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is one of the many proteins that is involved in recombination cross over events and during recombination repair in response to single strand DNA breaks. RecA is a rather small monomer protein that can multiplex with itself up to thousands of RecA proteins in order to associate with either dsDNA or ssDNA. The structure of RecA was determined through x-ray crystallography and each monomer contains very distinct structural components. These are a largely helical 30-residue N-terminal region, a 240-residue α/ß ATPase core, and a 64-residue C-terminal globular domain.

The process of recruiting new RecA monomers is carried out through an ATP dependent process. This occurs through the binding of ATP to two adjacent on subsequent RecA monomers. In order to properly grow the crystal that was used to determine structure through x-ray crystallography, a non-hydrolyzsable analog of ATP is used. This analog has the shorthand formula of ADP-AlF4-Mg. Specifically, the aluminum tetraflouride is bound to the adenine diphosphate in the ɣ position. Several residues are involved in the hydrolysis of ATP in order to coordinate strand exchange after binding to ssDNA has occurred. On one of the RecA monomers, two lysine residues, Lys 248 and Lys 250, are responsible for coordinating with the ɣ phosphate and stabilizing it. Lys 250 has also been implicated to have an additional function, that being to coordinate a glutamic acid, Glu 96, on the adjacent RecA monomer. This coordination with Glu 96 is achieved through hydrogen bonding and is believed to be critical for the catalytic mechanism. Specifically, there is a very complex network of hydrogen bonding that is occurring between several other residues in order to rotate Glu 96 to a more favorable conformation enabling Glu 96 to act as a nucleophile. (ask how much detail about this is necessary. Could potentially be another scene)

Once several RecA monomers have coordinated to one another, they coordinate with ssDNA to form a a repeating structure that contains exactly three nucleotides for every RecA monomer. However, this does not mean that each nucleotide triplet only interacts with a single RecA monomer. In reality, each RecA monomer spans three nucleotides, but the nucleotide triplet interacts with the other two RecA surrounding it in both the 5' and 3' direction. Essentially, each nucleotide triplet is interacting with three different RecA monomers named RecA5', RecA0, and RecA3' based on their relative location to the nucleotide triplet. The first nucleotide of the triplet is bound by both RecA5' and RecA0, the second is bound only by Rec0 and the third is bound by both Rec0 and Rec3'. is responsible for stabilizing ssDNA within this conformation. Specifically, the phosphate backbone of the nucleotide triplet is what interacts with the RecA monomers. Interestingly, the hydrogen bonding interactions that are occurring do not always use the side chains, but often will interact with the amide groups on amino acid backbones. For example, the first phosphate group within a nucleotide triplet will interact with the backbone amide of Met 197 from RecA5' and the amide backbone of Asn 123 from RecA0. The second phosphate of the triplet interacts with Gly 211 and Gly 212 on RecA0. The third phosphate of the triplet is more unique as it actually interacts with the side chains of Ser 172 and Arg 176.