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Journal Article

Variation in transport explains polymorphism of histidine and urocanate utilization in a natural Pseudomonas population


Rainey,  Paul B.
External Scientific Member Group Experimental and Evolutionary Genetics, Max Planck Institute for Evolutionary Biology, Max Planck Society;

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Zhang, X.-X., Chang, H., Tran, S. L., Gauntlett, J. C., Cook, G. M., & Rainey, P. B. (2012). Variation in transport explains polymorphism of histidine and urocanate utilization in a natural Pseudomonas population. Environmental Microbiology, 14(8), 1941-1951. doi:10.1111/j.1462-2920.2011.02692.x.

Cite as: https://hdl.handle.net/11858/00-001M-0000-000F-EAA2-0
Phenotypic variation is a fundamental requirement for evolution by natural selection. While evidence of phenotypic variation in natural populations abounds, its genetic basis is rarely understood. Here we report variation in the ability of plant-colonizing Pseudomonas to utilize histidine, and its derivative, urocanate, as sole sources of carbon and nitrogen. From a population of 164 phyllosphere-colonizing Pseudomonas strains, 77% were able to utilize both histidine and urocanate (His+, Uro+) as growth substrates, whereas the remainder could utilize histidine, but not urocanate (His+, Uro-), or vice versa (His-, Uro+). An in silico analysis of the hut locus, which determines capacity to utilize both histidine and urocanate, from genomesequenced Pseudomonas strains, showed significant variation in the number of putative transporters. To identify transporter genes specific for histidine and urocanate, we focused on a single genotype of Pseudomonas fluorescens, strain SBW25, which is capable of utilizing both substrates. Site-directed mutagenesis, combined with [3H]histidine transport assays, shows that hutTu encodes a urocanatespecific transporter; hutTh encodes the major highaffinity histidine transporter; and hutXWV encodes an ABC-type transporter that plays a minor role in histidine uptake. Introduction of cloned copies of hutTh and hutTu from SBW25 into strains incapable of utilizing either histidine, or urocanate, complemented the defect, demonstrating a lack of functional transporters in these strains. Taken together our data show that variation in transport systems, and not in metabolic genes, explains a naturally occurring phenotypic polymorphism.