Fig. 5.

A schematic of a tentative mechanism for CAS activity in Chlamydomonas retrograde signalling. (A) Under low light/high CO₂ conditions, CAS is dispersed throughout the chloroplast. (B) Under high light/low CO₂, the ETC proteins become over-reduced, triggering the movement of CAS into the pyrenoid along the pyrenoid tubules. In the Ca²⁺-rich pyrenoid, CAS binds to Ca²⁺ and becomes activated. This form of CAS signals back to the nucleus to modulate target genes. It also induces an increase in intracellular Ca²⁺.