Control of cell nuclear DNA replication by chloroplast and mitochondrion

Abstract

α-Proteobacteria and cyanobacteria endosymbiosis has been crucial to the evolution of eukaryotic cells. The descendants of these bacteria gave rise to mitochondria and chloroplasts, and these organelles still retain their own genome proliferation systems. Coordination between the proliferation processes of these organelles and the eukaryotic cell cycle is indispensable for cellular maintenance, and we have studied this using the red alga Cyanidioschyzon merolae. During the cell cycle progression of C. merolae, organelle DNA replication (ODR) in both of the mitochondrion and the chloroplast occurs prior to nuclear DNA replication (NDR). We found that Mg-protoporphyrin IX (Mg-ProtoIX), a type of tetrapyrrole synthesized in the chloroplast, accumulates with the onset of ODR, thereby inducing NDR. Binding of the F-box protein Fbx3 to Mg-ProtoIX was also shown to be involved in the polyubiquitination of Cyclin 1, which activates cyclin-dependent kinase. Moreover, Mg-ProtoIX-Fbx3 binding inhibits Fbx3-mediated polyubiquitination of Cyclin 1. These results suggest that Fbx3 is a receptor for Mg-ProtoIX in the chloroplast signal to the nucleus and that it appears to function as a checkpoint for the coordination of ODR and NDR. In this chapter, we discuss the ODR and NDR coordination system in the cell cycle.