Fig. 6.

Model of interaction of SPX1 and SPX2 with PHR2 in response to the cellular Pi concentration for PSI transcription. In high cellular Pi, SPX1 and SPX2 interact with PHR2 at a high affinity in the nucleus, inhibiting PHR2 from binding to the P1BS motif of PSI genes. Thus, the transcription of PSI genes, including SPX1 and SPX2, is basal. However, in low cellular Pi, SPX1 and SPX2 showed a low affinity to PHR2 and P1BS motifs compete with SPX1 and SPX2 on interacting with PHR2, resulting in the up-regulation of PSI genes, including SPX1 and SPX2. Accumulated SPX1 and SPX2 proteins allow quick repression of the PHR2-dependent Pi signaling with Pi resupply. The thick lines with arrows represent enhancement.