Figure 1. Schematic alignment of RecA protein and its homologues from yeast and human cells. Boxed in green is the region corresponding to the ATP-binding domain of RecA protein. Walker A- and B-type nucleotide binding motifs are indicated by blue boxes (not drawn to scale). The numbers above the proteins refer to amino acids.

Figure 2. The DNA substrates and expected reaction products of an in vitro assay to detect the strand-exchange activity of Escherichia coli RecA protein. The reaction makes use of circular single-stranded DNA (ssDNA) and homologous linear double-stranded DNA (dsDNA), usually of viral origin. RecA-mediated pairing of these substrates (reaction [I]) results in appearance of joint molecules followed by strand exchange (reaction [II]) leading to formation of nicked circular dsDNA and displaced ssDNA.

Figure 3. Directionality of yeast Rad51-mediated strand exchange. The reaction can be initiated from either end of linear dsDNA, given the presence of a single-stranded overhang (indicated by the same thickness of arrows). However, strand exchange is more effective when it is initiated at the 3¢ overhanging end and proceeds in the 5¢ to 3¢ direction with respect to circular ssDNA (pathway A, indicated by the thicker arrow).

Figure 4. Strand-exchange activity of human Rad51 protein. hRad51 catalyses formation of joint molecules, but only limited strand exchange is observed resulting in low yields of final products (indicated by thin arrow and grey cross).

Figure 5. Schematic presentation of a multiprotein complex involved in recombinational DNA repair in (a) yeast and (b) human cells. The degraded 3¢ end of a DNA double-strand break is shown as a substrate to assemble these putative complexes. In yeast cells, Rad51 interacts with Rad57 through Rad55. In human cells, Rad51 interacts directly with both Rad51C and Xrcc3, and Rad51C and Xrcc3 also interact with each other.

Figure 6. Major steps in meiotic recombination in yeast. Two homologous chromosomes, each consisting of two sister chromatids, are presented in different colours. At step 1, a meiosis-specific double-strand break is produced by Spo11 protein; protein complex composed of Mre11, Rad50 and Xrs2 proteins is also required for initiation of meiotic recombination. At step 2, the ends of the double-strand break are processed by 5¢ to 3¢ degradation resulting in the formation of 3¢ overhangs; an exonuclease activity associated with Mre11 protein is most likely responsible for this degradation. At step 3, 3¢-ended overhangs invade homologous duplex leading to formation of joint molecules and strand exchange. A multiprotein complex consisting of at least six proteins, including Rad51, is required for these reactions. DNA repair synthesis and branch migration followed by resolution of Holliday junctions (steps 4, 5 etc.) will eventually result in the formation of mature recombination products.