Epstein–Barr virus (EBV) replicates in its latent phase once per cell cycle in proliferating B cells. The latent origin of DNA replication, oriP, supports replication and stable maintenance of the EBV genome. OriP comprises two essential elements: the dyad symmetry (DS) and the family of repeats (FR), both containing clusters of binding sites for the transactivator EBNA1. The DS element appears to be the functional replicator. It is not yet understood how oriP‐dependent replication is integrated into the cell cycle and how EBNA1 acts at the molecular level. Using chromatin immunoprecipitation experiments, we show that the human origin recognition complex (hsORC) binds at or near the DS element. The association of hsORC with oriP depends on the DS element. Deletion of this element not only abolishes hsORC binding but also reduces replication initiation at oriP to background level. Co‐immunoprecipitation experiments indicate that EBNA1 is associated with hsORC in vivo. These results indicate that oriP might use the same cellular initiation factors that regulate chromosomal replication, and that EBNA1 may be involved in recruiting hsORC to oriP.

The Cdc6 protein is essential for the assembly of pre-replicative complexes (pre-RCs) at origins of DNA replication in the budding yeast Saccharomyces cerevisiae. This reaction is blocked in vivo by the cyclin-dependent kinase Cdc28p, together with its regulatory subunits, the B type cyclins that are present throughout S, G2, and M phases. Because the destruction of B type cyclins and the consequent inactivation of the kinase are essential for exit from mitosis, pre-RC formation can only occur after passage through mitosis. Therefore, pre-RC formation has been proposed to be essential for coupling S phase and mitosis and for limiting DNA replication to once per cell cycle. The Mcm2-7 family of proteins has been implicated in limiting replication to once per cell cycle from experiments with Xenopus egg extracts. Here we show that the Mcm proteins of budding yeast are abundant and are quantitatively found in a chromatin-enriched fraction specifically during the G1 phase of the cell cycle. This chromatin binding depends on the de novo synthesis of Cdc6p, providing evidence that a conserved biochemical pathway plays a critical role in coordinating DNA replication with mitosis in both yeast and higher eukaryotes. Cdc6p and the origin recognition complex can be selectively removed from this chromatin-enriched fraction without removing the Mcm proteins. From these results, we propose that Cdc6p (and the origin recognition complex) nucleates the binding of Mcm proteins to chromatin, but once bound, the Mcm proteins appear to interact tightly with some other component of chromatin.

DNA replication is a pivotal event in the cell cycle and, as a consequence, is tightly controlled in eukaryotic cells. The initiation of DNA replication is dependent upon the completion of mitosis and upon the commitment to complete the cell cycle made during G(1). Characterisation of the protein factors required for initiating DNA replication is essential to understand how the cell cycle is regulated. Recent results indicate that initiation complexes assemble in multiple stages during the cell cycle. First, origins are bound by the multisubunit origin recognition complex (ORC) which is essential for DNA replication in vivo. ORC, present at little more than one complete complex per replication origin, binds to origins immediately after initiation in the previous cell cycle. ORC binding occurs by the recognition of a bipartite sequence that includes the essential ARS consensus sequence (ACS) and the functionally important B1 element adjacent to the ACS. A novel pre-replicative complex (pre-RC) assembles at origins at the end of mitosis in actively cycling cells and remains at origins until DNA replication initiates. Finally, Dbf4, which is periodically synthesised at the end of G(1), interacts with replication origins. Dbf4-origin interaction requires an intact ACS strongly suggesting that interaction occurs through ORC. Dbf4 interacts with and is required for the activation of the Cdc7 protein kinase and together, Dbf4 and Cdc7 are required for the G(1)-S transition. Separate regions of Dbf4 are required for Cdc7- and origin-interaction suggesting that Dbf4 may act to recruit Cdc7 to replication origins where phosphorylation of some key component may cause origin firing.

Characterization of the proteins that interact with replication origins, as well as characterization of the mechanisms by which the levels and activities of these proteins are regulated during the cell cycle, is required to understand the initiation of chromosomal DNA replication in eukaryotic cells. We have previously shown that the first detectable step in the assembly of initiation complexes in vivo involves the binding of the multisubunit origin recognition complex (ORC) and the general transcription/replication factor ABF1 protein to origins. In this paper we show that ORC is present in cells at low levels, corresponding to little more than one complete complex per replication origin, indicating that in vivo origin recognition by ORC is extremely efficient, We show that this efficient recognition requires two sequence elements, the essential A element containing the ARS consensus sequence and the functionally important B1 element, both in vitro and in vivo. Moreover, we show that origin binding by ORC in vivo does not require any other functional sequence element, indicating that it occurs independently of the binding of other factors, such as ABF1, Our results suggest a model for the roles of the individual elements of yeast replication origins.

The development of the polymerase chain reaction (PCR) has allowed the rapid isolation of DNA sequences utilizing the hybridization of two oligonucleotide primers and subsequent amplification of the intervening sequences by Taq polymerase. There are many applications of this technique. One of the most useful is the screening of large numbers of samples in the search for mutations at a defined locus, for example in clinical studies or in the analysis of cultured cell lines (1–3). In the absence of PCR, this can only be achieved by isolating DNA from each individual, and making and screening a library. The use of PCR means that whereas previously it may have taken a month to examine each individual sample, it is now possible to examine many samples in a few days. In addition, by using redundant primers, it is even possible to isolate related novel genes.

THE HMR E silencer represses transcription of silent mating-type genes in the budding yeast Saccharomyces cerevisiae and contains three redundant regulatory elements A, E and B (ref. 1). The A element contains the 11 base pair consensus sequence that is essential for the firing of DNA replication origins2. A multisubunit protein called the origin recognition complex (ORC) binds specifically to this consensus sequence within yeast origins in vitro3 and in vivo4. We isolated mutants in A element-mediated silencing and report here that one of the genes we identified, RRR1, encodes ORC2, the 72K subunit of ORC. RRR1/ORC2 is an essential gene, but the rrr1-316 allele, which is viable, is defective in the replication of nuclear DNA and the maintenance of the 2-mum episomal DNA. This is, to our knowledge, the first genetic evidence that ORC is involved in DNA replication and silencing.

The nature of recessive mutations at the autosomal locus encoding the purine salvage enzyme adenine phosphoribosyl transferase (APRT) was analyzed in a highly malignant human tumor cell line (the colorectal carcinoma line SW620). Mutant strains resistant to the purine analog 8-azaadenine were obtained in two steps. The first step selection for partial drug resistance produced strains hemizygous for APRT as a result of high frequency loss of one allele. In the second step selection, low frequency base substitutions, small deletions, or insertions produced complete azaadenine resistance. Luria-Delbruck fluctuation analysis of each step of this process indicated that the rate of mutation resulting from allele loss was over 100-fold greater than the rate of mutation resulting from base substitution. There was no reproducible difference in the rate of loss of either of the two APRT alleles even though one maps to a rearranged chromosome. Similarly base substitution rates for the two alleles were not significantly different. Polymorphic loci surrounding APRT on chromosome 16 band q24 were lost together with the selected gene in all isolates while polymorphic loci on the short arm of the chromosome were retained. Thus the high frequency loss of APRT in SW620 appears to be the result of multilocus deletions. SW620 derivatives behaving as heterozygotes were also obtained in the first step selections, but these constituted only 5% of isolates.

We analyzed the nature of spontaneous mutations at the autosomal locus coding for adenine phosphoribosyltransferase in the human colorectal carcinoma cell line SW620 to establish whether distinctive mutational pathways exist that might underlie the more complex genome rearrangements arising in tumor cells. Point mutations occur at a low rate in aprt hemizygotes derived from SW620, largely as a result of base substitutions at G.C base pairs to yield transversions and transitions. However, a novel pathway is evident in the form of multiple dispersed mutations in which two errors, separated by as much as 1,800 bp, fall in the same mutant gene. Such mutations could be the result of error-prone DNA synthesis occurring during normal replication or during long-patch excision-repair of spontaneously arising DNA lesions. This process could also contribute to the chromosomal instability evident in these tumor cells.

The usefulness of the purine and pyrimidine salvage pathways in the study of the mechanisms of mutation and in the selection of cell lines stably transformed by vectors expressing these genes is well documented. Unfortunately, many investigators are deterred from selecting new host strains deficient in these enzymes because of the difficulties inherent in isolating recessive mutations of autosomal genes. Furthermore, considerable suspicion was cast over somatic cell genetics by the so-called epigenetic nature of some phenotypic changes (1). However, given the clear molecular basis of the vast majority of mutant phenotypes, such apprehensions are largely unwarranted, provided that careful, clean selections are employed (e.g., see. 2).