Mouse homolog of the yeast origin recognition complex subunit ORC1 and chromosomal localization of the cognate mouse gene Orc1

Interaction and assembly of murine pre-replicative complex proteins in yeast and mouse cells

Eukaryotic cells coordinate chromosome duplication by the assembly of protein complexes at origins of DNA replication by sequential binding of member proteins of the origin recognition complex (ORC), CDC6, and minichromosome maintenance (MCM) proteins. These pre-replicative complexes (pre-RCs) are activated by cyclin-dependent kinases and DBF4/CDC7 kinase. Here, we carried out a comprehensive yeast two-hybrid screen to establish sequential interactions between two individual proteins of the mouse pre-RC that are probably required for the initiation of DNA replication. The studies revealed multiple interactions among ORC subunits and MCM proteins as well as interactions between individual ORC and MCM proteins. In particular CDC6 was found to bind strongly to ORC1 and ORC2, and to MCM7 proteins. DBF4 interacts with the subunits of ORC as well as with MCM proteins. It was also demonstrated that CDC7 binds to different ORC and MCM proteins. CDC45 interacts with ORC1 and ORC6, and weakly with MCM3, -6, and -7. The three subunits of the single-stranded DNA binding protein RPA show interactions with various ORC subunits as well as with several MCM proteins. The data obtained by yeast two-hybrid analysis were paradigmatically confirmed in synchronized murine FM3A cells by immunoprecipitation of the interacting partners. Some of the interactions were found to be cell-cycle-dependent; however, most of them were cell-cycle-independent. Altogether, 90 protein-protein interactions were detected in this study, 52 of them were found for the first time in any eukaryotic pre-RC. These data may help to understand the complex interplay of the components of the mouse pre-RC and should allow us to refine its structural architecture as well as its assembly in real time.

Initiation of eukaryotic DNA replication: regulation and mechanisms

The accurate and timely duplication of the genome is a major task for eukaryotic cells. This process requires the cooperation of multiple factors to ensure the stability of the genetic information of each cell. Mutations, rearrangements, or loss of chromosomes can be detrimental to a single cell as well as to the whole organism, causing failures, disease, or death. Because of the size of eukaryotic genomes, chromosomal duplication is accomplished in a multiparallel process. In human somatic cells between 10,000 and 100,000 parallel synthesis sites are present. This raises fundamental problems for eukaryotic cells to coordinate the start of DNA replication at each origin and to prevent replication of already duplicated DNA regions. Since these general phenomena were recognized in the middle of the 20th century the regulation and mechanisms of the initiation of eukaryotic DNA replication have been intensively investigated. These studies were carried out to find the essential factors involved in the process and to determine their functions during DNA replication. These studies gave rise to a model of the organization and the coordination of DNA replication within the eukaryotic cell. The elegant experiments carried out by Rao and Johnson (1970) (1), who fused cells in different phases of the cell cycle, showed that G1 cells are competent for replication of their chromosomes, but lack a specific diffusible factor required to activate their replicaton machinery and showed that G2 cells are incompetent for DNA replication. These findings suggested that eukaryotic cells exist in two states. In G1 phase, cells are competent to initiate DNA replication, which is subsequently triggered in S phase. After completion of S phase, cells in G2 are no longer able to initiate DNA replication and they require a transition through mitosis to reenable initiation of DNA replication to take place in the next S phase. The Xenopus cell-free replication system has proved a good model system in which to study DNA replication in vitro as well as the mechanism preventing rereplication within a single cell cycle (2). Studies using this system resulted in the development of a model postulating the existence of a replication licensing factor, which binds to chromatin before the G1-S transition and which is displaced during replication (2, 3). These results were supported by genetic and biochemical experiments in Saccharomyces cerevisiae (budding yeast) and Schizosaccharomyces pombe (fission yeast) (4, 5). The investigation of cell division cycle mutants and the budding yeast origin of replication resulted in the concept of a prereplicative and a postreplicative complex of initiation proteins (6-9). These three individual concepts have recently started to merge and it has become obvious that initiation in eukaryotes is generally governed by the same ubiquitous mechanisms.

Genes involved in the initiation of DNA replication in yeast

Replication and segregation of the information contained in genomic DNA are strictly regulated processes that eukaryotic cells alternate to divide successfully. Experimental work on yeast has suggested that this alternation is achieved through oscillations in the activity of a serine/threonine kinase complex, CDK, which ensures the timely activation of DNA synthesis. At the same time, this CDK-mediated activation sets up the basis of the mechanism that ensures ploidy maintenance in eukaryotes. DNA synthesis is initiated at discrete sites of the genome called origins of replication on which a prereplicative complex (pre-RC) of different protein subunits is formed during the G1 phase of the cell division cycle. Only after pre-RCs are formed is the genome competent to be replicated. Several lines of evidence suggest that CDK activity prevents the assembly of pre-RCs ensuring single rounds of genome replication during each cell division cycle. This review offers a descriptive discussion of the main molecular events that a unicellular eukaryote such as the budding yeast Saccharomyces cerevisiae undergoes to initiate DNA replication.

Identification and chromosomal localization of murine ORC3, a new member of the mouse origin recognition complex

A new member of the murine origin recognition complex (ORC) related to Saccharomyces cerevisiae ORC3 has been cloned. Transcription of ORC3 is not suppressed in mouse NIH3T3 fibroblasts made quiescent by serum starvation. The transcription level of the ORC3 gene is constantly high in all phases of the cell cycle. Murine ORC3 protein contains a putative nuclear localization signal and a non-basic helix-loop-helix motif. Both motifs are conserved in eukaryotes. A potential dimerization partner of ORC3p in the murine ORC complex is ORC1p which also contains an HLH motif. This HLH motif is also highly conserved in all eukaryotic ORC1 proteins. Comparison of murine ORC3p with other ORC3-related proteins shows high amino acid homology and motif conservation leading to the conclusion that ORC3p is part of the initiation machinery conserved in eukaryotes. The mouse ORC3 gene Orc3 was assigned to mouse chromosome 4A3 by fluorescence in situ hybridization (FISH) analysis.

Identification and characterization of a mouse homolog to yeast Cdc6p

Periodic expression of the Cdc6 protein is essential for the entry of budding yeast cells into S phase, and also for participating in checkpoint controls that ensure that DNA replication is completed before mitosis is initiated. We have identified a mouse protein closely related to Cdc6p (MmCdc6p) as well as to its human and Xenopus homologs. The gene coding for MmCdc6p (Cdc6) is located at band D on murine chromosome 11. Analysis of its genomic region revealed that the 13-kb Cdc6 gene is divided into 12 exons by 11 introns. MmCdc6p has putative cyclin-dependent phosphorylation sites, a destruction box, nuclear localization signals, a nucleotide triphosphate-binding motif, and a potential leucine zipper. None of these consensus motifs except the leucine-zipper and the destruction box overlaps an intron. Expression of MmCdc6 mRNA and protein is suppressed in mouse NIH3T3 fibroblasts made quiescent by serum starvation. Upon replenishment of the medium, transcript and protein levels increase during progression through G(1), peaking as cells enter S phase. MmCdc6p is phosphorylated in vitro by cdk1/cyclin B, cdk4/cyclin D, cdk2/cyclin E, and cdk2/cyclin A, respectively at serine-residues. In vivo however, phosphorylation of MmCdc6p is carried out by cdk2/cyclin A at serine-residues exclusively. Conservation of structures among members of the Cdc6-related proteins suggests that these proteins play a key role in the regulation of DNA replication during the cell cycle in all eukaryotes. These results strongly suggest, that Cdc6p plays an important role in cell cycle regulation and replication licensing.