Abstract
Many bacteria exist in a state of feast or famine where high nutrient availability leads to periods of growth followed by nutrient scarcity and growth stagnation. To adapt to the constantly changing nutrient flux, metabolite acquisition systems must be able to function over a broad range. This, however, creates difficulties as nutrient concentrations vary over many orders of magnitude, requiring metabolite acquisition systems to simultaneously balance ligand specificity and the dynamic range in which a response to a metabolite is elicited. Here we present how a gene duplication of a periplasmic binding protein in a mannose ATP-binding cassette transport system potentially resolves this dilemma through gene functionalization. Determination of ligand binding affinities and specificities of the gene duplicates with fluorescence and circular dichroism demonstrates that although the binding specificity is maintained the Kd values for the same ligand differ over three orders of magnitude. These results suggest that this metabolite acquisition system can transport ligand at both low and high environmental concentrations while preventing saturation with related and less preferentially metabolized compounds. The x-ray crystal structures of the β-mannose-bound proteins help clarify the structural basis of gene functionalization and reveal that affinity and specificity are potentially encoded in different regions of the binding site. These studies suggest a possible functional role and adaptive advantage for the presence of two periplasmic-binding proteins in ATP-binding cassette transport systems and a way bacteria can adapt to varying nutrient flux through functionalization of gene duplicates.
Highlights
The functional role of gene duplicates in a bacterial metabolite transport system is unknown
Determination of ligand binding affinities and specificities of the gene duplicates with fluorescence and circular dichroism demonstrates that the binding specificity is maintained the Kd values for the same ligand differ over three orders of magnitude
These studies suggest a possible functional role and adaptive advantage for the presence of two periplasmic-binding proteins in ATP-binding cassette transport systems and a way bacteria can adapt to varying nutrient flux through functionalization of gene duplicates
Summary
The functional role of gene duplicates in a bacterial metabolite transport system is unknown. Determination of ligand binding affinities and specificities of the gene duplicates with fluorescence and circular dichroism demonstrates that the binding specificity is maintained the Kd values for the same ligand differ over three orders of magnitude These results suggest that this metabolite acquisition system can transport ligand at both low and high environmental concentrations while preventing saturation with related and less preferentially metabolized compounds. In another variation of phosphate uptake, Rhizobium tropici contains duplicate ABC phosphate transport operons that have transport Km values that are known to differ by almost two orders of magnitude [11] In both of these systems, gene duplication appears to confer an adaptive advantage by allowing for simultaneous optimization of affinity and specificity, the molecular mechanisms by which this is achieved have yet to be determined. These studies demonstrate how functionalization of duplicated PBPs provides an adaptive advantage by greatly increasing the dynamic range of ligand binding affinities and further demonstrate the remarkable adaptability of the PBP superfamily
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