In sexually reproducing organisms, recombination during Meiosis plays an important role. In an evolutionary point of view, it is a primary source of genetic variation by allowing exchange of genetic material from its homologous chromosome, giving selective advantage over asexual reproduction. Recombination also plays a mechanical role during meiosis as reciprocal exchange between homologous chromosome forms a physical connection that allow homologous pair of chromosome to accurately align themselves in the meiotic spindle for first meiotic division. Unsurprisingly, owing to its significance, process of recombination takes much of the prophase of meiosis, and is regulated tightly by interaction with multiple proteins, which ensures fidelity of the whole process. A protein of particular interest in recombination is Spo11, which is known to produce double-strand breaks (DSBs) in the DNA, which is an important step in initiating the meiotic recombination . Failure to initiate DSB required for recombination, the chromosomes segregates aberrantly resulting in aneuploid gametes .
Spo11 gene in humans is located in chromosome 20, and it is comprised of 396 amino acid sequence. To this date, there is no crystal structure of Spo11 protein, and much of the structural studies are deduced from observation of the subunit of archaeal topoisomerase VI (Top6A), which shows similarity in sequence and its mechanism of function .
There are two distinct structural regions:
- 5Y-CAP motif: Similar to catabolite gene activator protein (CAP) DNA binding domain and contains catalytic tyrosine residue.
- Toprim domain: Four-stranded parallel beta sheet sandwiched between two pair of alpha-helices. Accommodates Mg2+ ions but its function is not known.
Key role of Spo11 protein is to generate DSB between homologous chromosomes to initiate meiotic recombination. The meiotic recombination is best understood in yeast, which is a good model species for understanding eukaryotic function.
- Firstly, an intact DNA duplex is cleaved by Spo11 protein, which occurs leptotene of meiosis 1. This reaction is mediated by nucleophilic attack of the DNA phosphodiester backbone by tyrosine side chain on Spo11. This forms a phosphodiester bond between Spo11 and 5’ terminal strand and generating free 3’ OH-terminus . This experimental observation is further supported by the fact that point mutation of the tyrosine residue in Spo11 has failed to produce DSB 
- The determination of particular sequence for DSBs is not known, however open chromatin structures and repeated sequence are common targets for Spo11 gene . In addition, site-directed mutagenesis of Spo11 gene showed reduced cleavage specificity of the enzyme. This highlights the fact that Spo11 is also involved in directing specificity of its function .
- The free 3’ OH-terminus produced by DSBs repairs with the homologue rather than the sister chromatid, as a template, and thereby successfully producing recombinant homologous chromosomes.
- After the DSBs are formed, the Spo11 attached to 5’ end of the DNA is cleaved from the chain by Mre11-catalysed endonucleolytic cleavage, along with short oligonucleotide still covalently bound to Spo11 .
Mutation of the SPO11 gene shows distinct phenotype changes that can be related back to its function. Below outlines some of consequence and its significance Spo11 Mutation .
- Male germ line cells arrested their meiosis prior to pachytene with little or no synapsis between homologous chromosomes. The arrested meiotic germ line cells then undergo apoptosis and both male and female species are infertile.
- DSB formation is absent and consequently nondisjunction is observed in first meiotic division, which highlights the need for Spo11 to initiate recombination to allow accurate homologous chromosome segregation in meiosis 1.
- The Spo11 mutations exhibit other phenotypes that may not be derived by loss of DSB function. For example, mutant germ line cell accelerates cell cycle progression through meiotic prophase .
Protein-Protein Interactions with Spo11
Although Spo11 is the catalytic unit that is involved in forming DSB, its activity requires interaction with other DSB proteins. So far products of at least 9 different genes have been indentified that interacts with Spo11: RAD30, MRE11, XRS2, MEI4, MER2, REC102, REC104, REC114, and SKI8. Whilst some of them are meiosis specific, others are expressed in mitotic cycle as well . Mutations in these associated protein leads to defects similar defects in Spo11, highlighting their importance in recombination.
As to date relatively little is known about the functional interaction between Spo11 and these associated proteins, however key interaction is summarised below and in figure (2) .
- Ski8: direct partner of Spo11 and upon interaction with Spo11, it relocalizes from cytoplasm to nucleus and associates with meiotic chromosomes. This in turn allows Spo11 to recruit Rec102 and Rec104 to interact with to chromosomes.
- Mer2: It has a phosphorylation site that can be phosphorylated by the cyclin-Cdk complex. Phosphorylation of Mer2 residue modulates its interaction with Spo11 and other related proteins.
- Rec102/ Rec104: interact physically and functionally as a single functional unit. Residues in Rec104 can be phosphorylated; however its functional significance is unknown.
- Recombination: process by which genetic material is broken and joined to other genetic material
- Homologous Chromosome: non-identical chromosomes that both contain information for the same biological features
- Meiotic Spindle: structure involved in separation of chromosomes during mitosis and meiosis
- Prophase: early phase of meiosis and mitosis
- Aneuploid: abnormal number of chromosome derived from errors in meiosis
- leptotene: first phase of prophase of meiosis, where chromosomes condense into thin threads
- Sister chromatid: identical copies of a chromosome connected by a centromere
- Synapsis: paring of the two homologous chromosome that occurs in meiosis 1
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