Nick-closing enzymes

A Sugino, CL Peebles, KN Kreuzer, NR Cozzarelli. Mechanism of action of nalidixic acid purification of Escherichia coli nalA gene product and its relationship to DNA gyrase and a novel nicking-closing enzyme. Proc Natl Acad Sci (USA) 74 4767-4771, 1977. 

Pulleyblank, D.E., Shure, D.E., Tang, D., Vinograd, J., Vosberg, H.-P. 1975. Action of nicking-closing enzyme on supercoiled and nonsupercoiled closed circular DNA formation of a Boltzmann distribution of topological isomers. Proc. Natl. Acad. Sci. USA 72 4280-4284. 

The following enzymes described in the literature are now known to be type I T E. coll w-protein (identical with Eco DNA T., . coU swivelase, and E. cell type IT.), untwisting enzymes, nicking-closing enzymes, Relaxation protein (see), DNA-Relaxing enzyme (see). 

S.V. 40, minicol nick-close enzyme, gel el, CsCl banding... 

First, it is found that in nicked DNA E. coli topoisomerase I binds close to the site of the nick and, when allowed to cleave the DNA, cuts the intact strand nearly opposite the nick. Second, an important feature of the catenation reaction is that at least one DNA partner must contain a nick or a single-stranded gap. If the enzyme binds at the nick and allows passage of DNA through the intact strand, one can conclude that the... 

Based on the close proximity of the two types of hot spot motifs, it is possible that AID-catalyzed deamination of G residues in WRG could therefore provide a mechanism for loading an EP pol to copy nearby WA motifs. Gonversion of G —> U by AID, followed by U removal by UNG + APE enzymes, results in a nick at the 5 -end of the abasic moiety. If an error-prone repair polymerase (e.g., pol rf) binds and carries out strand displacement synthesis (long-patch BER), then misincorporation of G opposite T (Fig. 3) will yield A —> G transitions in the nontranscribed strand at WA hotspots, in accordance with pol r/ s mutational specificity (Pavlov et al, 2002), as shown (in red) in Fig. 3. 

Step 3. Closing the nicks One daughter DNA strand is synthesized without any nicks, but the Okazaki fragments of the other strand must be joined together. An enzyme called DNA ligase catalyzes this final step of DNA replication. The result is two DNA double-helical molecules that are identical. 

One of the most intriguing, but well-established, properties of poly(ADP-ribose) synthetase is its requirement for DNA [20-24]. As reported by Benjamin and Gill [23] and Yoshihara s group ([21] for review see [24]), the enzyme shows preference for DNA ends . Thus, closed circular dsDNA (ColEi or pBR322) shows no activating potential on the enzyme. However, this ability increases proportionally with the number of nicks introduced by DNAase I treatment [21]. 

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