Sgs1, also known asslow growth suppressor 1,[1] is a DNAhelicase protein found inSaccharomyces cerevisiae. It is ahomolog of the bacterialRecQ helicase. Like the other members of the RecQ helicase family, Sgs1 is important forDNA repair. In particular, Sgs1 collaborates with other proteins to repair double-strand breaks duringhomologous recombination in eukaryotes.[2]
A current model of meiotic recombination, initiated by a double-strand break or gap, followed by pairing with an homologous chromosome and strand invasion to initiate the recombinational repair process. Repair of the gap can lead to crossover (CO) or non-crossover (NCO) of the flanking regions. CO recombination is thought to occur by the Double Holliday Junction (DHJ) model, illustrated on the right, above. NCO recombinants are thought to occur primarily by the Synthesis Dependent Strand Annealing (SDSA) model, illustrated on the left, above. Most recombination events appear to be the SDSA type.
The Sgs1(BLM) helicase is anortholog of the humanBloom syndrome protein. It appears to be a central regulator of most of therecombination events that occur duringS. cerevisiaemeiosis.[3] During normalmeiosis Sgs1(BLM) is responsible for directing recombination towards the alternate formation of either early non-crossover recombinants (NCOs) orHolliday junction joint molecules, the latter being subsequently resolved as crossovers (COs) (see Figure).[3] The several roles of Sgs1 in meiotic recombination were reviewed by Klein and Symington.[4] Primarily, Sgs1 displaces the strand invasion intermediate that initiates recombination, thus facilitating NCO recombination (seeHomologous recombination andBloom syndrome protein).
Sgs1 also has a role in a pathway leading to CO recombinants. Sgs1 together withEXO1 andMLH1-MLH3 heterodimer (MutL gamma) define a joint molecule resolution pathway that produces the majority of crossovers in budding yeast, and by inference, in mammals.[5]
