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Gene Review

Sxl  -  Sex lethal

Drosophila melanogaster

Synonyms: CG14425, CG18350, CG33070, CG43770, DmSxl, ...
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Disease relevance of Sxl

  • Suppression of sterility by Wolbachia discriminates markedly among similar germline-specific Sxl alleles, and is not observed for mutations in other genes that produce similar 'tumorous ovary' phenotypes, including one that blocks Sxl germline expression [1].
  • Here we report that infection by Wolbachia restores fertility to Drosophila melanogaster mutant females prevented from making eggs by protein-coding lesions in Sex-lethal (Sxl), the master regulator of sex determination [1].
  • Small cell lung cancer (SCLC) is known to express the HuD protein, the neuronal antigen homologous to Drosophila Elav and Sxl genes involved in neuronal and sex development [2].

Psychiatry related information on Sxl

  • In addition, dosage compensation is controlled by Sex-lethal-mediated translational regulation while dFMR1 (the Drosophila homologue of the fragile X mental retardation protein) controls translation of various mRNAs which function in the nervous system [3].

High impact information on Sxl

  • Female flies must inhibit msl-2 mRNA translation for survival, and this inhibition is mediated by Sex-lethal (SXL) binding to sites in both the 5' and the 3' untranslated regions (UTRs) [4].
  • SXL binding to the 3'UTR regulatory region inhibits the recruitment of 43S ribosomal preinitiation complexes to the mRNA [4].
  • An unusual sequence arrangement of two AG dinucleotides and an intervening polypyrimidine (Py)-tract at the 3' end of intron 2 is important for Sxl autoregulation [5].
  • Unexpectedly, removal of the Sxl gene in the zygote mitigates both the migration and mitotic defects of nos- germ cells [6].
  • Supporting the conclusion that Sxl is an important target for nos repression, ectopic, premature expression of Sxl protein in germ cells disrupts migration and stimulates mitotic activity [6].

Biological context of Sxl

  • In hybrid males from D. melanogaster mothers, there is an abnormal expression of Sxl and a failure of localization of the male-specific lethal (MSL) complex to the X chromosome, which causes changes in gene expression [7].
  • Introduction of a Sxl mutation into this hybrid genotype will allow expression of the MSL complex but there is no sequestration to the X chromosome [7].
  • Alternative splicing of the Sex-lethal pre-mRNA has long served as a model example of a regulated splicing event, yet the mechanism by which the female-specific SEX-LETHAL RNA-binding protein prevents inclusion of the translation-terminating male exon is not understood [8].
  • Instead it appears to autoregulate in a manner similar to the autoregulation seen with DmSxl [9].
  • Sxl is a master switch gene that controls its own pre-mRNA splicing as well as splicing for subordinate switch genes that regulate sex determination and dosage compensation [10].

Anatomical context of Sxl

  • Our findings indicate that while Sxl is the master sex determination gene in somatic cells, it appears to play a more subsidiary role in the germ line [11].
  • Conceivably, this regulation safeguards the inadvertent activation of the Sxl autoregulatory feedback loop in the male zygote [12].
  • Using dominant suppressor mutations that relieve this early block in Sxl(fs) mutant females, we discovered additional requirements of Sxl for normal meiotic differentiation of the oocyte [13].
  • Female flies mutant for alleles of Sex-lethal, defective in sex determination, or null alleles of transformer or transformer-2 are converted into phenotypic males that formed male-specific abdominal muscles [14].
  • It corresponds to the syncytial blastoderm stage and coincides with the time when the X:A signal regulates Sxl [15].

Associations of Sxl with chemical compounds

  • Psoralen-mediated crosslinking experiments suggest that SXL binding to this uridine-rich sequence inhibits recognition of the 5' splice site by U1 snRNP in HeLa nuclear extracts [16].
  • We now report that a second pyrimidine-rich sequence containing 11 consecutive uridines immediately downstream from the 5' splice site is required for efficient splicing inhibition by SXL [16].
  • Unlike bulk Snf protein, which sediments slowly in sucrose gradients, the Snf protein associated with Sxl is in a large, rapidly sedimenting complex [17].
  • Similar results were obtained with the Sxl homologue of Ceratitis capitata (Saccone, G., Peluso, I., Artiaco, D. , Giodano, E., Bopp, D. and Polito, L. C. (1998) Development 125, 1495-1500) suggesting that, in these non-drosophilid species, Sxl performs a function different from that in sex determination [18].
  • Previous work demonstrated that Sxl monomers interact cooperatively upon binding to target RNAs and that the cooperativity depends on the glycine-rich N terminus [19].

Physical interactions of Sxl

  • We have previously shown that SXL blocks the binding of U2 auxiliary factor (U2AF) to the polypyrimidine (Py)-tract associated with the 3' splice site of the regulated intron [16].
  • Here we report that splicing inhibition of msl-2 requires a binding site for SXL at the polypyrimidine (poly(Y)) tract associated with the 3' splice site, and an unusually long distance between the poly(Y) tract and the conserved AG dinucleotide at the 3' end of the intron [20].
  • Moreover, we find that female Sxl-encoded protein binds specifically to the tra transcript at or near the non-sex-specific acceptor site, implying that the female Sxl gene product is the trans-acting factor that regulates the alternative splicing [21].
  • Co-immunoprecipitation and mass spectrometry results indicate that SXL and FL (2)D form a complex and that the protein VIRILIZER and a Ran-binding protein implicated in protein nuclear import are also present in complexes containing FL (2)D [22].
  • We examined SISB-Da binding at Sxl by using footprinting and gel mobility shift assays and found that SISB-Da binds numerous clustered sites in the establishment promoter Sxl(Pe) [23].

Regulatory relationships of Sxl

  • The position of the otu locus in the regulatory cascade of germ line sex determination has been studied by using mutations that constitutively express the feminizing activity of the Sxl gene [24].
  • Previous studies have suggested that snf plays a role in regulating Sex-lethal splicing [25].
  • virilizer regulates Sex-lethal in the germline of Drosophila melanogaster [26].
  • Both loss-of-function and gain-of-function mutations in snf define a role for snRNP proteins in regulating Sex-lethal pre-mRNA splicing in Drosophila development [27].
  • The protein Sex-lethal (Sxl) activates a female-specific 3' splice site in the first intron of transformer (tra) pre-mRNA while repressing an alternative non-sex-specific site [28].
  • We show that although STAT directly regulates SxlPe, it is dispensable for promoter activation [29].

Other interactions of Sxl

  • Modulation of msl-2 5' splice site recognition by Sex-lethal [16].
  • The female-specific expression of this gene's function is regulated by the Sex lethal (Sxl) gene, through a mechanism involving sex-specific alternative splicing of tra pre-mRNA [30].
  • Finally, we demonstrate that the germ line function of Sxl depends on the activity of a specific OTU isoform [11].
  • Thus, with respect to the development of the germline the fl(2)d mutations mimic the behavior of loss-of-function mutations at the gene Sxl [31].
  • Fragments of the Sxl and dsx orthologous were isolated with RT-PCR [32].

Analytical, diagnostic and therapeutic context of Sxl

  • Using in situ hybridization, we determined the time course of sis-a, sis-b, and Sxl transcription during the first few hours after fertilization [33].
  • Western blot (immunoblot) analysis demonstrates that this difference in protein level correlates directly with the activity state of the Sxl gene [34].
  • Using pole cell transplantation, we have tested whether these genes are also needed to activate Sxl in the germ line [35].
  • Two interaction domains, the Sxl N terminus and the Sxl first RNA binding domain, were suggested by the yeast two-hybrid assay [19].
  • In the present study, we used site-directed mutagenesis to improve the solubility of the didomain fragment of Sxl, and confirmed that this mutant fragment forms hydrogen bonds with the target RNA in the same manner as that of the wild-type fragment [36].


  1. A host parasite interaction rescues Drosophila oogenesis defects. Starr, D.J., Cline, T.W. Nature (2002) [Pubmed]
  2. Molecular analysis of the HuD gene encoding a paraneoplastic encephalomyelitis antigen in human lung cancer cell lines. Sekido, Y., Bader, S.A., Carbone, D.P., Johnson, B.E., Minna, J.D. Cancer Res. (1994) [Pubmed]
  3. Mechanisms of translational regulation in Drosophila. Wilhelm, J.E., Smibert, C.A. Biol. Cell (2005) [Pubmed]
  4. A dual inhibitory mechanism restricts msl-2 mRNA translation for dosage compensation in Drosophila. Beckmann, K., Grskovic, M., Gebauer, F., Hentze, M.W. Cell (2005) [Pubmed]
  5. Splicing regulation at the second catalytic step by Sex-lethal involves 3' splice site recognition by SPF45. Lallena, M.J., Chalmers, K.J., Llamazares, S., Lamond, A.I., Valcárcel, J. Cell (2002) [Pubmed]
  6. Novel functions of nanos in downregulating mitosis and transcription during the development of the Drosophila germline. Deshpande, G., Calhoun, G., Yanowitz, J.L., Schedl, P.D. Cell (1999) [Pubmed]
  7. Misregulation of sex-lethal and disruption of male-specific lethal complex localization in Drosophila species hybrids. Pal Bhadra, M., Bhadra, U., Birchler, J.A. Genetics (2006) [Pubmed]
  8. Sex-lethal splicing autoregulation in vivo: interactions between SEX-LETHAL, the U1 snRNP and U2AF underlie male exon skipping. Nagengast, A.A., Stitzinger, S.M., Tseng, C.H., Mount, S.M., Salz, H.K. Development (2003) [Pubmed]
  9. Masters change, slaves remain. Graham, P., Penn, J.K., Schedl, P. Bioessays (2003) [Pubmed]
  10. Functioning of the Drosophila integral U1/U2 protein Snf independent of U1 and U2 small nuclear ribonucleoprotein particles is revealed by snf(+) gene dose effects. Cline, T.W., Rudner, D.Z., Barbash, D.A., Bell, M., Vutien, R. Proc. Natl. Acad. Sci. U.S.A. (1999) [Pubmed]
  11. Molecular characterization of ovarian tumors in Drosophila. Bae, E., Cook, K.R., Geyer, P.K., Nagoshi, R.N. Mech. Dev. (1994) [Pubmed]
  12. Expression of the Sex-lethal gene is controlled at multiple levels during Drosophila oogenesis. Bopp, D., Horabin, J.I., Lersch, R.A., Cline, T.W., Schedl, P. Development (1993) [Pubmed]
  13. Recombination and disjunction in female germ cells of Drosophila depend on the germline activity of the gene sex-lethal. Bopp, D., Schütt, C., Puro, J., Huang, H., Nöthiger, R. Development (1999) [Pubmed]
  14. Differentiation of a male-specific muscle in Drosophila melanogaster does not require the sex-determining genes doublesex or intersex. Taylor, B.J. Genetics (1992) [Pubmed]
  15. The sisterless-b function of the Drosophila gene scute is restricted to the stage when the X:A ratio determines the activity of Sex-lethal. Torres, M., Sánchez, L. Development (1991) [Pubmed]
  16. Modulation of msl-2 5' splice site recognition by Sex-lethal. Förch, P., Merendino, L., Martínez, C., Valcárcel, J. RNA (2001) [Pubmed]
  17. Sex-lethal interacts with splicing factors in vitro and in vivo. Deshpande, G., Samuels, M.E., Schedl, P.D. Mol. Cell. Biol. (1996) [Pubmed]
  18. Sex-lethal, the master sex-determining gene in Drosophila, is not sex-specifically regulated in Musca domestica. Meise, M., Hilfiker-Kleiner, D., Dübendorfer, A., Brunner, C., Nöthiger, R., Bopp, D. Development (1998) [Pubmed]
  19. Sex-lethal interactions with protein and RNA. Roles of glycine-rich and RNA binding domains. Wang, J., Dong, Z., Bell, L.R. J. Biol. Chem. (1997) [Pubmed]
  20. Inhibition of msl-2 splicing by Sex-lethal reveals interaction between U2AF35 and the 3' splice site AG. Merendino, L., Guth, S., Bilbao, D., Martínez, C., Valcárcel, J. Nature (1999) [Pubmed]
  21. Binding of the Drosophila sex-lethal gene product to the alternative splice site of transformer primary transcript. Inoue, K., Hoshijima, K., Sakamoto, H., Shimura, Y. Nature (1990) [Pubmed]
  22. Biochemical function of female-lethal (2)D/Wilms' tumor suppressor-1-associated proteins in alternative pre-mRNA splicing. Ortega, A., Niksic, M., Bachi, A., Wilm, M., Sánchez, L., Hastie, N., Valcárcel, J. J. Biol. Chem. (2003) [Pubmed]
  23. Interpretation of X chromosome dose at Sex-lethal requires non-E-box sites for the basic helix-loop-helix proteins SISB and daughterless. Yang, D., Lu, H., Hong, Y., Jinks, T.M., Estes, P.A., Erickson, J.W. Mol. Cell. Biol. (2001) [Pubmed]
  24. The role of the ovarian tumor locus in Drosophila melanogaster germ line sex determination. Pauli, D., Oliver, B., Mahowald, A.P. Development (1993) [Pubmed]
  25. The Drosophila sex determination gene snf encodes a nuclear protein with sequence and functional similarity to the mammalian U1A snRNP protein. Flickinger, T.W., Salz, H.K. Genes Dev. (1994) [Pubmed]
  26. virilizer regulates Sex-lethal in the germline of Drosophila melanogaster. SchüŁtt, C., Hilfiker, A., Nöthiger, R. Development (1998) [Pubmed]
  27. Both loss-of-function and gain-of-function mutations in snf define a role for snRNP proteins in regulating Sex-lethal pre-mRNA splicing in Drosophila development. Salz, H.K., Flickinger, T.W. Genetics (1996) [Pubmed]
  28. The protein Sex-lethal antagonizes the splicing factor U2AF to regulate alternative splicing of transformer pre-mRNA. Valcárcel, J., Singh, R., Zamore, P.D., Green, M.R. Nature (1993) [Pubmed]
  29. Drosophila JAK/STAT pathway reveals distinct initiation and reinforcement steps in early transcription of Sxl. Avila, F.W., Erickson, J.W. Curr. Biol. (2007) [Pubmed]
  30. Interspecific comparison of the transformer gene of Drosophila reveals an unusually high degree of evolutionary divergence. O'Neil, M.T., Belote, J.M. Genetics (1992) [Pubmed]
  31. Evidence of a dual function in fl(2)d, a gene needed for Sex-lethal expression in Drosophila melanogaster. Granadino, B., San Juán, A., Santamaria, P., Sánchez, L. Genetics (1992) [Pubmed]
  32. Isolation and characterization of the Bactrocera oleae genes orthologous to the sex determining Sex-lethal and doublesex genes of Drosophila melanogaster. Lagos, D., Ruiz, M.F., Sánchez, L., Komitopoulou, K. Gene (2005) [Pubmed]
  33. A bZIP protein, sisterless-a, collaborates with bHLH transcription factors early in Drosophila development to determine sex. Erickson, J.W., Cline, T.W. Genes Dev. (1993) [Pubmed]
  34. scute (sis-b) function in Drosophila sex determination. Deshpande, G., Stukey, J., Schedl, P. Mol. Cell. Biol. (1995) [Pubmed]
  35. Sex determination in the germ line of Drosophila melanogaster: activation of the gene Sex-lethal. Granadino, B., Santamaria, P., Sánchez, L. Development (1993) [Pubmed]
  36. NMR analysis of the hydrogen bonding interactions of the RNA-binding domains of the Drosophila sex-lethal protein with target RNA fragments with site-specific [3-15N]uridine substitutions. Kim, I., Muto, Y., Inoue, M., Watanabe, S., Kitamura, A., Yokoyama, S., Hosono, K., Takaku, H., Ono, A., Kainosho, M., Sakamoto, H., Shimura, Y. Nucleic Acids Res. (1997) [Pubmed]
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