A possible interaction with a periplasmatic Aldo-Keto-Reductase indicates a new physiological function of bacterial nucleotide transporters

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Nucleotide transporters (NTTs) are a characteristic feature of Chlamydiales and Rickettsiales. These obligate intracellular bacteria completely lost their nucleotide synthesis capacity but alternatively evolved NTTs to exploit energy and nucleotides from the host. To our surprise, we identified NTT-sequences in few species not related to Chlamydiales or Rickettsiales, not impaired in nucleotide generation, and not commonly known as intracellular bacteria. So far it is unclear why free living bacteria, like i.e. Pseudomonas putida or Ralstonia metallidurans recruit an NTT. P.putida and R.metallidurans are important soil bacteria and it might be speculated that they use NTTs to gain metabolites (nucleotides) that are liberated from plant roots or from other soil microorganisms. Interestingly, first molecular data argue for a so far not observed correlation of proteobacterial NTTs with a periplasmatic aldo-keto-reductase (AKR). This might point to a new function of these NTTs, namely as an NAD/NADH shuttle that provides cosubstrates for the periplasmatic AKR. R.metallidurans and P.putida are ecologically important bacteria and a role of the NTT/AKR in biodegradation of pollutants and complex carbohydrate compounds is imaginable. In the proposed project we want to clarify the physiological relevance of NAD-transporters in free living bacteria and investigate the interaction with the AKR in more detail.