Sandbox reserved 1754

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 what would be the ɣ position. Several residues are involved in the normal hydrolysis of ATP in order to coordinate strand exchange after binding to ssDNA has occurred. On one of the RecA monomers, two lysine residues, Lys248 and Lys250, are responsible for coordinating with the ɣ phosphate and stabilizing it. Lys 250 has been implicated to have an additional function as well, that being to coordinate a glutamic acid, Glu96, on the adjacent RecA monomer. This coordination with Glu96 is achieved through hydrogen bonding and is believed to be critical for the catalytic mechanism. Specifically, the hydrogen bond formed between Lys250 and Glu96 causes Glu96 to change positioning to face the ɣ phosphate providing a more favorable conformation enabling Glu96 to act as a nucleophile. (explain more about how hydrolysis is achieved during strand exchange, and how it destabilizes the complex)

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 on both the 5' and 3' sides. Essentialy, each nucleotide triplet is interacting with three different RecA monomers. When ssDNA is bound to RecA in this manner it take a unique conformation becoming very similar to normal B-DNA structure, however, the bound DNA is stretched slightly from normal parameters.For example, normal B-DNA has a 36° twist while bound ssDNA appears to have a 42° twist from the first to second base and then a 60° twist from the second to third base.