DNA helicase

Lohman, T. M., Thorn, K., and Vale, R. D., 1998. Staying on track Common features of DNA helicases and microtnbnle motors.. Cell 93 9-12. 

Examples (a) nucleosome K Huger, AW Mader, RK Richmond, DF Sargent, TJ Richmond. Nature 389 251-260, 1997 (b) DNA polymerases CA Brautigam, TA Steitz. Curr. Opin. Struct. Biol. 8 54-63, 1998 (c) single-stranded binding protein Y Shamoo, AM Friedman, MR Parsons, WH Konigsberg, TA Steitz. Nature 376 362-366, 1995 (d) restriction endonucleases RA Kovall, BW Matthews. Curr. Opin. Chem. Biol. 3 578-583, 1999 (e) DNA lig-ase S Shuman. Structure 4 653-656, 1996 (f) DNA helicases MC Hall, SW Matson. Mol. Microbiol. 34 867-877, 1999 (g) zinc-finger proteins Y Choo, JW Schwabe. Nat. Struct. Biol. 5 253-255, 1998. 

A replication fotk consists of four components that form in the following sequence (1) the DNA helicase unwinds a short segment of the patental duplex DNA ... 

Mehes G, Pajor L (1995) Nucleolin and fibrillarin expression in stimulated lymphocytes and differentiating HL-60 cells. A flow cytometric assay. Cell Prolif 28 329-336 Nasirudin KM, Ehtesham NZ, Tuteja R, Sopory SK, Tuteja N (2005) The Gly-Arg-rich C-terminal domain of pea nucleolin is a DNA helicase that catalytically translocates in the 5 - to 3 -direction. Arch Biochem Biophys 434 306-315... 

Tuteja N, Huang NW, Skopac D, Tuteja R, Hrvatic S, Zhang J, Pongor S, Joseph G, Faucher C, Amalric F et al (1995) Human DNA helicase IV is nucleolin, an RNA helicase modulated by phosphorylation. Gene 160 143—148... 

Figure 20.14 Preparation for DNA replication. At Least three proteins are involved DNA helicases disrupt hydrogen bonds between bases to allow the two strands to unwind single-strand DNA-binding proteins stabilise the unbound strands in preparation for base pairings DNA topoisomerase relaxes strain in the strands to facilitate polymerisation.

Travers, A.A. (1992) The reprogramming of transcriptional competence. Cell 69, 573-575. Velankar, S.S., Soultanas, P., Dillingham, M.S., Subramanya, H.S., and Wigley, D.B. (1999) Crystal structures of complexes of PcrA DNA helicase with a DNA substrate indicate an inch-worm mechanism. Cell 97, 75-84. 

Lee CM, Sedman J, Neupert W, Stuart RA (1999) The DNA helicase, Hmilp, is transported into mitochondria by a C-terminal cleavable targeting signal. J Biol Chem 274 20937-20942... 

Elongation The elongation phase of replication includes two distinct but related operations leading strand synthesis and lagging strand synthesis. Several enzymes at the replication fork are important to the synthesis of both strands. Parent DNA is first unwound by DNA helicases, and the resulting topological stress is relieved by topo-isomerases. Each separated strand is then stabilized by... 

Dam methylase MutH, MutL, MutS proteins DNA helicase II SSB... 

The p-dependent terminators lack the sequence of repeated A residues in the template strand but usually include a CA-rich sequence called a rut (rho rhilization) element. The p protein associates with the RNA at specific binding sites and migrates in the 5 —>3 direction until it reaches the transcription complex that is paused at a termination site. Here it contributes to release of the RNA transcript. The p protein has an ATP-depend-ent RNA-DNA helicase activity that promotes translocation of the protein along the RNA, and ATP is hydrolyzed by p protein during the termination process. The detailed mechanism by which the protein promotes the release of the RNA transcript is not known. 

The process of eukaryotic DNA replication closely follows that of prokaryotic DNA synthesis. Some differences, such as the multiple origins of replication in eukaryotic cells versus single origins of replication in prokaryotes, have already been discussed. Eukaryotic single-stranded DNA-binding proteins and ATP-dependent DNA helicases have been identified, whose functions are analogous to those of the prokaryotic enzymes previously discussed. In contrast, RNA primers are removed by RNase H. 

While defects in protein XPD often cause typical XP symptoms, some defects in the same protein lead to trichothiodystrophy (TTD, brittle hair disease). The hair is sulfur deficient, and scaly skin (ichthyosis, Box 8-F), mental retardation, and other symptoms are observed.0 Like their yeast counterparts (proteins RAD3 and RAD25), XPB and XPD are both DNA helicases.0 They also constitute distinct subunits of the human transcription factor TFIIHP, which is discussed in Chapter 28. It seems likely that XPD is involved in transcription-coupled repair (TCR) of DNA.° °i-s This is a subpathway of the nucleotide excision repair (NER) pathway, which allows for rapid repair of the transcribed strand of DNA. This is important in tissues such as skin, where the global NER process may be too slow to keep up with the need for rapid protein synthesis. Transcription-coupled repair also appears to depend upon proteins CSA and CSB, defects which may result in the rare cockayne syndrome.13 0 4 11 Patients are not only photosensitive but have severe mental and physical retardation including skeletal defects and a wizened appearance. 

Why is a single-strand binding protein or a DNA helicase not required for transcription as they are for replication ... 

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