Branch migration

[1] Branch migration is the second step of genetic recombination, following the exchange of two single strands of DNA between two homologous chromosomes.

[2] The process is random, and the branch point can be displaced in either direction on the strand, influencing the degree of which the genetic material is exchanged.

[2] In E. coli, the proteins RuvA and RuvB come together and form a complex that facilitates the process in a number of ways.

[2] Rad54, a highly conserved eukaryotic protein, is reported to oligomerize on Holliday junctions to promote branch migration.

[5] A helicase (designated Saci-0814) isolated from the thermophilic crenarchaeon Sulfolobus acidocaldarius dissociated DNA Holliday junction structures, and showed branch migration activity in vitro.

Based on this evidence it appears that Saci-0814 is employed in homologous recombination in S. acidocaldarius and functions as a branch migration helicase.

[6] Homologous recombination appears to be an important adaptation in hyperthermophiles, such as S. acidocaldarius, for efficiently repairing DNA damage.

Diagram illustrating the movement of a branch point between two homologous pieces of DNA. Migration travels to the left and stops when it reaches the end of the homologous region. The second branch point on the right is free to move in either direction as well.
Open X structure of a Holliday junction. RuvA binds to the DNA and fits in between the double strands on all four sides. RuvA also has a domain that fits inside the centre of the junction.
The Holliday junction converts between the open X structure (top) and the stacked X structure (bottom) depending on the Mg 2+ concentration.