The intracellular inositol (pyro)phosphate receptor AtSPX1 reciprocally binds to P1BS DNA.
Whitfield HL., Gilmartin M., Riley AM., Shipton ML., Potter BVL., Hemmings AM., Brearley CA.
The response of plants to phosphate starvation engages PHR (CC-MYB-PHOSPHATE STARVATION RESPONSE) transcription factors that bind to P1BS (GNATATNC) promoter elements of phosphate-starvation induced (PSI) genes. The encoded proteins include single-domain SPX (SYG1/Pho81/XPR1) proteins. SPX proteins bind PHR proteins. Current models of SPX1: PHR interaction define only a high-phosphate role for SPX1, as an inositol (pyro)phosphate-dependent negative regulator of PHR. Here, by combination of chemical synthesis, orthogonal binding assays and molecular modeling we report that full-length SPX1 binds P1BS promoter elements and inositol (pyro)phosphates with similar affinity. Inositol (pyro)phosphates and DNA are reciprocally competing ligands of SPX1. Structural models of SPX1: inositol (pyrophosphate) and of SPX1: P1BS interaction are provided beside a working hypothesis of SPX1: PHR1 interaction. The results reveal the low-phosphate function of SPX1. These findings proffer a fundamentally different perspective of SPX involvement in the phosphate starvation response (PSR).