A candidate NAD+ transporter in an intracellular bacterial symbiont related to Chlamydiae

Bacteria living within eukaryotic cells can be essential for the survival or reproduction of the host(1,2) but in other cases are among the most successful pathogens(3,4). Environmental Chlamydiae, including strain UWE25, thrive as obligate intracellular symbionts within protozoa; are recently discovered relatives of major bacterial pathogens of humans; and also infect human cells(4-7). Genome analysis of UWE25 predicted that this symbiont is unable to synthesize the universal electron carrier nicotinamide adenine dinucleotide (NAD(+))(7). Compensation of limited biosynthetic capacity in intracellular bacteria is usually achieved by import of primary metabolites(8-11). Here, we report the identification of a candidate transporter protein from UWE25 that is highly specific for import of NAD(+) when synthesized heterologously in Escherichia coli. The discovery of this candidate NAD(+)/ADP exchanger demonstrates that intact NAD(+) molecules can be transported through cytoplasmic membranes. This protein acts together with a newly discovered nucleotide transporter and an ATP/ADP translocase(12), and allows UWE25 to exploit its host cell by means of a sophisticated metabolic parasitism.