Question

How does the general model of strand invasion, branch migration, and resolution of the Holliday junction work?

Answer 1

Free ends of each broken strand then migrate across to the other DNA helix. There, the invading strands are joined to the free ends they encounter, resulting in the Holliday junction. As each crossover strand reanneals to its original partner strand, it displaces the original complementary strand ahead of it.

Branch migration is the process by which base pairs on homologous DNA strands are consecutively exchanged at a Holliday junction, moving the branch point up or down the DNA sequence. Branch migration is the second step of genetic recombination, following the exchange of two single strands of DNA between two homologous chromosomes. 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. Branch migration can also be seen in DNA repair and replication, when filling in gaps in the sequence. It can also be seen when a foreign piece of DNA invades the strand.

In budding yeast Saccharomyces cerevisiae, Holliday junctions can be resolved by four different pathways that account for essentially all Holliday junction resolution in vivo. The pathway that produces the majority of crossovers in S. cerevisiae budding yeast, and possibly in mammals, involves proteins EXO1, MLH1-MLH3 heterodimer (called MutL gamma) and SGS1 (ortholog of Bloom syndrome helicase). The MLH1-MLH3 heterodimer binds preferentially to Holliday junctions. It is an endonuclease that makes single-strand breaks in supercoiled double-stranded DNA. The MLH1-MLH3 heterodimer promotes the formation of crossover recombinants. While the other three pathways, involving proteins MUS81-MMS4, SLX1 and YEN1, respectively, can promote Holliday junction resolution in vivo, absence of all three nucleases has only a modest impact on formation of crossover products.

Double mutants deleted for both MLH3 (major pathway) and MMS4 (minor pathway) showed dramatically reduced crossing over compared to wild-type (6- to 17-fold); however spore viability was reasonably high (62%) and chromosomal disjunction appeared mostly functional.