FIGURE 1.

Evolution of the coupling between ODR and NDR. (A) Prior to endosymbiosis, the ancestral cyanobacteria were living phototrophically and dependent on light, while the host eukaryotic cells were proliferating independent of light conditions. After the engulfment, the primitive chloroplast would have required light for proliferation, while the host did not. Therefore, the uniquely acquired chloroplast may have been lost if the host cell could proliferate in the dark. A newly evolved checkpoint system, “light checkpoint” in the figure, prevented host proliferation in the dark to maintain the endosymbiotic association. (B) Architecture of the light checkpoint. Under dark conditions, an F-box component of E3-ubiquitin ligase, Fbx3, recognizes Cyclin1, resulting in ubiquitination and prompt degradation. Cyclin1 is the cyclin that forms a complex with CDKA and activates NDR, and thus, degradation of Cyclin1 prevents NDR in the dark. Light illumination somehow activates ODR and results in the accumulation of Mg-ProtoIX in the cytosol. Interaction of Mg-ProtoIX with Fbx3 prevents the ubiquitination of Cyclin1 and induces the activation of CDKA and NDR.