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

核苷酸转运体（NTTs）是衣原体目和立克次体目的特征。这些专性细胞内细菌完全失去了它们的核苷酸合成能力，但替代地进化出NTT以利用来自宿主的能量和核苷酸。令我们惊讶的是，我们在一些与衣原体目或立克次体目无关的物种中鉴定出了NTT序列，这些物种在核苷酸生成方面没有受损，并且通常不被称为细胞内细菌。到目前为止，尚不清楚为什么自由生活的细菌，如恶臭假单胞菌或耐金属罗尔斯通氏菌招募NTT。恶臭假单胞菌（P.putida）和耐金属腐殖菌（R.metallidurans）是重要的土壤细菌，可以推测它们使用NTTs来获得从植物根部或其他土壤微生物释放的代谢物（核苷酸）。有趣的是，第一个分子数据论证了迄今为止未观察到的蛋白细菌NTT与周质醛酮还原酶（AKR）的相关性。这可能指向这些NTTs的新功能，即作为NAD/NADH穿梭体，为周质AKR提供共底物。R.metallidurans和P.putida是生态学上重要的细菌，NTT/AKR在污染物和复杂碳水化合物的生物降解中的作用是可以想象的。在拟议的项目中，我们希望澄清NAD-转运蛋白在自由生活的细菌中的生理相关性，并更详细地研究与AKR的相互作用。

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.