The present application describes the complete 1.66-megabase pair genome sequence of an autotrophic archaeon, Methanococcus jannaschii, and its 58- and 16-kilobase pair extrachromosomal elements. Also described are 1738 predicted protein-coding genes.

https://science.sciencemag.org/content/sci/273/5278/1058.full.pdf

COMPLETE GENOME SEQUENCE OF THE METHANOGENIC ARCHAEON, $i(METHANOCOCCUS JANNASCHII)

▪ Abstract The maintenance of the eukaryotic genome requires precisely coordinated replication of the entire genome each time a cell divides. To achieve this coordination, eukaryotic cells use an ordered series of steps to form several key protein assemblies at origins of replication. Recent studies have identified many of the protein components of these complexes and the time during the cell cycle they assemble at the origin. Interestingly, despite distinct differences in origin structure, the identity and order of assembly of eukaryotic replication factors is highly conserved across all species. This review describes our current understanding of these events and how they are coordinated with cell cycle progression. We focus on bringing together the results from different organisms to provide a coherent model of the events of initiation. We emphasize recent progress in determining the function of the different replication factors once they have been assembled at the origin.

DNA replication in eukaryotic cells

Evidence obtained from studies with yeast and Xenopus indicate that the initiation of DNA replication is a multistep process. The origin recognition complex (ORC), Cdc6p, and minichromosome maintenance (MCM) proteins are required for establishing prereplication complexes, upon which initiation is triggered by the activation of cyclin-dependent kinases and the Dbf4p-dependent kinase Cdc7p. The identification of human homologues of these replication proteins allows investigation of S-phase regulation in mammalian cells. Using centrifugal elutriation of several human cell lines, we demonstrate that whereas human Orc2 (hOrc2p) and hMcm proteins are present throughout the cell cycle, hCdc6p levels vary, being very low in early G(1) and accumulating until cells enter mitosis. hCdc6p can be polyubiquitinated in vivo, and it is stabilized by proteasome inhibitors. Similar to the case for hOrc2p, a significant fraction of hCdc6p is present on chromatin throughout the cell cycle, whereas hMcm proteins alternate between soluble and chromatin-bound forms. Loading of hMcm proteins onto chromatin occurs in late mitosis concomitant with the destruction of cyclin B, indicating that the mitotic kinase activity inhibits prereplication complex formation in human cells.

Chromatin Association of Human Origin Recognition Complex, Cdc6, and Minichromosome Maintenance Proteins during the Cell Cycle: Assembly of Prereplication Complexes in Late Mitosis

The plant Arabidopsis thaliana (Arabidopsis) has become an important model species for the study of many aspects of plant biology. The relatively small size of the nuclear genome and the availability of extensive physical maps of the five chromosomes provide a feasible basis for initiating sequencing of the five chromosomes. The YAC (yeast artificial chromosome)-based physical map of chromosome 4 was used to construct a sequence-ready map of cosmid and BAC (bacterial artificial chromosome) clones covering a 1.9-megabase (Mb) contiguous region, and the sequence of this region is reported here. Analysis of the sequence revealed an average gene density of one gene every 4.8 kilobases (kb), and 54% of the predicted genes had significant similarity to known genes. Other interesting features were found, such as the sequence of a disease-resistance gene locus, the distribution of retroelements, the frequent occurrence of clustered gene families, and the sequence of several classes of genes not previously encountered in plants.

Analysis of 1.9 Mb of contiguous sequence from chromosome 4 of Arabidopsis thaliana

The initiation of DNA replication in eukaryotic cells is a highly regulated process that leads to the duplication of the genetic information for the next cell generation. This requires the ordered assembly of many proteins at the origins of DNA replication to form a competent, pre-replicative chromosomal state. In addition to this competent complex, at least two cell cycle regulated protein kinase pathways are required to affect a transition to a post-replicative chromosomal state. Protein kinases required to establish mitosis prevent re-replication of the DNA. As cells exit mitosis, the cell cycle is reset, allowing the establishment of a new, competent replication state.

https://science.sciencemag.org/content/sci/274/5293/1659.full.pdf

Cell Cycle Control of DNA Replication

The origin recognition complex (ORC), a multisubunit protein identified in Saccharomyces cerevisiae, binds to chromosomal replicators and is required for the initiation of cellular DNA replication Complementary DNAs (cDNAs) encoding proteins related to the two largest subunits of ORC were cloned from various eukaryotes The cDNAs encoding proteins related to S cerevisiae Orc1p were cloned from the budding yeast Kluyveromyces lactis, the fission yeast Schizosaccharomyces pombe, and human cells These proteins show similarity to regulators of the S and M phases of the cell cycle Genetic analysis of orc1+ from S pombe reveals that it is essential for cell viability The cDNAs encoding proteins related to S cerevisiae Orc2p were cloned from Arabidopsis thaliana, Caenorhabditis elegans, and human cells The human ORC-related proteins interact in vivo to form a complex These studies studies suggest that ORC subunits are conserved and that the role of ORC is a general feature of eukaryotic DNA replication

http://science.sciencemag.org/content/sci/270/5242/1667.full.pdf

Kimberley A. Gavin

Papers

Conserved Initiator Proteins in Eukaryotes