Abstract
The uptake of phosphate from the environment is an important process for all organisms including cyanobacteria. Phosphate is a fundamental component of DNA, RNA, and ATP, which are integral for cell survival. The uptake of phosphate from the environment by cyanobacteria has been well characterised previously, and has been shown to be regulated by a two-component signalling system, centred around a histidine kinase, SphS, and its response regulator SphR. This system also relies on a negative regulator SphU. Phosphate uptake in the model cyanobacteria is achieved by two high-affinity ABC-type transporter systems termed Pst1 and Pst2. Previous studies have also shown that both SphU and SphS interact with the Pst1 transport system as part of the regulatory two-component system and hence the pho regulon genes including the high-affinity transport systems. Interestingly \emph{Synechocystis} sp. PCC 6803 contains four copies of the periplasmic-binding subunit of the high-affinity transport systems, and it is hypothesised that this phenomenon is involved in the contribution of the Pst1 transport system to the regulation of the two-component system and pho regulon. The results of gene knockout experiments undertaken in this study have shown that the PstS1 periplasmic-binding protein is the probable extracellular phosphate sensor of \emph{Synechocystis} sp. PCC 6803, in combination with interactions with SphU, as the inactivation of PstS1 results in constitutive expression of the pho regulon and inability to increase the rate of phosphate uptake in response to phosphate-limiting conditions. There is also evidence that the transport process itself is involved in the regulation process. The results also indicated a possible role of another periplasmic-binding protein, SphX, in the response to changing extracellular phosphate concentrations, as it exhibited a reduced phosphate uptake response when grown in phosphate-limiting conditions.