Eukaryotic cells arose from an ancient endosymbiotic association of prokaryotes, with plant cells harboring 3 genomes as the remnants of such evolution. In plant cells, plastid and mitochondrial DNA replication [organelle DNA replication (ODR)] occurs in advance of the subsequent cell cycles composed of nuclear DNA replication (NDR) and cell division. However, the mechanism by which replication of these genomes with different origins is coordinated is largely unknown. Here, we show that NDR is regulated by a tetrapyrrole signal in plant cells, which has been suggested as an organelle-to-nucleus retrograde signal. In synchronized cultures of the primitive red alga Cyanidioschyzon merolae, specific inhibition of A-type cyclin-dependent kinase (CDKA) prevented NDR but not ODR after onset of the cell cycle. In contrast, inhibition of ODR by nalidixic acid also resulted in inhibition of NDR, indicating a strict dependence of NDR on ODR. The requirement of ODR for NDR was bypassed by addition of the tetrapyrrole intermediates protoporphyrin IX (ProtoIX) or Mg-ProtoIX, both of which activated CDKA without inducing ODR. This scheme was also observed in cultured tobacco cells (BY-2), where inhibition of ODR by nalidixic acid prevented CDKA activation and NDR, and these inhibitions were circumvented by Mg-ProtoIX without inducing ODR. We thus show that tetrapyrrole-mediated organelle-nucleus replicational coupling is an evolutionary conserved process among plant cells.Cyanidioschyzon merolae ͉ retrograde signal ͉ tobacco BY-2 T he origin of eukaryotic cells is still unclear, but it is now generally accepted that mitochondria and plastids arose from endosymbiosis of rickettsia-like ␣-proteobacteria and cyanobacteria-like photosynthetic bacteria, respectively (1). Many of the genes of these endosymbionts, including those for DNA replication and maintenance of genomic integrity, were subsequently lost or transferred to the nuclear genome [endosymbiotic gene transfer (EGT)]. According to current dogma, the transfer of genes of endosymbiotic origin to the nucleus that are required for organelle DNA replication (ODR) has resulted in loss of the independence of the cell cycles of the endosymbionts and in their integration into the eukaryotic control system that is mediated largely by cyclins and cyclin-dependent kinase (CDK). However, coordination of cell-cycle events such as DNA replication would have been essential for establishing integrity of eukaryotic cells, at least during the early stages of endosymbiotic association. Although there is little evidence for discrete cell-cycle control of ODR in animal and fungal cells, studies with algae and flowering plants have shown that ODR precedes the subsequent cell proliferation cycles composed of nuclear DNA replication (NDR) and cell division (2-4).C. merolae is an unicellular red alga, each cell of which contains 1 plastid, 1 mitochondrion, and 1 nucleus. The advantage of studying nuclear and organelle proliferation cycles in C. merolae is that the cultures can be highly synchro...