A nucleobase cation symporter 2, EaXanP, from Erwinia amylovora transports xanthine

Abstract

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.