Licensing factor

Last updated September 13, 2020

A licensing factor is a protein or complex of proteins that allows an origin of replication to begin DNA replication at that site. Licensing factors primarily occur in eukaryotic cells, since bacteria use simpler systems to initiate replication. However, many archaea use homologues of eukaryotic licensing factors to initate replication.^[1]

Function of licensing factors

Origins of replication represent start sites for DNA replication and so their "firing" must be regulated to maintain the correct karyotype of the cell in question. The origins are required to fire only once per cell cycle, an observation that led to the postulated existence of licensing factors by biologists in the first place. If the origins were not carefully regulated then DNA replication could be restarted at that origin giving rise to multiple copies of a section of DNA. This could be damaging to cells and could have detrimental effects on the organism as a whole.

The control that licensing factors exert over the cycle represents a flexible system, necessary so that different cell types in an organism can control the timing of DNA replication to their own cell cycles.

Location of licensing factors

The factors themselves are found in different places in different organisms. For example in metazoan organisms, they are commonly synthesised in the cytoplasm of the cell to be imported into the nucleus when required. The situation is different in yeast where the factors present are degraded and resynthesised throughout the cell cycle but are found in the nucleus for most of their existence.

Example of licensing factor action in Saccharomyces cerevisiae, brewers' yeast

Immediately after mitosis has finished the cell cycle starts again, entering G1 phase of the cycle. At this point protein synthesis of various products required for the rest of the cycle begins. Two of the proteins synthesised are called Cdc6 and Cdt1 and are only synthesised in G1 phase. These two together bind to the origin recognition complex (ORC), which is already bound at the origin and in fact never leaves these sites throughout the cycle. Now we have a so-called pre-replication complex, which then allows a heterohexameric protein complex of proteins MCM2 to 7 to bind. This entire hexamer acts as a helicase unwinding the double stranded DNA. At this point Cdc6 leaves the complex and is inactivated, by being degraded in yeast but by being exported from the nucleus in metazoans, triggered by CDK-dependent phosphorylation. The next steps included the loading of a variety of other proteins like MCM10, a CDK, DDK and Cdc45, the latter directly required for loading the DNA polymerase. During this period Cdt1 is released from the complex and the cell leaves G1 phase and enters S phase when replication starts.

From the above sequence we can see that Cdc6 and Cdt1 fulfill the role of licensing factors. They are only produced in G1 phase, in addition to which binding of all the proteins in this process excludes binding of additional copies. In this way their mode of action is limited to starting replication once, since once they have been ejected from the complex by other proteins, the cell enters S phase, during which they are not re-produced or re-activated. Thus they act as licensing factors, but only together. It has been suggested that the whole pre-replication complex be called the licensing factor since the whole is required for assembling additional proteins to initiate replication.

Related Research Articles

The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA and some of its organelles, and subsequently the partitioning of its cytoplasm and other components into two daughter cells in a process called cell division.

In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms acting as the most essential part for biological inheritance. The cell possesses the distinctive property of division, which makes replication of DNA essential.

The origin of replication is a particular sequence in a genome at which replication is initiated. Propagation of the genetic material between generations requires timely and accurate duplication of DNA by semiconservative replication prior to cell division to ensure each daughter cell receives the full complement of chromosomes. This can either involve the replication of DNA in living organisms such as prokaryotes and eukaryotes, or that of DNA or RNA in viruses, such as double-stranded RNA viruses. Synthesis of daughter strands starts at discrete sites, termed replication origins, and proceeds in a bidirectional manner until all genomic DNA is replicated. Despite the fundamental nature of these events, organisms have evolved surprisingly divergent strategies that control replication onset. Although the specific replication origin organization structure and recognition varies from species to species, some common characteristics are shared.

A pre-replication complex (pre-RC) is a protein complex that forms at the origin of replication during the initiation step of DNA replication. Formation of the pre-RC is required for DNA replication to occur. Complete and faithful replication of the genome ensures that each daughter cell will carry the same genetic information as the parent cell. Accordingly, formation of the pre-RC is a very important part of the cell cycle.

S phase (Synthesis Phase) is the phase of the cell cycle in which DNA is replicated, occurring between G₁ phase and G₂ phase. Since accurate duplication of the genome is critical to successful cell division, the processes that occur during S-phase are tightly regulated and widely conserved.

Cell cycle checkpoints are control mechanisms in the eukaryotic cell cycle which ensure its proper progression. Each checkpoint serves as a potential termination point along the cell cycle, during which the conditions of the cell are assessed, with progression through the various phases of the cell cycle occurring only when favorable conditions are met. There are many checkpoints in the cell cycle, but the three major ones are: the G1 checkpoint, also known as the Start or restriction checkpoint or Major Checkpoint; the G2/M checkpoint; and the metaphase-to-anaphase transition, also known as the spindle checkpoint. Progression through these checkpoints is largely determined by the activation of cyclin-dependent kinases by regulatory protein subunits called cyclins, different forms of which are produced at each stage of the cell cycle to control the specific events that occur therein.

A DNA unwinding element is the initiation site for the opening of the double helix structure of the DNA at the origin of replication for DNA synthesis. It is A-T rich and denatures easily due to its low helical stability, which allows the single-strand region to be recognized by origin recognition complex.

In molecular biology, origin recognition complex (ORC) is a multi-subunit DNA binding complex that binds in all eukaryotes and archaea in an ATP-dependent manner to origins of replication. The subunits of this complex are encoded by the ORC1, ORC2, ORC3, ORC4, ORC5 and ORC6 genes. ORC is a central component for eukaryotic DNA replication, and remains bound to chromatin at replication origins throughout the cell cycle.

Eukaryotic DNA replication is a conserved mechanism that restricts DNA replication to once per cell cycle. Eukaryotic DNA replication of chromosomal DNA is central for the duplication of a cell and is necessary for the maintenance of the eukaryotic genome.

The minichromosome maintenance protein complex (MCM) is a DNA helicase essential for genomic DNA replication. Eukaryotic MCM consists of six gene products, Mcm2–7, which form a heterohexamer. As a critical protein for cell division, MCM is also the target of various checkpoint pathways, such as the S-phase entry and S-phase arrest checkpoints. Both the loading and activation of MCM helicase are strictly regulated and are coupled to cell growth cycles. Deregulation of MCM function has been linked to genomic instability and a variety of carcinomas.

DNA replication licensing factor MCM7 is a protein that in humans is encoded by the MCM7 gene.

DNA replication licensing factor MCM2 is a protein that in humans is encoded by the MCM2 gene.

Cell division control protein 6 homolog is a protein that in humans is encoded by the CDC6 gene.

CDT1 is a protein that in humans is encoded by the CDT1 gene. It is a licensing factor that functions to limit DNA from replicating more than once per cell cycle.

Cell division cycle 7-related protein kinase is an enzyme that in humans is encoded by the CDC7 gene. The Cdc7 kinase is involved in regulation of the cell cycle at the point of chromosomal DNA replication. The gene CDC7 appears to be conserved throughout eukaryotic evolution; this means that most eukaryotic cells have the Cdc7 kinase protein.

Cdc6, or cell division cycle 6, is a protein in eukaryotic cells. It is mainly studied in the budding yeast Saccharomyces cerevisiae. It is an essential regulator of DNA replication and plays important roles in the activation and maintenance of the checkpoint mechanisms in the cell cycle that coordinate S phase and mitosis. It is part of the pre-replicative complex (pre-RC) and is required for loading minichromosome maintenance (MCM) proteins onto the DNA, an essential step in the initiation of DNA synthesis. In addition, it is a member of the family of AAA+ ATPases and highly related to ORC1.

A series of biochemical switches control transitions between and within the various phases of the cell cycle. The cell cycle is a series of complex, ordered, sequential events that control how a single cell divides into two cells, and involves several different phases. The phases include the G1 and G2 phases, DNA replication or S phase, and the actual process of cell division, mitosis or M phase. During the M phase, the chromosomes separate and cytokinesis occurs.

In cell biology, eukaryotes possess a regulatory system that ensures that DNA replication occurs only once per cell cycle.

Origin recognition complex subunit 1 is a protein that in humans is encoded by the ORC1 gene.

DNA re-replication is an undesirable and possibly fatal occurrence in eukaryotic cells in which the genome is replicated more than once per cell cycle. Rereplication is believed to lead to genomic instability and has been implicated in the pathologies of a variety of human cancers. To prevent rereplication, eukaryotic cells have evolved multiple, overlapping mechanisms to inhibit chromosomal DNA from being partially or fully rereplicated in a given cell cycle. These control mechanisms rely on cyclin-dependent kinase (CDK) activity. DNA replication control mechanisms cooperate to prevent the relicensing of replication origins and to activate cell cycle and DNA damage checkpoints. DNA rereplication must be strictly regulated to ensure that genomic information is faithfully transmitted through successive generations.

References

↑ Ausiannikava, Darya; Allers, Thorsten (February 2017). "Diversity of DNA Replication in the Archaea". Genes. 8 (2): 56. doi:10.3390/genes8020056. PMC 5333045 . PMID 28146124.

External links

Recent paper on licensing in human cells

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[1] Ausiannikava, Darya; Allers, Thorsten (February 2017). "Diversity of DNA Replication in the Archaea". Genes. 8 (2): 56. doi:10.3390/genes8020056. PMC 5333045 . PMID 28146124.