Ammonium and the analogue methylammonium are taken into the cell by active transport systems which constitute a family of transmembrane proteins that have been identified in fungi, bacteria, plants, and animals. Two genes from Aspergillus nidulans, mepA and meaA, which encode ammonium transporters with different affinities have been characterized. The MepA transporter exhibits the highest affinity for methylammonium (Km, 44.3 microM); in comparison, the Km for MeaA is 3.04 mM. By use of targeted gene replacement strategies, meaA and mepA deletion mutants were created. Deletion of both meaA and mepA resulted in the inability of the strain to grow on ammonium concentrations of less than 10 mM. The single meaA deletion mutant exhibited reduced growth at the same concentrations, whereas the mepA deletion mutant displayed wild-type growth. Interestingly, multiple copies of mepA were found to complement the methylammonium resistance phenotype conferred by the deletion of meaA. The expression profiles for mepA and meaA differed; the mepA transcript was detected only in nitrogen-starved cultures, whereas meaA was expressed under both ammonium-sufficient and nitrogen starvation conditions. Together, these results indicate that MeaA constitutes the major ammonium transport activity and is required for the optimal growth of A. nidulans on ammonium as the sole nitrogen source and that MepA probably functions in scavenging low concentrations of ammonium under nitrogen starvation conditions.

译文

铵和类似物甲基铵被主动转运系统带入细胞,这些转运系统构成了在真菌,细菌,植物和动物中已鉴定的跨膜蛋白家族。已经鉴定了来自构巢曲霉的两个基因mepA和meaA,它们编码具有不同亲和力的铵转运蛋白。MepA转运蛋白对甲基铵表现出最高的亲和力 (Km,44.3微米); 相比之下,MeaA的Km为3.04 mM。通过使用靶向基因替代策略,创建了meaA和mepA缺失突变体。meaA和mepA的缺失均导致菌株无法在小于10 mM的铵浓度下生长。单个meaA缺失突变体在相同浓度下表现出降低的生长,而mepA缺失突变体表现出野生型生长。有趣的是,发现多个mepA副本补充了meaA缺失所赋予的甲基铵抗性表型。mepA和meaA的表达谱有所不同; mepA转录物仅在缺乏氮的培养物中检测到,而meaA在铵充足和氮饥饿条件下均表达。总之,这些结果表明,MeaA构成了主要的铵转运活性,并且是在铵上最佳生长所必需的。作为唯一的氮源,MepA可能在氮饥饿条件下清除低浓度的铵。

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