Replication initiation

Cdc4 S. cerevisiae Cdc6 Replication initiation protein... 

Chaudhuri B, Xu H, Todorov 1, Dutta A, Yates JL (2001) Human DNA replication initiation factors, ORC and MCM, associate with oriP of Epstein-Barr virus. Proc Natl Acad Sci USA 98 10085-10089... 

T. Nenguke, M. I. Aladjem, J. F. Gusella, N. S. Wexler, and N. Arnheim, Candidate DNA replication initiation regions at human trinucleotide repeat disease loci. Hum. Mol. Genet. 12, 1021-1028 (2003). 

O. Hyrien, and M. Mechali, Chromosomal replication initiates and terminates at random sequences but at regular intervals in the ribosomal DNA of Xenopus early embryos. EMBO J. 12, 4511 520 (1993). 

C. Girard-Reydet, D. Gregoire, Y. Vassetzky, and M. Mechali, DNA replication initiates at domains overlapping with nuclear matrix attachment regions in the Xenopus and mouse c-myc promoter. Gene 332, 129-138 (2004). 

Komori, H., Matsunaga, E, Higuchi,Y, Ishiai, M., Wada, C. and Miki, K. (1999). Crystal structure of a prokaryotic replication initiator protein bound to DNA at 2.6 A resolution. EMBO J. 18,4597-d607. 

Control of origin activity occiu s via specific protein complexes that are boimd at certain times of the cell cycle to a replication origin. For replication initiation, two states of this protein complex are important, known as the pre-rephcation complex and the post-replication complex. 

The timing of replication initiation is affected by DNA methylation and interactions with the bacterial plasma membrane. The oriC DNA is methylated by the Dam methylase (Table 25-3), which methylates the Na position of adenine within the palindromic sequence (5 )GATC. (Dam is not a biochemical expletive it stands for DNA adenine methylation.) The oriC region of E. coli is highly enriched in GATC sequences—it has 11 of them in its 245 bp, whereas the average frequency of GATC in the E. coli chromosome as a whole is 1 in 256 bp. 

Boye, E., Lobner-Olesen, A., Skarstad, K. (2000) Limiting DNA replication to once and only once. EMBO Rep. 1, 479 83. Good summary of the mechanisms by which replication initiation is regulated. 

The basic mechanisms of replication implied in Eq. 27-3 seems to be universal, but several questions had to be asked. "Is replication initiated at a fixed point or points in a chromosome " and "Does replication occur in one direction only or do two forks form at the point... 

Amplification of DNA of chromosomes. During formation of oocytes parts of the DNA are "amplified" by repeated replication. This provides a way for the ovum to accumulate ribosomal RNA and various proteins in large amounts. Similarly, genes for two abundant proteins of the egg shell or chorion of insects are amplified. Bidirectional replication initiated at discrete positions yields an "onion skin" structure containing many copies of an 90-kb sequence containing the two genes. The polyploidy observed in some highly specialized cells such as the Purkinje cells... 

Two aspects of E. coli chromosomal replication still to be considered are initiation and termination. From what has been said we conjecture that replication initiates at a unique site, proceeds bidirectionally, and terminates at a point where the two oppositely advancing growth forks meet. To study initiation it was first necessary to isolate that segment... 

With circular minichromosomes containing this unique segment termed oriC, it has been possible to study the initiation and replication process in vitro. First, in crude extracts it was possible to show that replication initiates within or near the oriC sequence and proceeds bidirectionally. About 13 different proteins participate in the oriC-directed DNA replication as judged by their activity in a reconstituted replication reaction. 

Viruses of subgroup B replicate Initially In the nuclei and, later, also In the cytoplasm. Midgut and fat body are the principal tissues affected, but replication can occur also In hemocytes, tracheal epithelium, and epidermis. 

KP Lemon, I Kurtser, J Wu, AD Grossman. Control of initiation of sporulation by replication initiation genes in Bacillus subtilis. J Bacteriol 182 2989-2991, 2000. 

Under all nutrient conditions at a fixed temperature, the rate of replication-fork movement remains fairly constant. The overall rate of DNA replication is determined by the frequency of initiation at oriC. To accelerate replication, initiation at daughter origins occurs before the ongoing cycle of replication is completed. This results in the formation of a multiforked chromosome, as shown in Fig. 16-6. Note that a multiforked chromosome contains at least four copies of oriC. 

More recently, a covalently linked system exploits the replication initiator of bacteriophage P2, which covalently attaches the P2A protein to its own DNA phosphate backbone.306 This method is droplet independent, theoretically allowing libraries as large as those used in mRNA display but at present the system suffers from low complex formation efficiency ( 3%) that needs to be improved. 

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