An Erwinia amylovora uracil transporter mutant retains virulence on immature apple and pear fruit

Substrate identification of putative NCS1 and NCS2 nucleobase transporters in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen that can salvage nucleobases from the environment to conserve nutrients that would otherwise be spent on de novo nucleotide biosynthesis. However, little is known regarding the substrate specificity of the 13 putative nucleobase transporters in P. aeruginosa. Here, using a combination of genetic and chemical approaches, we report substrate identifications for 10 putative nucleobase transporters in P. aeruginosa. Specifically, we individually expressed each transporter in a genetic background lacking all 13 putative nucleobase transporters and quantified growth on a panel of 10 nucleobases as sole nitrogen sources. We confirmed these expression-based substrate identifications using targeted genetic knockouts. In a complementary approach, we utilized four toxic nucleobase antimetabolites to characterize antimicrobial activity in these same strains. We identified the sole allantoin transporter as well as transporters for guanine, xanthine, uric acid, cytosine, thymine, uracil, and dihydrouracil. Furthermore, we associated at least five nucleobase transporters with hypoxanthine, which has been recently reported to be an antibiofilm cue in P. aeruginosa. These results provide an initial characterization of the putative nucleobase transporters in P. aeruginosa, significantly advancing our understanding of nucleobase transport in this clinically relevant organism.

IMPORTANCE

Pseudomonas aeruginosa is a frequently multidrug-resistant opportunistic pathogen and one of the most common causes of healthcare-acquired infections. While nucleobases are known to support growth in nutrient-limited conditions, recent work showed that adenine and hypoxanthine can also decrease P. aeruginosa biofilm formation by disrupting c-di-GMP metabolism. Thus, nucleobase transport may be relevant to multiple aspects of P. aeruginosa biology and pathogenesis. However, there is currently little known about the transport of nucleobases in P. aeruginosa. Our work reports initial substrate identifications for 10 putative nucleobase transporters in P. aeruginosa, providing new tools to address previously difficult-to-test hypotheses relating to nucleobase transport in this organism.

The guanine-hypoxanthine permease GhxP of Erwinia amylovora facilitates the influx of the toxic guanine derivative 6-thioguanine

AIM

Erwinia amylovora is the causal agent of fire blight, a devastating disease of apples and pears. This study determines whether the E. amylovora guanine-hypoxanthine transporter (EaGhxP) is required for virulence and if it can import the E. amylovora produced toxic analogue 6-thioguanine (6TG) into cells.

METHODS AND RESULTS

Characterization of EaGhxP in guanine transport deficient Escherichia coli reveals that it can transport guanine, hypoxanthine and the toxic analogues 8-azaguanine (8AG) and 6TG. Similarly, EaGhxP transports 8AG and 6TG into E. amylovora cells. EaGhxP has a high affinity for 6TG with a K_i of 3·7 µmol l^-1 . An E. amylovora ⊿ghxP::Cam^r strain shows resistance to growth on 8AG and 6TG. Although EaGhxP is expressed during active disease propagation, it is not necessary for virulence as determined on immature apple and pear assays.

CONCLUSIONS

EaGhxP is not required for virulence, but it does import 6TG into E. amylovora cells.

SIGNIFICANCE AND IMPACT OF THE STUDY

As part of the disease establishment process, E. amylovora synthesizes and exports a toxic guanine derivative 6TG. Our results are counter intuitive and show that EaGhxP, an influx transporter, can move 6TG into cells raising questions regarding the role of 6TG in disease establishment.