  For millennia, cyanobacteria have evolved strategies for acclimatization to
dynamic environments- this requires a molecular response system. Phosphate is
essential for cellular integrity and metabolism, yet inorganic phosphate is
often the limiting nutrient for most aquatic environments. This study
characterizes two periplasmic phosphate-binding proteins, namely SphX and SphZ,
which are essential for the functional response of the SphS-SphR signal
transduction system in Synechocystis sp. PCC 6803 and incorporates both into a
model. SphX offers a competitive role so as to ensure inorganic phosphate is
not depleted from the environment at a faster rate than is necessary for
metabolism and, therefore, hinders luxury uptake; whereas, SphZ, encoded by
sll0540, is the auxiliary sensor necessary for the SphS response under
phosphate limitation in tandem with a protein complex association with the
phosphate-specific transport 2 system and the negative regulator, SphU.
Cross-regulation between the pho regulon and the photosynthetic apparatus is
presented as well as a parameterized analysis for low-temperature fluorescence
emission spectroscopy.
