A conserved RNA-binding protein that regulates sexual fates in the C. elegans hermaphrodite germ line

Germ-line regulation of the Caenorhabditis elegans sex-determining gene tra-2

The Caenorhabditis elegans sex-determining gene tra-2 promotes female development of the XX hermaphrodite soma and germ line. We previously showed that a 4.7-kb tra-2 mRNA, which encodes the membrane protein TRA-2A, provides the primary feminizing activity of the tra-2 locus. This paper focuses on the germ-line activity and regulation of tra-2. First, we characterize a 1.8-kb tra-2 mRNA, which is hermaphrodite-specific and germ-line-dependent. This mRNA encodes TRA-2B, a protein identical to a predicted intracellular domain of TRA-2A. We show that the 1.8-kb mRNA is oocyte-specific, suggesting that it is involved in germ-line or embryonic sex determination. Second, we identify a tra-2 maternal effect on brood size that may be associated with the 1.8-kb mRNA. Third, we investigate seven dominant tra-2(mx) (for mixed character) mutations that sexually transform hermaphrodites to females by eliminating hermaphrodite spermatogenesis. Each of the tra-2(mx) mutants possesses a nonconserved missense change in a 22-amino-acid region common to both TRA-2A and TRA-2B, called the MX region. We propose that the MX region mediates a posttranslational regulation of tra-2 essential for the onset of hermaphrodite spermatogenesis. Finally, we discuss aspects of tra-2 function and regulation that are specific to the unusual control of cell fate in the hermaphrodite germ line.

Repression by the 3' UTR of fem-3, a sex-determining gene, relies on a ubiquitous mog-dependent control in Caenorhabditis elegans

The fem-3 sex-determining gene is repressed post-transcriptionally via a regulatory element in its 3' untranslated region (UTR) to achieve the switch from spermatogenesis to oogenesis in the Caenorhabditis elegans hermaphrodite germ line. In this paper, we investigate the fem-3 3' UTR control in somatic tissues using transgenic reporter assays, and we also identify six genes essential for this control. First, we find that a reporter transgene bearing a wild-type fem-3 3' UTR is repressed in somatic tissues, whereas one bearing a mutant fem-3 3' UTR is derepressed. Moreover, control by mutant 3' UTRs is temperature sensitive as predicted from the temperature sensitivity of the fem-3 gain-of-function (gf) mutations. Secondly, we find a fem-3 3' UTR RNA-binding activity in somatic tissues, in addition to the previously reported germ-line-specific binding by FBF. Thirdly, we find that each of six genes, mog-1-mog-6, is required for repression by the fem-3 3' UTR. Therefore, the mog genes not only affect the sperm/oocyte switch in the germ line, but also function in somatic tissues. We suggest that the mog genes may encode components of a ubiquitous machinery that is used for fem-3 3' UTR-mediated repression and the sperm/oocyte switch.

Developmental genetics of Caenorhabditis elegans sex determination

The nematode Caenorhabditis elegans has two naturally occurring sexes: a self-fertile XX hermaphrodite that first produces sperm, then oocytes, and an XO male. The primary determinant of sex is the X:A ratio, the number of X chromosomes to sets of autosomes. The X:A ratio regulates not only sex determination, but also dosage compensation. In the intervening years since the identification of the X:A ratio, most of the key regulatory genes that respond to the X:A ratio have been genetically identified and ordered into regulatory hierarchies. Advances have also been made in identifying the X chromosome numerator elements of the X:A ratio. This review highlights the genetic, molecular, and biochemical approaches that have led to an understanding of how these genes interact to control sex determination and dosage compensation. The review also discusses the differences between the control of sexual cell fate in the soma and germ line of C. elegans and addresses the role of germ-line-specific regulation in controlling the sperm-oocyte decision in the hermaphrodite germ line. Finally, strategies that take advantage of the availability of the entire C. elegans genome sequence, which is expected to be completed in 1998, are discussed for identifying hitherto unidentified genes that may play a role in the control of sexual cell fate.

Changing styles in C. elegans genetics

The past 30 years have taken the nematode Caenorhabditis elegans from obscurity, as a nondescript member of a large but unglamorous invertebrate phylum, to a position as one of the major model organisms. This year, it will acquire a particular celeberity as the owner of the first animal genome to be sequenced in its entirety. In this review we consider the ways in which genetical investigations of this species have begun to change and what some of the consequences of the completion of the sequence are likely to be.

Caenorhabditis elegans Akt/PKB transduces insulin receptor-like signals from AGE-1 PI3 kinase to the DAF-16 transcription factor

A neurosecretory pathway regulates a reversible developmental arrest and metabolic shift at the Caenorhabditis elegans dauer larval stage. Defects in an insulin-like signaling pathway cause arrest at the dauer stage. We show here that two C. elegans Akt/PKB homologs, akt-1 and akt-2, transduce insulin receptor-like signals that inhibit dauer arrest and that AKT-1 and AKT-2 signaling are indispensable for insulin receptor-like signaling in C. elegans. A loss-of-function mutation in the Fork head transcription factor DAF-16 relieves the requirement for Akt/PKB signaling, which indicates that AKT-1 and AKT-2 function primarily to antagonize DAF-16. This is the first evidence that the major target of Akt/PKB signaling is a transcription factor. An activating mutation in akt-1, revealed by a genetic screen, as well as increased dosage of wild-type akt-1 relieves the requirement for signaling from AGE-1 PI3K, which acts downstream of the DAF-2 insulin/IGF-1 receptor homolog. This demonstrates that Akt/PKB activity is not necessarily dependent on AGE-1 PI3K activity. akt-1 and akt-2 are expressed in overlapping patterns in the nervous system and in tissues that are remodeled during dauer formation.

Binding of the human immunodeficiency virus type 1 Gag protein to the viral RNA encapsidation signal in the yeast three-hybrid system

We have used the yeast three-hybrid system (D. J. SenGupta, B. Zhang, B. Kraemer, P. Pochart, S. Fields, and M. Wickens, Proc. Natl. Acad. Sci. USA 93:8496-8501, 1996) to study binding of the human immunodeficiency virus type 1 (HIV-1) Gag protein to the HIV-1 RNA encapsidation signal (HIVPsi). Interaction of these elements results in the activation of a reporter gene in the yeast Saccharomyces cerevisiae. Using this system, we have shown that the HIV-1 Gag protein binds specifically to a 139-nucleotide fragment of the HIVPsi signal containing four stem-loop structures. Mutations in either the Gag protein or the encapsidation signal that have been shown previously to impair this interaction reduced the activation of the reporter gene. Interestingly, the nucleocapsid portion of Gag retained the RNA binding activity but lost its specificity compared to the full-length Gag. These results demonstrate the utility of this system and suggest that a variety of genetic analyses could be performed to study Gag-encapsidation signal interactions.

Detection of internal repeats: how common are they?

The exponential growth in the amount of genomic data published in recent years has led to increased efforts in analysing genomes for the presence of repeated sequences, which has in turn fostered the development of novel repeat recognition methods. This has resulted in a deepened understanding of the importance and abundance of protein and nucleotide repeats. In the past year, a shift in focus has taken place--from the significance of repeats to protein structure and function, mostly at the protein domain level, to the implication of generally much shorter repeated fragments in genetic diseases and protein malfunctioning.

The Pumilio RNA-binding domain is also a translational regulator

Posterior patterning in the Drosophila embryo requires the action of Nanos (Nos) and Pumilio (Pum), which collaborate to regulate the translation of maternal hunchback (hb) mRNA. Previous work demonstrated that Pum recognizes sites in the 3' UTR of hb mRNA. In this report, we first define the RNA-binding domain of Pum and then show that residues essential for translational repression are embedded within this domain. We also show that Nos and Pum can repress cap-independent translation from an internal ribosome entry site (IRES) in vivo, suggesting that they act downstream of the initial steps of normal, cap-dependent translation.

Gametogenesis: keeping the male element under control

Caenorhabditis elegans hermaphrodites switch from making sperm to oocytes. This switch involves repression of fem-3 mRNA, mediated by a protein that binds RNA through a conserved motif; a similar motif mediates RNA binding by the Drosophila pattern-regulatory protein Pumilio.