The binding of RecA protein to duplex DNA molecules is directional and is promoted by a single stranded region.

RecA protein from E. coli binds more strongly to single stranded DNA than to duplex molecules. Using duplex DNA that contains single stranded gaps, we have studied the protection by RecA protein at various concentrations, of restriction sites as a function of their distance from the single stranded region. We show that the binding of RecA protein, initiated in the single stranded region, extends progressively along the adjoining duplex in the 5' to 3' direction with respect to the single stranded region. The strand exchange reaction is known to proceed in the same direction.     

Role of SSB protein in RecA promoted branch migration reactions

Summary The RecA protein ofEscherichia coli is essential for genetic recombination and postreplicational repair of DNA. In vitro, RecA protein promotes strand transfer reactions between full length linear duplex and single stranded circular DNA of X174 to form heteroduplex replicative form II-like structures (Cox and Lehman 1981a). In a similar way, it transfers one strand of a short duplex restriction fragment to a single stranded circle. Both reactions require RecA and single strand binding protein (SSB) in amounts sufficient to saturate the ssDNA. The rate and extent of strand transfer is enhanced considerably when SSB is added after preincubation of the DNA with RecA protein. In contrast, SSB protein is not required for RecA protein catalysed reciprocal strand exchanges between regions of duplex DNA. These results indicate that while SSB is necessary for efficient transfer between linear duplex and ssDNA to form a single heteroduplex, it is not required for branch migration reactions between duplex molecules that form two heteroduplexes.Abbreviations SSB single strand binding protein - ssDNA single stranded DNA - X phage X174 - bp base pairs - ATP[S] adenosine 5-O-(gamma-thiotriphosphate)     

Biomechanics of chelipeds in some decapod crustaceans

The major chelipeds of five species of decapod crustaceans were studied with reference to lever system mechanical advantage, pattern of occlusive geometry, and force/pressure developed during cheliped closure by intact animals. Every cheliped type was found to possess a linear array of two to four distinctive regions of occlusion. The factors responsible for the differences in occlusive design are discussed. It is suggested that crustacean major chelipeds must be regarded as regionally-specialized, multifunctional appendages.