Spo11
Spo11 is a protein that in humans is encoded by the SPO11 gene. Spo11, in a complex with mTopVIB, creates double strand breaks to initiate meiotic recombination.[5][6] Its active site contains a tyrosine which ligates and dissociates with DNA to promote break formation. One Spo11 protein is involved per strand of DNA, thus two Spo11 proteins are involved in each double stranded break event.
Genetic exchange between two DNA molecules by homologous recombination begins with a break in both strands of DNA—called a double-strand break—and recombination is started by an endonuclease enzyme that cuts the DNA molecule that "receives" the exchanged DNA. In meiosis the enzyme is SPO11, which is related to DNA topoisomerases. Topoisomerases change DNA by transiently breaking one or both strands, passing the unbroken DNA strand or strands through the break and repairing the break; the broken ends of the DNA are covalently linked to topoisomerase. SPO11 is similarly attached to the DNA when it forms double-strand breaks during meiosis.[7]
Meiotic recombination
SPO11 is considered to play a predominant role in initiating meiotic recombination. However, recombination may also occur by alternative SPO11-independent mechanisms that can be studied experimentally using spo11 mutants.
In the budding yeast Saccharomyces cerevisiae, the meiotic defects in recombination and chromosome disjunction of spo11 mutants are alleviated by X-irradiation.[8] This finding indicates that X-ray induced DNA damages can initiate crossover recombination leading to proper disjunction independently of SPO11.
In the worm Caenorhabditis elegans, a homolog of spo11 is ordinarily employed in the initiation of meiotic recombination. However, radiation induced-breaks can also initiate recombination in mutants deleted for this spo11 homolog.[9]
Deamination of cytosine resulting in the dU:dG mismatch is one of the most common single-base-altering lesions in non-replicating DNA. Spo11 mutants of the fission yeast Schizosaccharomyces pombe and C. elegans undergo meiotic crossover recombination and proper chromosome segregation when dU:dG lesions are produced in their DNA.[10] This crossover recombination does not involve the formation of large numbers of double-strand breaks, but does require uracil DNA-glycosylase, an enzyme that removes uracil from the DNA phosphodiester backbone and initiates base excision repair. Thus, it was proposed that base excision repair of DNA damage such as a uracil base, an abasic site, or a single-strand nick is sufficient to initiate meiotic crossover recombination in S. pombe and C. elegans.[10]
In S. pombe, a mutant defective in the spo11 homolog Rec12 is deficient in meiotic recombination. However recombination can be restored to near normal levels by a deletion in rad2, a gene that encodes an endonuclease involved in Okazaki fragment processing (Farah et al., 2005). Both crossover and non-crossover recombination were increased but double-strand breaks were undetectable. On the basis of the biochemical properties of the rad2 deletion, it was proposed that meiotic recombination can be initiated by DNA lesions other than double-strand breaks, such as nicks and gaps which accumulate during premeiotic DNA replication when Okasaki fragment processing is deficient.[11]
The above findings indicate that DNA damages arising from a variety of sources can be repaired by meiotic recombination and that such a process can occur independently of SPO11.
Absence in some sexual species
The most recent common ancestor of the social amoeba genera Dictyostelium, Polysphondylium and Acytostelium, appears to have lacked the Spo11 gene.[12][13] Such an ancestor likely lived several hundred million years ago.[14] Dictyostelium discoideum and Polysphondylium pallidum are both capable of meiotic sexual reproduction (see D. discoideum sexual reproduction and P. pallidum sexual reproduction). Bloomfield[12] speculated that dormant cells in the soil might be exposed to many kinds of stress, such as desiccation or radiation, that could induce spontaneous DNA damage. Such damage would make the induction of double-strand breaks by Spo11 redundant for the initiation of recombination during meiosis, and thus explain its absence in this group.
References
- ^ a b c GRCh38: Ensembl release 89: ENSG00000054796 – Ensembl, May 2017
- ^ a b c GRCm38: Ensembl release 89: ENSMUSG00000005883 – Ensembl, May 2017
- ^ "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
- ^ "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
- ^ Vrielynck N, Chambon A, Vezon D, Pereira L, Chelysheva L, De Muyt A, et al. (February 2016). "A DNA topoisomerase VI-like complex initiates meiotic recombination". Science. 351 (6276): 939–43. Bibcode:2016Sci...351..939V. doi:10.1126/science.aad5196. PMID 26917763. S2CID 206643600.
- ^ Robert T, Nore A, Brun C, Maffre C, Crimi B, Bourbon HM, de Massy B (February 2016). "The TopoVIB-Like protein family is required for meiotic DNA double-strand break formation". Science. 351 (6276): 943–9. Bibcode:2016Sci...351..943R. doi:10.1126/science.aad5309. PMID 26917764. S2CID 9445593.
- ^ Lewin's Genes X (10th ed.). Jones and Bartlett Publishers, Inc. 2011. pp. 353–354. ISBN 978-0-7637-7992-4.
- ^ Thorne LW, Byers B (May 1993). "Stage-specific effects of X-irradiation on yeast meiosis". Genetics. 134 (1): 29–42. doi:10.1093/genetics/134.1.29. PMC 1205431. PMID 8514137.
- ^ Dernburg AF, McDonald K, Moulder G, Barstead R, Dresser M, Villeneuve AM (August 1998). "Meiotic recombination in C. elegans initiates by a conserved mechanism and is dispensable for homologous chromosome synapsis". Cell. 94 (3): 387–98. doi:10.1016/s0092-8674(00)81481-6. PMID 9708740. S2CID 10198891.
- ^ a b Pauklin S, Burkert JS, Martin J, Osman F, Weller S, Boulton SJ, et al. (May 2009). "Alternative induction of meiotic recombination from single-base lesions of DNA deaminases". Genetics. 182 (1): 41–54. doi:10.1534/genetics.109.101683. PMC 2674839. PMID 19237686.
- ^ Farah JA, Cromie G, Davis L, Steiner WW, Smith GR (December 2005). "Activation of an alternative, rec12 (spo11)-independent pathway of fission yeast meiotic recombination in the absence of a DNA flap endonuclease". Genetics. 171 (4): 1499–511. doi:10.1534/genetics.105.046821. PMC 1456079. PMID 16118186.
- ^ a b Bloomfield G (June 2016). "Atypical ploidy cycles, Spo11, and the evolution of meiosis". Seminars in Cell & Developmental Biology. 54: 158–64. doi:10.1016/j.semcdb.2016.01.026. PMID 26811992.
- ^ Malik SB, Pightling AW, Stefaniak LM, Schurko AM, Logsdon JM (August 2007). "An expanded inventory of conserved meiotic genes provides evidence for sex in Trichomonas vaginalis". PLOS ONE. 3 (8): e2879. doi:10.1371/journal.pone.0002879. PMC 2488364. PMID 18663385.
- ^ Fiz-Palacios O, Romeralo M, Ahmadzadeh A, Weststrand S, Ahlberg PE, Baldauf S (2013). "Did terrestrial diversification of amoebas (amoebozoa) occur in synchrony with land plants?". PLOS ONE. 8 (9): e74374. Bibcode:2013PLoSO...874374F. doi:10.1371/journal.pone.0074374. PMC 3770592. PMID 24040233.