Strand exchange proteins

In related reactions a single-stranded SSB-coated circular DNA will bind to RecA protein, then exchange strands with a linear duplex (Eq. 27-12). The strand exchange requires ATP and advances in the 5 to 3 ... 

Figure 27-25 A possible mechanism for homologous pairing of an ssDNA with a duplex DNA and strand exchange. The ssDNA (right) binds together with a hydrogen-bonded duplex (left). The RecA protein rotates the bases into the heteroduplex configuration, where hydrogen bonds may be formed in many of the base pairs. After Howard-Flanders, West, and Stassiak.536...

Reactions catalyzed by purified recA protein in vitro. RecA catalyzes a number of different reactions between DNA strands, all of them involving the unwinding and winding of base-paired structures, (a) D-loop formation by interaction between supercoiled circular duplex DNA and single-stranded DNA. (b) Strand exchange between a gapped circular duplex structure and a linear duplex structure, (c) Complex formation between two helices, one of which is gapped. 

The fact that mutations in recA completely eliminate recombination, as well as the finding that the recA-related protein catalyzes strand exchange reactions like those required in the recombination models we have been discussing, makes it highly likely that recA plays a key role in these processes in vivo. If this is so, then recA protein is clearly situated at the hub of activities in the recombination complex. 

Radding, C.M. (1988). Homologous pairing and strand exchange promoted by Escherichia coli RecA protein. In Genetic recombination, G.R. Smith (Ed.) pp. 193-229. American Society for Microbiology, Washington, D.C. 

The biochemical process of recombination occurs by breaking and rejoining DNA strands. The key reaction is strand displacement initiated at a nick in the chromosome. Then a protein called RecA (which stands for recombination rec bacteria are unable to recombine their DNA information and therefore are abnormally sensitive to UV radiation) binds to a single-stranded DNA fragment and catalyzer its exchange with the same sequence of the duplex. RecA protein is a strand displacement protein. See Figure 8-16. 

Sung P. Catalysis of ATP-dependent homologous DNA pairing and strand exchange by yeast RadSl protein. Science 1994 265(5176) 1241-1243. 

DNA-strand exchange between a ssDNA and a duplex in which all G and C residues have been replaced by 2 -deoxyisoguanosine (iG) and 2 -deoxy-5-methyl-isocytidine MiC) by the E. coli RecA protein in vitro occurred at a similar rate and efficiency to unmodified DNA. This provides further potential for the role of iG and MiC in an expanded genetic code. Using ODNs rich in isoguanosine residues, Chaput and Switzer have shown that iG quintet structures may be formed from a metal-assisted hydrogen bond-mediated self-assembly process. The structures were stabilised particularly in the presence of Cs+ ions. 

BRCA1 interacts specifically with the Rad51 protein which catalyzes strand exchange during recombination processes. By this property, BRCA1 is assumed to participate in DNA double-strand break repair. Furthermore, BRCA1 shows a DNA-binding activity specific for branched DNAs. 

Figure 25.23 RecA protein-mediated DNA strand exchange reactions. 

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