP, CTP, UTP, and NAD. In contrast, the human pathogen Chlamydia trachomatis encodes only two nucleotide transporters, the ATP/ADP translocase C. trachomatis Npt1 (Npt1(Ct)) and the nucleotide uniporter Npt2(Ct), which transports GTP, UTP, CTP, and ATP. The notable absence of a NAD transporter, coupled with the lack of alternative nucleotide transporters on the basis of bioinformatic analysis of multiple C. trachomatis genomes, led us to re-evaluate the previously characterized transport properties of Npt1(Ct). Using [adenylate-(32)P]NAD, we demonstrate that Npt1(Ct) expressed in Escherichia coli enables the transport of NAD with an apparent K(m) and V(max) of 1.7 μM and 5.8 nM mg(-1) h(-1), respectively. The K(m) for NAD transport is comparable to the K(m) for ATP transport of 2.2 μM, as evaluated in this study. Efflux and substrate competition assays demonstrate that NAD is a preferred substrate of Npt1(Ct) compared to ATP. These results suggest that during reductive evolution, the pathogenic chlamydiae lost individual nucleotide transporters, in contrast to their environmental endosymbiont relatives, without compromising their ability to obtain nucleotides from the host cytosol through relaxation of transport specificity. The novel properties of Npt1Ct and its conservation in chlamydiae make it a potential target for the development of antimicrobial compounds and a model for studying the evolution of transport specificity.