Abstract
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.