![]() Moreover, other studies revealed the presence of a divergent ORC4 subunit in Trypanosoma, and also the absence of ORC subunits in other lineages ( Tiengwe et al. However, the position of excavates with respect to other eukaryotic groups is still uncertain ( Adl et al. Indeed, this was proposed as a synapomorphy in favor of an earlier origin of the lineage leading to Trypanosoma compared with other eukaryotes ( Cavalier-Smith 2010). However, this scenario was disputed by the finding of a simpler ORC in Trypanosoma (Excavata), consisting only of a CDC6/ORC1 protein and hence resembling that of Archaea ( Godoy et al. Given the presence of orthologs of CDC6 and all ORC1-5 subunits in distantly related lineages such as animals and plants, it was proposed that the whole multiprotein complex was likely to be a conserved feature, common to all eukaryotes ( Duncker et al. The ORC1, 2, 3, 4, and 5 subunits (ORC1-5) as well as the CDC6 protein possess an AAA+ ATPase domain and presumably evolved from an ancestral CDC6/ORC1 archaeal sequence ( Duncker et al. In eukaryotes, potential ORIs are specified by the formation of prereplication complexes by binding of the origin recognition complex (ORC) and the sequential assembly of Cell Division Cycle 6 (CDC6), CDC10-dependent transcription factor 1 (CDT1), and the minichromosome maintenance (MCM) protein complex ( Yeeles et al. ORIs in Bacteria and Archaea are marked by the DnaA and the CDC6/ORC1 AAA+ ATPases, respectively ( Marques et al. 2015), a process where the AAA+ ATPases are crucial and a common feature of Bacteria, Archaea, and eukaryotes ( Duderstadt and Berger 2008). The first event in DNA replication is the specification of potential DNA replication origins (ORIs) by the formation of a stable complex of initiator proteins ( Yeeles et al. Origin recognition complex (ORC), DNA replication, eukaryotic evolution, centriole, gene loss, parasitism, whole genome duplication IntroductionĭNA replication is essential for the maintenance of the genetic integrity in any cellular lineage. This, together with the PACT region not being significantly more conserved in centriole-bearing eukaryotes, supports the notion that this neofunctionalization of ORC1 would be a recent acquisition rather than an ancestral eukaryotic feature. We found that the proportion of centrioles to flagella and nuclei was not dramatically affected. In particular, we tested the consequences of reducing ORC1 levels in the centriole-containing green alga Chlamydomonas reinhardtii. We also investigated the evolutionary origin of the ORC1 role in centriole homeostasis mediated by the PACT region in human cells. Interestingly, parasites show significantly lower number of subunits than free-living eukaryotes, especially those with the lowest genome size and gene content metrics. Contrary to previous expectations, we found a global tendency in eukaryotes to increase or decrease the number of subunits as a consequence of genome duplications or streamlining, respectively. First, we identified ORC subunits previously undetected in divergent lineages, which allowed us to propose a series of parsimonious scenarios for the origin of this multiprotein complex. We have addressed these questions by reconstructing the distribution and evolutionary history of ORC1-5/CDC6 in a taxon-rich eukaryotic data set. Also, it is unclear whether the ancestral diversification of ORC in eukaryotes was accompanied by the neofunctionalization of some subunits, for example, role of ORC1 in centriole homeostasis. However, various uncertainties have arisen concerning ORC subunit composition in a variety of lineages. The conservation of orthologs of most subunits of the origin recognition complex (ORC) has served to propose that the whole complex is common to all eukaryotes.
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