Abstract
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.
