The solute transport profile of two Aza-guanine transporters from the Honey bee pathogenPaenibacillus larvae

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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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