VTC complex catalyzes polyphosphate synthesis in C. reinhardtii during the transition from P-deprivation to P-replete conditions

Phosphorus (P) assimilation and polyphosphate (polyP) synthesis were investigated in Chlamydomonas reinhardtii by supplying phosphate to P depleted cultures of a wild-type and mutants with defects in genes involved in the Vacuolar Transporter Chaperone (VTC) complex. Wild-type C. reinhardtii assimilated P and stored polyP within minutes of adding P to cultures that were P-deprived, demonstrating that these cells were metabolically primed to assimilate and store P. In contrast, vtc1 and vtc4 mutant lines assayed under the same conditions never accumulated polyP, which confirmed the bioinformatics inferences and previous experimental work that the VTC complex of C. reinhardtii has a polyP polymerase function. RNA-seq was carried out on C. reinhardtii cells that were either P-depleted (control) or supplied with P (treatment) in order to identify changes in the levels of mRNAs correlated with the P status. This analysis showed that transcript levels from VTC1 and VTC4 were strongly reduced at 5 and 24 h after the addition of P. The observation that polyP granules were synthesized during this 24 h period suggests that VTC complex functions for more than 5 h after supplying the cells with P.