In late mitosis and early G1, Mcm2-7 complexes are loaded onto DNA to license replication origins for use in the upcoming S phase. However, the amount of Mcm2-7 loaded is in significant excess over the number of origins normally used. We show here that in human cells, excess chromatin-bound Mcm2-7 license dormant replication origins that do not fire during normal DNA replication, in part due to checkpoint activity. Dormant origins were activated within active replicon clusters if replication fork progression was inhibited, despite the activation of S-phase checkpoints. After lowering levels of chromatin-bound Mcm2-7 in human cells by RNA interference (RNAi), the use of dormant origins was suppressed in response to replicative stress. Although cells with lowered chromatin-bound Mcm2-7 replicated at normal rates, when challenged with replication inhibitors they had dramatically reduced rates of DNA synthesis and reduced viability. These results suggest that the use of dormant origins licensed by excess Mcm2-7 is a new and physiologically important mechanism that cells utilize to maintain DNA replication rates under conditions of replicative stress. We propose that checkpoint kinase activity can preferentially suppress initiation within inactive replicon clusters, thereby directing new initiation events toward active clusters that are experiencing replication problems.[Keywords: Replication origins; dormant origins; Mcm2-7; genetic stability] Supplemental material is available at http://www.genesdev.org.

Background: 1,25(OH) 2 D 3 inhibits NF-B activation by an undefined mechanism. Results: Vitamin D receptor protein binds to IKK␤ protein, blocking TNF␣-induced IKK complex formation and NF-B activity. Conclusion: The vitamin D receptor suppresses NF-B activation by directly interacting with IKK␤.Significance: This is a novel mechanism whereby 1,25(OH) 2 D 3 -VDR inhibits NF-B.

Many replication origins that are licensed by loading MCM2-7 complexes in G1 are not normally used. Activation of these dormant origins during S phase provides a first line of defence for the genome if replication is inhibited. When replication forks fail, dormant origins are activated within regions of the genome currently engaged in replication. At the same time, DNA damage response kinases activated by the stalled forks preferentially suppress the assembly of new replication factories, thereby ensuring that chromosomal regions experiencing replicative stress complete synthesis before new regions of the genome are replicated. Mice expressing reduced levels of MCM2-7 have fewer dormant origins, are cancer prone and are genetically unstable, thus demonstrating the importance of dormant origins for preserving genome integrity. Here we review the function of dormant origins, the molecular mechanism of their regulation and their physiological implications. The problem of ensuring precise genome duplicationDuring S phase of the metazoan cell cycle, replication forks are initiated at replication origins that are organised into clusters, each comprising 2-5 adjacent origins. A timing programme sequentially activates different clusters, thereby leading to the complete duplication of the genome (Figure 1, 'normal replication'). To preserve genome integrity, it is critical that these origins are properly regulated. Unless a sufficient number of origins and origin clusters are activated, there is a danger that sections of the genome remain unreplicated when cells enter mitosis . It is also critical that replication origins fire no more than once, and never fire on sections of DNA that have already been replicated, otherwise DNA would be amplified in the vicinity of the over-firing origin (Figure 1,. Cells prevent re-replication of sections of DNA by dividing the process of replication into two non-overlapping phases ( Figure 2) [1][2][3]. From late mitosis until the end of G1, before DNA synthesis begins, cells license replication origins for use in the upcoming S phase by loading them with double hexamers of the MCM2-7 (minichromosome maintenance) proteins. During S phase, MCM2-7 complexes are activated to form a central part of the helicase that unwinds DNA at the replication fork [4]. As active MCM2-7 complexes move with the replication fork, replicated origins are converted to the unlicensed state. Because no more MCM2-7 can be loaded onto DNA once S phase has started, no origin can fire more than once in a single S phase [1,2]. Cells rely on the presence of MCM2-7 to mark origin DNA that has not been replicated in the current cell cycle.Corresponding author: Julian Blow (j.j.blow@dundee.ac.uk). Thus, it is important for cells to ensure that sufficient origins are licensed before entering S phase. This is accomplished by a checkpoint (the "licensing checkpoint") which monitors the number of licensed origins in G1, and delays entry into S phase if the number is insufficient [5,6]. In addition to being regulated...

At low levels of replication stress, Chk1 favors resolving problems at stalled replication forks over initiating origin firing in unreplicated areas of the genome.