EvoprintHD of Sxl

BIOLOGICAL OVERVIEW

Sex determination in the developing fly is like a one ring circus that occasionally complicates an already exciting scene by adding a second and a third ring, providing the audience a variety of different acts taking place at different times, and occasionally, a choice of viewing among simultaneous events (see Schematic of the sex determination hierarchy in Control of male sexual behavior in Drosophila by the sex determination pathway, Billeter, 2006). Whatever goes on, the circus master in charge is Sex lethal (Sxl). At least three acts get top billing in this circus under the direction of Sxl: the development of female somatic fate; the dosage compensation in males during embryonic development, and finally, the sex specific development of female and male germ lines.

Act 1: Immediately after fertilization there is an assessment of the ratio of X chromosomes to autosomes. Three genes are X linked: when there are two X chromosomes, as in females, the ratio of their gene products like Runt (Torres, 1994), Sisterless-A and Sisterless-B (also known as Scute) is higher than in males. These three genes bind the Sex-lethal promoter and induce activation. In the case of males, where there is only one X chromosome, the autosomal proteins Daughterless, Deadpan and Extramachrochaete, absent sufficient activator proteins, act as repressors of Sex lethal. Thus, despite Sex lethal's central position as ringmaster in this developmental circus, it is the crowd of at least a half dozen transcription factors that regulate Sex lethal's activities.

Sex lethal is an RNA splicing enzyme. Sex lethal's immediate target is Transformer mRNA. Since Sex lethal is not transcribed in males, its action on Transformer is restricted to females. Sex lethal acts positively in the functional splicing of Transformer mRNA. Transformer is another splice factor, acting in turn on downstream RNAs that require sex-specific splicing (Sosnowski, 1994). Transformer protein thus determines female developmental fate.

Act 2. In the spotlight here is dosage compensation, regulated by Sex lethal. The immediate target of SXL is male specific lethal-2 (MSL-2), a transcription factor. In the presence of SXL, MSL-2 is spliced into an inactive form, one that cannot function in dosage compensation. In the absence of SXL (in males), MSL-2 splicing is productive, and the active MSL-2 transcription factor effectively carries out dosage compensation.

A word about dosage compensation is in order. The ratio of sex chromosomes to autosomes in females (1:1) is different from the ratio in males (0.5:1). This is because males, by definition, have one X chromosome and not two. This presents a dosage problem. The ratio of gene products coded for by the sex chromosome will be different in males and females, unless some compensatory action is taken. The sex chromosome carries a lot of genes that are simply along for the ride, and the creation of a dosage imbalance spells catastrophe for development. What to do?

Two alternatives are possible. One of the X chromosomes could be shut off, inactivated in females. This is the solution humans and other higher vertebrates have employed. A second option would be to heighten the activity of the single X chromosome in males. This is the route flies take. MSL-2, spliced into a functional form in males, serves to heighten transcriptional activation of the solitary X chromosome. MSL-2 acts in concert with three partners in this task: Maleless, Male-specific lethal-1 and Male specific lethal-3. They bind to about one hundred sites on the male chromosome, modifying the chromatin structure to permit heightened gene activation (Bashaw, 1995). The action of these male specific transcription factors is very similar to proteins of the trithorax complex.

Act 3. In the center ring here is the regulation by Sex lethal of sex specific RNA splicing in ovaries. Ovaries represent a completely different tissue milieu from somatic cells, and consequently Sex-lethal splicing in these tissues will demand a different kind of regulation (Granadino, 1993). Germ line Sex-lethal function requires an XX karyotype, a female soma, and action of the genes ovp, otu and snf (sans fille). SNF is an RNA binding protein and an integral component of the machinery required for splice site recognition (Flickinger, 1994 and Salz, 1996). ovo, a zinc finger protein and presumably a transcription factor, has a higher level of transcription in females than in males, responding to the number of sex chromosomes in the cell (Oliver, 1994). otu (ovarian tumor) gene also acts upstream of Sex lethal (Pauli, 1993 and Bopp, 1993). Thus ovarian Sex lethal activation and splicing is regulated by a completely different set of proteins (with the exception of SNF, whose function is general) from those in the embryo.

Biological systems operate at a level of complexity that continually astounds the student. A tiny fly carries a whole circus at the core of its development, as far as sex determination is concerned.

GENE STRUCTURE

Sex-lethal expresses a set of three early transcripts and a set of seven late transcripts occurring from midembryogenesis through adulthood. Among the late transcripts, male-specific mRNAs have been distinguished from their female counterparts by the presence of an extra exon interrupting an otherwise long open reading frame (ORF). The late transcripts appear to use a common 5' end but differ at their 3' ends by the use of alternative polyadenylation sites. Two of these sites lack canonical AATAAA sequences, and their use correlates in females with the presence of a functional germ line, suggesting possible tissue-specific polyadenylation. A number of non-sex-specific splicing variants have been observed. In females, the various forms of late SXL transcript potentially encode up to six slightly different polypeptides (Samuels, 1991).

Sex lethal has two transcripts, early and late, with different promoters and dramatically different splicing patterns. The Sxl early transcripts are activated transiently in early embryos by a female-specific promoter and have a unique 5' exon (E1) located between late exons 1 and 2. Exon E1 is spliced to exon 4, which is common to all SXL transcripts, skipping both exons 2 and 3 (Keyes, 1992). In contrast, the late SXL transcripts derive from an essentially constitutive promoter but are spliced sex specifically. The male-specific exon, exon 3, is included by default in all male transcripts and contains in-frame nonsense codons that block SXL protein production. In the presence of SXL protein, the late transcripts skip exon 3 and splice in the female pattern. No embryo-specific splicing factors are needed for the early splice. Neither are sex-specific factors required. Instead, the early splicing pattern is dependent on whether the 5' splice site region originates from exon E1 or exon 2 (Zhu, 1997).

PROTEIN STRUCTURE

Amino Acids - 354 for product of female cDNA

Structural domains

The female SXL protein has large duplicate domains each with 74 residues that are 35% identical to one another. These domains are common to RNA binding proteins. SXL shows greatest homology to yeast poly A binding protein (Bell, 1988).

date revised: 20 June 98

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