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Abstract:
Pseudomonas proliferate in a wide spectrum of harsh
and variable environments. In many of these environments,
amino acids, such as histidine, are a valuable
source of carbon, nitrogen and energy. Here, we demonstrate
that the histidine uptake and utilization (hut)
pathway of Pseudomonas aeruginosa PAO1 contains
two branches from the intermediate formiminoglutamate
to the product glutamate. Genetic analysis
revealed that the four-step route is dispensable as
long as the five-step route is present (and vice versa).
Mutants with deletions of either the four-step (HutE)
or five-step (HutFG) branches were competed against
each other and the wild-type strain to test the hypothesis
of ecological redundancy; that is, that the presence
of two pathways confers no benefit beyond that
delivered by the individual pathways. Fitness assays
performed under several environmental conditions
led us to reject this hypothesis; the four-step pathway
can provide an advantage when histidine is the sole
carbon source. An IclR-type regulator (HutR) was
identified that regulates the four-step pathway. Comparison
of sequenced genomes revealed that
P. aeruginosa strains and P. fluorescens Pf-5 have
branched hut pathways. Phylogenetic analyses suggests
that the gene encoding formiminoglutamase
(hutE) was acquired by horizontal gene transfer from
a Ralstonia-like ancestor. Potential barriers to interspecies
transfer of the hutRE module were explored
by transferring it from P. aeruginosa PAO1 to P. fluorescens
SBW25. Transfer of the operon conferred theability to utilize histidine via the four-step pathway in
a single step, but the fitness cost of acquiring this
new operon was found to be environment dependent.
