  The chemotaxis pathway of the bacterium Rhodobacter sphaeroides has many
similarities to the well-studied pathway in Escherichia coli. It exhibits
robust adaptation and has several homologues of the latter's chemotaxis
proteins. Recent theoretical results have been able to correctly predict that
the chemotactic response of Escherichia coli exhibits the same output behavior
in response to scaled ligand inputs, a dynamic property known as fold-change
detection (FCD), or input-scale invariance. In this paper, we present
theoretical assumptions on the R. sphaeroides chemotaxis sensing dynamics that
can be analytically shown to yield FCD behavior in a specific ligand
concentration range. Based on these assumptions, we construct two models of the
full chemotaxis pathway that are able to reproduce experimental time-series
data from earlier studies. To test the validity of our assumptions, we propose
a series of experiments in which our models predict robust FCD behavior where
earlier models do not. In this way, we illustrate how a dynamic phenotype such
as FCD can be used for the purposes of discriminating between two models that
reproduce the same experimental time-series data.
