Initiation of DNA replication

Blow, J. J., and Nurse, P. (1990). Acdc2-like protein is involved in the initiation of DNA replication in Xenopus egg extracts. Cell 62 855-862. 

In summary, we showed that DNA damage can elicit a variety of responses in Xenopus early embryos. While y-irradiation induces apoptosis in the embryos, DSB-containing DNA prevents initiation of DNA replication in cell-free extracts derived from eggs. 

Primer A short length of single-stranded DNA or sometimes RNA (usually 20 bases which can be synthesized) which is complementary to a known DNA sequence so that it can bind there and serve for the initiation of DNA replication. Promoter Usually a specific region of DNA at which RNA-polymerase binds and initiates transcription. 

Montagnoli A, ValsasrnaB, CrociVet al (2008) A Cdc7 kinase inhibitor restricts initiation of DNA replication and has antitumor activity. Nat Chem Biol 4 357-365... 

Artificial chromosomes (Chapter 9) have been constructed as a means of better understanding the functional significance of many structural features of eukaryotic chromosomes. A reasonably stable artificial linear chromosome requires only three components a centromere, telomeres at each end, and sequences that allow the initiation of DNA replication. Yeast artificial chromosomes (YACs see Fig. 9-8) have been developed as a research tool in biotechnology. Similarly, human artificial chromosomes (HACs) are being developed for the treatment of genetic diseases by somatic gene therapy. 

Donachie WD (1968) Relationship between cell size and time of initiation of DNA replication. Nature 219 1077-1079... 

Rifampicin was first shown by Hartmann et al. 54 to have a specific inhibitory effect on RNA polymerase from E. coli. Later, other active ansamycins were found and RNA polymerases from a large variety of bacteria other than E. coli proved to be sensitive to the drug. More recently, an RNA polymerase from E. coli containing only one subunit and probably involved in the initiation of DNA replication (dna G gene product) has been shown to be resistant to rifampicin5 s This holds true also for the various mammalian RNA polymerases. In contrast to non-specific inhibitors of transcription such as actinomycin and mitomycin, rifampicin interacts specifically with the bacterial enzyme itself. With the aid of 14C-labelled rifampicin it could be shown that the drug forms a very stable complex with the enzyme in a molar ratio of 1 1S6> 57 The dissociation constant of this complex is 10-9 M at 37 °C and... 

Martins, S., Eikvar, S., Fumkawa, K. and Collas, P. (2003) HA95 and LAP2 beta mediate a novel chromatin-nuclear envelope interaction implicated in initiation of DNA replication. J. Cell Biol. 160, 177-188. 

Sharova, N.P., and E.B. Abramova. Initiation of DNA replication in eukaryotes is an intriguing cascade of protein interactions. Biochemistry 2002 67 1217-1223. 

The polymerase holoenzyme assembles. The DNA polymerase ITT holoen-zyme assembles on the prepriming complex, initiated by interactions between DnaB and the sliding clamp subunit of DNA polymerase 111. These interactions also trigger ATP hydrolysis within the DnaA subunits, signaling the initiation of DNA replication. The breakup of the DnaA assembly prevents additional rounds of replication from beginning at the replication origin. 

Cyclin A is assigned a special role in the progess of S phase and transition into G2 phase. Cyclin A binds and activates CDK2. The CDK2-cyclin A complex binds to the transcription factor E2F-1 and phosphorylates its DP-1 subunit. As a consequence, the DNA-binding capacity of the transcription factor is reduced and the transcription-activating function is inhibited. Furthermore, cyclin A-CDK2 complexes are involved in the phosphorylation of protein complexes involved in the initiation of DNA replication. 

Masai, H. and Arai, K. (2002) Cdc7 kinase complex a key regulator in the initiation of DNA replication. J. Cell Physiol, 190, 287-296. 

Primer synthesis. The formation of short RNA segments called primers, required for the initiation of DNA replication, is catalyzed by primase, an RNA polymerase. Primase is a 60-kD polypeptide product of the dnaG gene. A multienzyme complex containing primase and several auxiliary proteins is called the primosome. 

Entry into the S phase is defined by the Initiation of DNA replication. In S. cerevisiae cells this occurs when the Sid inhibitor is precipitously degraded following Its polyubiqui-tination by a distinct ubiquitin ligase called SCF (Figure... 

S. cerevisiae by regulated proteolysis of the S-phase inhibitor Sicl. The S-phase cyclin-CDK complexes (Clb5-CDK and Clb6-CDK) begin to accumulate in G-, but are inhibited by Sicl. This inhibition prevents initiation of DNA replication until the cell is fully prepared. G- cyclin-CDK complexes assembled in late G- ... 

Initiation of DNA replication occurs at each origin, but only once, until a cell proceeds through anaphase when activation of APC leads to the degradation of B-type cyclins. The block on reinitiation of DNA replication until replicated chromosomes have segregated assures that daughter cells contain the proper number of chromosomes per cell. 

Initiation of DNA replication in eukaryotes appears to be more complex than what has been found in bacteria. 

DNA replication at the ARS origin in yeast. This sequence corresponds to a binding site for the yeast transcription factor ABF1. Studies with eukaryotic viral origins of replication have suggested that some of the proteins involved in RNA synthesis (also called transcription factors, see Chapter 24) have dual functions in transcription and initiation of DNA replication. Current models suggest that transcription factors may be necessary to recruit replication proteins to the origin. 

See also Glycerophospholipid Pathway, Glycerophospholipids, Glycerol, Phosphatidylglycerol. Initiation of DNA Replication... 

In prokaryotic DNA the major methylated bases are N -methyladenine (mA) and to a lesser extent Nl-methylcytosine. Methylation in bacteria occurs at specific sites. In E. coli, methylation of A residues in the sequence 5 -GATC-3 is involved in mismatch error correction, and it plays a role in controlling initiation of DNA replication. Methylation at other sites protects DNA against cleavage by restriction endonucleases (described here). Structural studies on a bacterial DNA methylase have shown that the bases undergoing methylation rotate completely out of the DNA duplex and into a catalytic pocket within the enzyme structure. Other enzymes that work on bases, such as uracil-N-glycosylase, operate similarly. 

N -Methyladenine is a methylated base found in prokaryotic DNA. In E. coli, methylation of A residues in the sequence 5 -GATC-3 is involved in mismatch error correction, and it plays a role in controlling initiation of DNA replication. Methylation at other sites protects DNA against cleavage by restriction endonucleases (described here). 

See also DNA Methylation, Prokaryotic Mismatch Repair, Initiation of DNA Replication, Ni-Methylcytosine... 

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