Sequencing specific enzymes

The repair of dimer lesions, induced with the aid of light of relatively long wavelength that is not absorbed by the dimer sites (2 300-400 nm), is based on photoreceptor action, as dealt with in Section 8.2.2.3 above. It occurs if DNA photolyases, i.e. structure-specific (not sequence-specific) enzymes are present in the system during the irradiation [6]. Photolyases are proteins of 450-550 amino acids containing two non-covalently bound chromophore cofactors (see Chart 8.6). 

Figure 7.8 Sequence-specific recognition sites in the major groove of DNA for three restriction enzymes—Eco RI, Bal I, and Sma I. The DNA sequences that are recognized by these enzymes ate represented by tbe color code defined in Figure 7.7.

Nielsen P.E., Egholm M., Berg R. H., Buchardt O. Sequence specific inhibition of DNA restriction enzyme cleavage by PNA. Nucleic Acids Res. 1993 21 197-200. 

As discussed above, alternative recombinant DNA techniques are necessary to efficiently generate genome-scale clone sets. One alternative exploits the ability of the Vaccinia virus DNA topoisomerase I to both cleave and rejoin DNA strands with high sequence specificity (Shuman, 1992a Shuman, 1992b). In the reaction, the enzyme recognizes the sequence 5 -CCCTT and cleaves at the final T whereby a covalent adduct is formed between the 3 phosphate of the cleaved strand and a tyrosine residue in the enzyme (Fig. 4.1). The covalent complex can combine with a heterologous acceptor DNA that has a 5 hydroxyl tail complementary to the sequence on the covalent adduct to create a recombinant molecule (Shuman, 1994). 

Restriction enzymes are sequence-specific endonucleases that cut double-stranded DNA at specific sites. 

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