Erwinia amylovora causes fire blight, one of the more serious diseases for apple and pear cultivation. Previous studies generated E. amylovora mutants in purine metabolic pathway that still retain the ability to grow on host tissue and produce limited disease symptoms. Here we show that the E. amylovora genome has a locus that encodes for a xanthine permease belonging to the nucleobase cation symporter 2 (NCS2) family and the encoded protein displays a high level of amino acid sequence similarity to the Escherichia coli XanP. Our hypothesis is to investigate if the lack of a xanthine transporter has an effect upon disease progression. Heterologous expression of EaXanP in nucleobase transporter-deficient E. coli strains, coupled with radiolabeled nucleobase uptake studies determined that EaXanP is a high affinity xanthine transporter with a Km of 1.2 μM + 0.1 μM that confers sensitivity to growth on caffeine (1,3,7-trimethylxanthine). An E. amylovora ΔxanP::Camr mutant shows resistance to growth on caffeine, while over expression of EaXanP increases growth sensitivity to caffeine. While the EaXanP gene is expressed in infected immature pear fruitlets, an E. amylovora xanthine transport mutant is still able to grow and cause disease symptoms on immature pears and apple fruitlets.

中文翻译：

---

来自梨火疫病菌的核碱基阳离子同向转运蛋白 2 EaXanP 转运黄嘌呤

梨火疫病菌会导致火疫病，这是苹果和梨种植中较为严重的病害之一。以前的研究在嘌呤代谢途径中产生了 E. amylovora 突变体，这些突变体仍然保留在宿主组织上生长的能力并产生有限的疾病症状。在这里，我们展示了梨火疫病菌基因组具有编码属于核碱基阳离子同向转运蛋白 2 (NCS2) 家族的黄嘌呤通透酶的位点，并且编码的蛋白质显示出与大肠杆菌 XanP 的高水平氨基酸序列相似性。我们的假设是调查缺乏黄嘌呤转运蛋白是否对疾病进展有影响。EaXanP 在缺乏核碱基转运蛋白的大肠杆菌菌株中的异源表达，加上放射性标记的核碱基摄取研究，确定 EaXanP 是一种 Km 为 1 的高亲和力黄嘌呤转运蛋白。2 μM + 0.1 μM，赋予咖啡因（1,3,7-三甲基黄嘌呤）生长敏感性。E. amylovora ΔxanP::Camr 突变体显示出对咖啡因生长的抗性，而 EaXanP 的过度表达会增加对咖啡因的生长敏感性。虽然 EaXanP 基因在受感染的未成熟梨果实中表达，但梨火疫病菌黄嘌呤转运突变体仍然能够在未成熟梨和苹果果实上生长并引起疾病症状。