Positive autoregulation of sex-lethal by alternative splicing maintains the female determined state in Drosophila

Feed-forward control of prostate growth: dihydrotestosterone induces expression of its own biosynthetic enzyme, steroid 5 alpha-reductase

Dihydrotestosterone, the primary mediator of prostate growth, is synthesized in target tissues from the circulating androgen testosterone through the action of steroid 5 alpha-reductase (EC 1.3.99.5). The expression of 5 alpha-reductase and the level of 5 alpha-reductase messenger RNA in rat ventral prostate are regulated by androgens. To determine whether this control is mediated by dihydrotestosterone or testosterone, we investigated the effect of finasteride, a potent inhibitor of steroid 5 alpha-reductase, on the expression of 5 alpha-reductase in the prostate. The administration of finasteride to intact rats for 7 days caused a 55% decrease in prostate weight and an 87% decrease in 5 alpha-reductase enzyme activity. Furthermore, the restoration of prostate growth after castration and the enhancement in 5 alpha-reductase enzyme activity and 5 alpha-reductase messenger RNA level by testosterone administration were blocked by finasteride, whereas the inhibitor had no effect on dihydrotestosterone-mediated increases in 5 alpha-reductase activity or messenger RNA level. These findings indicate that dihydrotestosterone itself controls prostate growth and 5 alpha-reductase activity. They further suggest that prostate growth is controlled by a feed-forward mechanism by which formation of trace amounts of dihydrotestosterone induces 5 alpha-reductase, thereby increasing dihydrotestosterone synthesis and triggering a positive developmental cascade.

Primary structure of the human splicing factor ASF reveals similarities with Drosophila regulators

We described previously the purification of a human protein, called alternative splicing factor (ASF), that can switch utilization of alternative 5' splice sites in an SV40 early pre-mRNA. We now report the isolation of a cDNA, designated ASF-1, that encodes this protein. ASF-1 consists of 248 amino acid residues, including an 80 residue RNA-binding domain at its N-terminus and a 50 residue C-terminal region that is 80% serine plus arginine. ASF-1 produced in E. coli can activate splicing in vitro and switch 5' splice-site utilization, establishing that the recombinant protein is sufficient to supply these activities. Analysis of additional cDNAs revealed that ASF pre-mRNA can itself be alternatively spliced, surprisingly, by utilization of a shared 5' splice site and two closely spaced 3' splice sites. Use of the upstream site results in a second mRNA (ASF-2) in which translation of the downstream exon occurs extensively in an alternative reading frame distinct from ASF-1.

The complex set of late transcripts from the Drosophila sex determination gene sex-lethal encodes multiple related polypeptides

Sex-lethal (Sxl), a key sex determination gene in Drosophila melanogaster, is known to express a set of three early transcripts arising during early embryogenesis and a set of seven late transcripts occurring from midembryogenesis through adulthood. Among the late transcripts, male-specific mRNAs were distinguished from their female counterparts by the presence of an extra exon interrupting an otherwise long open reading frame (ORF). We have now analyzed the structures of the late Sxl transcripts by cDNA sequencing, Northern (RNA) blotting, primer extension, and RNase protection. 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. Besides the presence of the male-specific exon, no additional sex-specific splicing events were detected, although a number of non-sex-specific splicing variants were observed. In females, the various forms of late Sxl transcript potentially encode up to six slightly different polypeptides. All of the protein-coding differences occur outside the previously defined ribonucleoprotein motifs. One class of Sxl mRNAs also includes a second long ORF in the same frame as the first ORF but separated from it by a single ochre codon. The function of this second ORF is unknown. Significant amounts of apparently partially processed Sxl RNAs were observed, consistent with the hypothesis that the regulated Sxl splices occur relatively slowly.

The complex set of late transcripts from the Drosophila sex determination gene sex-lethal encodes multiple related polypeptides

Sex-lethal (Sxl), a key sex determination gene in Drosophila melanogaster, is known to express a set of three early transcripts arising during early embryogenesis and a set of seven late transcripts occurring from midembryogenesis through adulthood. Among the late transcripts, male-specific mRNAs were distinguished from their female counterparts by the presence of an extra exon interrupting an otherwise long open reading frame (ORF). We have now analyzed the structures of the late Sxl transcripts by cDNA sequencing, Northern (RNA) blotting, primer extension, and RNase protection. 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. Besides the presence of the male-specific exon, no additional sex-specific splicing events were detected, although a number of non-sex-specific splicing variants were observed. In females, the various forms of late Sxl transcript potentially encode up to six slightly different polypeptides. All of the protein-coding differences occur outside the previously defined ribonucleoprotein motifs. One class of Sxl mRNAs also includes a second long ORF in the same frame as the first ORF but separated from it by a single ochre codon. The function of this second ORF is unknown. Significant amounts of apparently partially processed Sxl RNAs were observed, consistent with the hypothesis that the regulated Sxl splices occur relatively slowly.

Autoregulation of the splicing of transcripts from the transformer-2 gene of Drosophila

The Drosophila transformer-2 gene uses alternative promoters and splicing patterns to generate four different mRNAs that together encode three putative RNA-binding polypeptides. The transformer-2 products expressed in somatic tissues function to regulate the RNA splicing of the sex determination gene doublesex, whereas products expressed in the male germ line play an unknown, but essential, role in spermatogenesis. Two alternatively spliced transformer-2 transcripts, each encoding a different putative RNA-binding protein, are found only in the male germ line. These male germ line-specific mRNAs differ from each other by the presence or absence of a single intron called M1. We show that M1-containing transcripts make up a majority of transformer-2 germ-line transcripts in wild-type males but fail to accumulate in males homozygous for transformer-2 null mutations. Germ-line transformation experiments using a variety of reporter gene constructs demonstrate that specific polypeptide products of the transformer-2 gene itself normally repress M1 splicing in the male germ line. Thus, in addition to its role in the sex-specific control of doublesex RNA splicing in somatic tissues, the transformer-2 gene also regulates the splicing of its own transcripts in the male germ line. We propose that this autoregulatory function may serve in negative feedback control of transformer-2 activity during spermatogenesis. The finding that transformer-2 controls multiple splicing decisions suggests that a variety of different alternative splicing choices could be regulated by a relatively limited number of trans-acting factors.

Developmental distribution of female-specific Sex-lethal proteins in Drosophila melanogaster

The binary switch gene Sex-lethal (Sxl) must be on in females and off in males to allow the proper elaboration of the appropriate sexual developmental pathway in Drosophila melanogaster. Previous studies suggested a mechanism in which the on/off regulation of Sxl occurs post-transcriptionally at the level of RNA splicing. A critical prediction of this model is that functional Sxl proteins are absent in males but present in females. In this report we show that the expected full-length proteins are only present in female animals. Multiple forms of Sxl protein are found in females, some of which are expressed in a stage- and tissue-specific pattern. Consistent with a role of Sxl proteins in regulating alternate splicing, the proteins are localized in the nucleus where they exhibit a punctate staining pattern. Surprisingly, several minor Sxl proteins appear to be present in specific tissues of both sexes of adults. The possible origin of these species is discussed. We also show that Sxl expression in the early embryo is sex specific and depends on maternal daughterless and zygotic sisterless-b activity in accordance with the established roles of these genes as positive regulators of Sxl. The onset of Sxl expression in the germ line occurs later than that in the soma.