Here, we report that a novel nucleobase cation symporter 2 encoded in the genome of the honeybee bacterial pathogen Paenibacillus larvae reveals high levels of amino acid sequence similarity to the Escherichia coli and Bacillus subtilis uric acid and xanthine transporters. This transporter is named P. larvae uric acid permease‐like protein (PlUacP). Even though PlUacP displays overall amino acid sequence similarities, has common secondary structures, and shares functional motifs and functionally important amino acids with E. coli xanthine and uric acid transporters, these commonalities are insufficient to assign transport function to PlUacP. The solute transport and binding profile of PlUacP was determined by radiolabeled uptake experiments via heterologous expression in nucleobase transporter‐deficient Saccharomyces cerevisiae strains. PlUacP transports the purines adenine and guanine and the pyrimidine uracil. Hypoxanthine, xanthine, and cytosine are not transported by PlUacP, but, along with uric acid, bind in a competitive manner. PlUacP has strong affinity for adenine K m 7.04 ± 0.18 μ m , and as with other bacterial and plant NCS 2 proteins, PlUacP function is inhibited by the proton disruptor carbonyl cyanide m‐chlorophenylhydrazone. The solute transport and binding profile identifies PlUacP as a novel nucleobase transporter.

Two nucleobase transporters encoded in the genome of the Honey bee bacterial pathogen Paenibacillus larvae belong to the azaguanine-like transporters and are referred to as PlAzg1 and PlAzg2. PlAzg1 and 2 display significant amino acid sequence similarity, and share predicted secondary structures and functional sequence motifs with two Escherichia coli nucleobase cation symporter 2 (NCS2) members: adenine permease (EcAdeP) and guanine-hypoxanthine permease EcGhxP. However, similarity does not define function. Heterologous complementation and functional analysis using nucleobase transporter-deficient Saccharomyces cerevisiae strains revealed that PlAzg1 transports adenine, hypoxanthine, xanthine and uracil, while PlAzg2 transports adenine, guanine, hypoxanthine, xanthine, cytosine and uracil. Both PlAzg1 and 2 display high affinity for adenine with Km of 2.95 ± 0.22 and 1.92 ± 0.22 μM, respectively. These broad nucleobase transport profiles are in stark contrast to the narrow transport range observed for EcAdeP (adenine) and EcGhxP (guanine and hypoxanthine). PlAzg1 and 2 are similar to eukaryotic Azg-like transporters in that they share a broad solute transport profile, particularly the fungal Aspergillus nidulans AzgA (that transports adenine, guanine and hypoxanthine) and plant AzgA transporters from Arabidopsis thaliana and Zea mays (that collectively move adenine, guanine, hypoxanthine, xanthine, cytosine and uracil).

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Erwinia amylovora is the causal agent of fire blight, an economically important disease of apples and pears. As part of the infection process E. amylovora propagates on different plant tissues each with distinct nutrient environments. Here, the biochemical properties of the E. amylovora adenine permease (EaAdeP) are investigated. Heterologous expression of EaAdeP in nucleobase transporter-deficient E. coli strains, coupled with radiolabel uptake studies revealed that EaAdeP is a high affinity adenine transporter with a Km of 0.43 ± 0.09 μM. Both E. coli and E. amylovora carrying extra copies of EaAdeP are sensitive to growth on the toxic analog 8-azaadenine. EaAdeP is expressed during immature pear fruit infection. Immature pear and apple fruit virulence assays reveal that an E. amylovora ΔadeP::Camr mutant is still able to cause disease symptoms however with growth at a lower level, indicating that external adenine is utilized in disease establishment.