The in vivo functional significance of PUF hub partnerships in C. elegans germline stem cells.

PUF RNA-binding proteins are conserved stem cell regulators. Four PUF proteins govern self-renewal of Caenorhabditis elegans germline stem cells together with two intrinsically disordered proteins, LST-1 and SYGL-1. Based on yeast two-hybrid results, we previously proposed a composite self-renewal hub in the stem cell regulatory network, with eight PUF partnerships and extensive redundancy. Here, we investigate LST-1-PUF and SYGL-1-PUF partnerships and their molecular activities in their natural context - nematode stem cells. We confirm LST-1-PUF partnerships and their specificity to self-renewal PUFs by co-immunoprecipitation and show that an LST-1(AmBm) mutant defective for PUF-interacting motifs does not complex with PUFs in nematodes. LST-1(AmBm) is used to explore the in vivo functional significance of the LST-1-PUF partnership. Tethered LST-1 requires this partnership to repress expression of a reporter RNA, and LST-1 requires the partnership to co-immunoprecipitate with NTL-1/Not1 of the CCR4-NOT complex. We suggest that the partnership provides multiple molecular interactions that work together to form an effector complex on PUF target RNAs in vivo. Comparison of LST-1-PUF and Nanos-Pumilio reveals fundamental molecular differences, making LST-1-PUF a distinct paradigm for PUF partnerships.

LST-1 is a bifunctional regulator that feeds back on Notch-dependent transcription to regulate C. elegans germline stem cells.

Notch signaling regulates stem cells across animal phylogeny. C. elegans Notch signaling activates transcription of two genes, lst-1 and sygl-1, that encode potent regulators of germline stem cells. The LST-1 protein regulates stem cells in two distinct ways: It promotes self-renewal posttranscriptionally and also restricts self-renewal by a poorly understood mechanism. Its self-renewal promoting activity resides in its N-terminal region, while its self-renewal restricting activity resides in its C-terminal region and requires the Zn finger. Here, we report that LST-1 limits self-renewal by down-regulating Notch-dependent transcription. We detect LST-1 in the nucleus, in addition to its previously known cytoplasmic localization. LST-1 lowers nascent transcript levels at both lst-1 and sygl-1 loci but not at let-858, a Notch-independent locus. LST-1 also lowers levels of two key components of the Notch activation complex, the LAG-1 DNA binding protein and Notch intracellular domain (NICD). Genetically, an LST-1 Zn finger mutant increases Notch signaling strength in both gain- and loss-of-function GLP-1/Notch receptor mutants. Biochemically, LST-1 co-immunoprecipitates with LAG-1 from nematode extracts, suggesting a direct effect. LST-1 is thus a bifunctional regulator that coordinates posttranscriptional and transcriptional mechanisms in a single protein. This LST-1 bifunctionality relies on its bipartite protein architecture and is bolstered by generation of two LST-1 isoforms, one specialized for Notch downregulation. A conserved theme from worms to human is the coupling of PUF-mediated RNA repression together with Notch feedback in the same protein.

Notch-dependent DNA cis-regulatory elements and their dose-dependent control of C. elegans stem cell self-renewal.

A long-standing biological question is how DNA cis-regulatory elements shape transcriptional patterns during metazoan development. Reporter constructs, cell culture assays and computational modeling have made major contributions to answering this question, but analysis of elements in their natural context is an important complement. Here, we mutate Notch-dependent LAG-1 binding sites (LBSs) in the endogenous Caenorhabditis elegans sygl-1 gene, which encodes a key stem cell regulator, and analyze the consequences on sygl-1 expression (nascent transcripts, mRNA, protein) and stem cell maintenance. Mutation of one LBS in a three-element cluster approximately halved both expression and stem cell pool size, whereas mutation of two LBSs essentially abolished them. Heterozygous LBS mutant clusters provided intermediate values. Our results lead to two major conclusions. First, both LBS number and configuration impact cluster activity: LBSs act additively in trans and synergistically in cis. Second, the SYGL-1 gradient promotes self-renewal above its functional threshold and triggers differentiation below the threshold. Our approach of coupling CRISPR/Cas9 LBS mutations with effects on both molecular and biological readouts establishes a powerful model for in vivo analyses of DNA cis-regulatory elements.

The great small organisms of developmental genetics: Caenorhabditis elegans and Drosophila melanogaster.

Experimental embryologists working at the turn of the 19th century suggested fundamental mechanisms of development, such as localized cytoplasmic determinants and tissue induction. However, the molecular basis underlying these processes proved intractable for a long time, despite concerted efforts in many developmental systems to isolate factors with a biological role. That road block was overcome by combining developmental biology with genetics. This powerful approach used unbiased genome-wide screens to isolate mutants with developmental defects and to thereby identify genes encoding key determinants and regulatory pathways that govern development. Two small invertebrates were the pioneers: the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans. Their modes of development differ in many ways, but the two together led the way to unraveling the molecular mechanisms of many fundamental developmental processes. The discovery of the grand homologies between key players in development throughout the animal kingdom underscored the usefulness of studying these small invertebrate models for animal development and even human disease. We describe developmental genetics in Drosophila and C. elegans up to the rise of genomics at the beginning of the 21st Century. Finally, we discuss themes that emerge from the histories of such distinct organisms and prospects of this approach for the future.

An improved in vivo tethering assay with single molecule FISH reveals that a nematode Nanos enhances reporter expression and mRNA stability.

Robust methods are critical for testing the in vivo regulatory mechanism of RNA binding proteins. Here we report improvement of a protein-mRNA tethering assay to probe the function of an RNA binding protein in its natural context within the C. elegans adult germline. The assay relies on a dual reporter expressing two mRNAs from a single promoter and resolved by trans-splicing. The gfp reporter 3'UTR harbors functional binding elements for λN22 peptide, while the mCherry reporter 3'UTR carries mutated nonfunctional elements. This strategy enables internally controlled quantitation of reporter protein by immunofluorescence and mRNA by smFISH. To test the new system, we analyzed a C. elegans Nanos protein, NOS-3, which serves as a post-transcriptional regulator of germ cell fate. Unexpectedly, tethered NOS-3 enhanced reporter expression. We confirmed this enhancement activity with a second reporter engineered at an endogenous germline gene. NOS-3 enhancement of reporter expression was associated with its amino-terminal intrinsically disordered region, not its carboxy-terminal zinc fingers. RNA quantitation revealed that tethered NOS-3 enhances stability of the reporter mRNA. We suggest that this direct NOS-3 enhancement activity may explain a paradox: Classically Nanos proteins are expected to repress RNA, but nos-3 had been found to promote gld-1 expression, an effect that could be direct. Regardless, the new dual reporter dramatically improves in situ quantitation of reporter expression after RNA binding protein tethering to determine its molecular mechanism in a multicellular tissue.

Two classes of active transcription sites and their roles in developmental regulation.

The expression of genes encoding powerful developmental regulators is exquisitely controlled, often at multiple levels. Here, we investigate developmental expression of three conserved genes, Caenorhabditis elegans mpk-1, lag-1, and lag-3/sel-8, which encode homologs of ERK/MAPK and core components of the Notch-dependent transcription complex, respectively. We use single-molecule FISH (smFISH) and MATLAB to visualize and quantify nuclear nascent transcripts and cytoplasmic mRNAs as a function of position along the germline developmental axis. Using differentially labeled probes, one spanning an exceptionally long first intron and the other spanning exons, we identify two classes of active transcription sites (ATS). The iATS class, for "incomplete" ATS, harbors only partial nascent transcripts; the cATS class, for "complete" ATS, harbors full-length nascent transcripts. Remarkably, the frequencies of iATS and cATS are patterned along the germline axis. For example, most mpk-1 ATS are iATS in hermaphrodite germline stem cells, but most are cATS in differentiating stem cell daughters. Thus, mpk-1 ATS class frequencies switch in a graded manner as stem cell daughters begin differentiation. Importantly, the patterns of ATS class frequency are gene-, stage-, and sex-specific, and cATS frequency strongly correlates with transcriptional output. Although the molecular mechanism underlying ATS classes is not understood, their primary difference is the extent of transcriptional progression. To generate only partial nascent transcripts in iATS, progression must be slowed, paused, or aborted midway through the gene. We propose that regulation of ATS class can be a critical mode of developmental gene regulation.

The molecular basis of LST-1 self-renewal activity and its control of stem cell pool size.

PUF RNA-binding proteins have diverse roles in animal development, with a broadly conserved role in stem cells. Two paradigmatic PUF proteins, FBF-1 and FBF-2, promote both self-renewal and differentiation in the C. elegans germline. The LST-1 protein is a pivotal regulator of self-renewal and is oncogenic when mis-expressed. Here, we demonstrate that LST-1 self-renewal activity resides within a predicted disordered region that harbors two KXXL motifs. We find that the KXXL motifs mediate the binding of LST-1 to FBF, and that point mutations of these motifs abrogate LST-1 self-renewal activity. The LST-1-FBF partnership is therefore crucial to stem cell maintenance and is a key element in the FBF regulatory network. A distinct region within LST-1 determines its spatial expression and size of the GSC pool. Most importantly, the molecular understanding of how an IDR-rich protein works in an essential partnership with a conserved stem cell regulator and RNA-binding protein suggests broad new avenues for combinatorial control.

Unbiased screen of RNA tailing activities reveals a poly(UG) polymerase.

Ribonucleotidyl transferases (rNTases) add untemplated ribonucleotides to diverse RNAs. We have developed TRAID-seq, a screening strategy in Saccharomyces cerevisiae to identify sequences added to a reporter RNA at single-nucleotide resolution by overexpressed candidate enzymes from different organisms. The rNTase activities of 22 previously unexplored enzymes were determined. In addition to poly(A)- and poly(U)-adding enzymes, we identified a cytidine-adding enzyme that is likely to be part of a two-enzyme system that adds CCA to tRNAs in a eukaryote; a nucleotidyl transferase that adds nucleotides to RNA without apparent nucleotide preference; and a poly(UG) polymerase, Caenorhabditis elegans MUT-2, that adds alternating uridine and guanosine nucleotides to form poly(UG) tails. MUT-2 is known to be required for certain forms of RNA silencing, and mutants of the enzyme that result in defective silencing did not add poly(UG) tails in our assay. We propose that MUT-2 poly(UG) polymerase activity is required to promote genome integrity and RNA silencing.

SYGL-1 and LST-1 link niche signaling to PUF RNA repression for stem cell maintenance in Caenorhabditis elegans.

Central questions in regenerative biology include how stem cells are maintained and how they transition from self-renewal to differentiation. Germline stem cells (GSCs) in Caeno-rhabditis elegans provide a tractable in vivo model to address these questions. In this system, Notch signaling and PUF RNA binding proteins, FBF-1 and FBF-2 (collectively FBF), maintain a pool of GSCs in a naïve state. An open question has been how Notch signaling modulates FBF activity to promote stem cell self-renewal. Here we report that two Notch targets, SYGL-1 and LST-1, link niche signaling to FBF. We find that SYGL-1 and LST-1 proteins are cytoplasmic and normally restricted to the GSC pool region. Increasing the distribution of SYGL-1 expands the pool correspondingly, and vast overexpression of either SYGL-1 or LST-1 generates a germline tumor. Thus, SYGL-1 and LST-1 are each sufficient to drive "stemness" and their spatial restriction prevents tumor formation. Importantly, SYGL-1 and LST-1 can only drive tumor formation when FBF is present. Moreover, both proteins interact physically with FBF, and both are required to repress a signature FBF mRNA target. Together, our results support a model in which SYGL-1 and LST-1 form a repressive complex with FBF that is crucial for stem cell maintenance. We further propose that progression from a naïve stem cell state to a state primed for differentiation relies on loss of SYGL-1 and LST-1, which in turn relieves FBF target RNAs from repression. Broadly, our results provide new insights into the link between niche signaling and a downstream RNA regulatory network and how this circuitry governs the balance between self-renewal and differentiation.

C. elegans germ cells divide and differentiate in a folded tissue.

Knowing how stem cells and their progeny are positioned within their tissues is essential for understanding their regulation. One paradigm for stem cell regulation is the C. elegans germline, which is maintained by a pool of germline stem cells in the distal gonad, in a region known as the 'progenitor zone'. The C. elegans germline is widely used as a stem cell model, but the cellular architecture of the progenitor zone has been unclear. Here we characterize this architecture by creating virtual 3D models of the progenitor zone in both sexes. We show that the progenitor zone in adult hermaphrodites is organized like a folded epithelium. The progenitor zone in males is not folded. Analysis of germ cell division shows that daughter cells are born side-by-side along the epithelial-like surface of the germline tissue. Analysis of a key regulator driving differentiation, GLD-1, shows that germ cells in hermaphrodites differentiate along a folded path, with previously described "steps" in GLD-1 expression corresponding to germline folds. Our study provides a three-dimensional view of how C. elegans germ cells progress from stem cell to overt differentiation, with critical implications for regulators driving this transition.

PGL germ granule assembly protein is a base-specific, single-stranded RNase.

Cellular RNA-protein (RNP) granules are ubiquitous and have fundamental roles in biology and RNA metabolism, but the molecular basis of their structure, assembly, and function is poorly understood. Using nematode "P-granules" as a paradigm, we focus on the PGL granule scaffold protein to gain molecular insights into RNP granule structure and assembly. We first identify a PGL dimerization domain (DD) and determine its crystal structure. PGL-1 DD has a novel 13 α-helix fold that creates a positively charged channel as a homodimer. We investigate its capacity to bind RNA and discover unexpectedly that PGL-1 DD is a guanosine-specific, single-stranded endonuclease. Discovery of the PGL homodimer, together with previous results, suggests a model in which the PGL DD dimer forms a fundamental building block for P-granule assembly. Discovery of the PGL RNase activity expands the role of RNP granule assembly proteins to include enzymatic activity in addition to their job as structural scaffolds.

The PUF binding landscape in metazoan germ cells.

PUF (Pumilio/FBF) proteins are RNA-binding proteins and conserved stem cell regulators. The Caenorhabditis elegans PUF proteins FBF-1 and FBF-2 (collectively FBF) regulate mRNAs in germ cells. Without FBF, adult germlines lose all stem cells. A major gap in our understanding of PUF proteins, including FBF, is a global view of their binding sites in their native context (i.e., their "binding landscape"). To understand the interactions underlying FBF function, we used iCLIP (individual-nucleotide resolution UV crosslinking and immunoprecipitation) to determine binding landscapes of C. elegans FBF-1 and FBF-2 in the germline tissue of intact animals. Multiple iCLIP peak-calling methods were compared to maximize identification of both established FBF binding sites and positive control target mRNAs in our iCLIP data. We discovered that FBF-1 and FBF-2 bind to RNAs through canonical as well as alternate motifs. We also analyzed crosslinking-induced mutations to map binding sites precisely and to identify key nucleotides that may be critical for FBF-RNA interactions. FBF-1 and FBF-2 can bind sites in the 5'UTR, coding region, or 3'UTR, but have a strong bias for the 3' end of transcripts. FBF-1 and FBF-2 have strongly overlapping target profiles, including mRNAs and noncoding RNAs. From a statistically robust list of 1404 common FBF targets, 847 were previously unknown, 154 were related to cell cycle regulation, three were lincRNAs, and 335 were shared with the human PUF protein PUM2.

A nuclear Argonaute promotes multigenerational epigenetic inheritance and germline immortality.

Epigenetic information is frequently erased near the start of each new generation. In some cases, however, epigenetic information can be transmitted from parent to progeny (multigenerational epigenetic inheritance). A particularly notable example of this type of epigenetic inheritance is double-stranded RNA-mediated gene silencing in Caenorhabditis elegans. This RNA-mediated interference (RNAi) can be inherited for more than five generations. To understand this process, here we conduct a genetic screen for nematodes defective in transmitting RNAi silencing signals to future generations. This screen identified the heritable RNAi defective 1 (hrde-1) gene. hrde-1 encodes an Argonaute protein that associates with small interfering RNAs in the germ cells of progeny of animals exposed to double-stranded RNA. In the nuclei of these germ cells, HRDE-1 engages the nuclear RNAi defective pathway to direct the trimethylation of histone H3 at Lys 9 (H3K9me3) at RNAi-targeted genomic loci and promote RNAi inheritance. Under normal growth conditions, HRDE-1 associates with endogenously expressed short interfering RNAs, which direct nuclear gene silencing in germ cells. In hrde-1- or nuclear RNAi-deficient animals, germline silencing is lost over generational time. Concurrently, these animals exhibit steadily worsening defects in gamete formation and function that ultimately lead to sterility. These results establish that the Argonaute protein HRDE-1 directs gene-silencing events in germ-cell nuclei that drive multigenerational RNAi inheritance and promote immortality of the germ-cell lineage. We propose that C. elegans use the RNAi inheritance machinery to transmit epigenetic information, accrued by past generations, into future generations to regulate important biological processes.

NeuCode Labeling in Nematodes: Proteomic and Phosphoproteomic Impact of Ascaroside Treatment in Caenorhabditis elegans.

The nematode Caenorhabditis elegans is an important model organism for biomedical research. We previously described NeuCode stable isotope labeling by amino acids in cell culture (SILAC), a method for accurate proteome quantification with potential for multiplexing beyond the limits of traditional stable isotope labeling by amino acids in cell culture. Here we apply NeuCode SILAC to profile the proteomic and phosphoproteomic response of C. elegans to two potent members of the ascaroside family of nematode pheromones. By consuming labeled E. coli as part of their diet, C. elegans nematodes quickly and easily incorporate the NeuCode heavy lysine isotopologues by the young adult stage. Using this approach, we report, at high confidence, one of the largest proteomic and phosphoproteomic data sets to date in C. elegans: 6596 proteins at a false discovery rate ≤ 1% and 6620 phosphorylation isoforms with localization probability ≥75%. Our data reveal a post-translational signature of pheromone sensing that includes many conserved proteins implicated in longevity and response to stress.

Discovery of two GLP-1/Notch target genes that account for the role of GLP-1/Notch signaling in stem cell maintenance.

A stem cell's immediate microenvironment creates an essential "niche" to maintain stem cell self-renewal. Many niches and their intercellular signaling pathways are known, but for the most part, the key downstream targets of niche signaling remain elusive. Here, we report the discovery of two GLP-1/Notch target genes, lst-1 (lateral signaling target) and sygl-1 (synthetic Glp), that function redundantly to maintain germ-line stem cells (GSCs) in the nematode Caenorhabditis elegans. Whereas lst-1 and sygl-1 single mutants appear normal, lst-1 sygl-1 double mutants are phenotypically indistinguishable from glp-1/Notch mutants. Multiple lines of evidence demonstrate that GLP-1/Notch signaling activates lst-1 and sygl-1 expression in GSCs within the niche. Therefore, these two genes fully account for the role of GLP-1/Notch signaling in GSC maintenance. Importantly, lst-1 and sygl-1 are not required for GLP-1/Notch signaling per se. We conclude that lst-1 and sygl-1 forge a critical link between Notch signaling and GSC maintenance.

Mitosis-meiosis and sperm-oocyte fate decisions are separable regulatory events.

Germ cell fate decisions are poorly understood, despite their central role in reproduction. One fundamental question has been whether germ cells are regulated to enter the meiotic cell cycle (i.e., mitosis-meiosis decision) and to be sperm or oocyte (i.e., sperm-oocyte decision) through one or two cell fate choices. If a single decision is used, a male-specific or female-specific meiotic entry would lead necessarily toward spermatogenesis or oogenesis, respectively. If two distinct decisions are used, meiotic entry should be separable from specification as sperm or oocyte. Here, we investigate the relationship of these two decisions with tools uniquely available in the nematode Caenorhabditis elegans. Specifically, we used a temperature-sensitive Notch allele to drive germ-line stem cells into the meiotic cell cycle, followed by chemical inhibition of the Ras/ERK pathway to reprogram the sperm-oocyte decision. We found that germ cells already in meiotic prophase can nonetheless be sexually transformed from a spermatogenic to an oogenic fate. This finding cleanly uncouples the mitosis-meiosis decision from the sperm-oocyte decision. In addition, we show that chemical reprogramming occurs in a germ-line region where germ cells normally transition from the mitotic to the meiotic cell cycle and that it dramatically changes the abundance of key sperm-oocyte fate regulators in meiotic germ cells. We conclude that the C. elegans mitosis-meiosis and sperm-oocyte decisions are separable regulatory events and suggest that this fundamental conclusion will hold true for germ cells throughout the animal kingdom.

RNA targets and specificity of Staufen, a double-stranded RNA-binding protein in Caenorhabditis elegans.

The Staufen family consists of proteins that possess double-stranded RNA-binding domains (dsRBDs). Staufen proteins of Drosophila and mammals regulate mRNA localization, translation, and decay. We report analysis of Staufen in Caenorhabditis elegans, which we have designated STAU-1. We focus on its biochemical properties, mRNA targets, and possible role in RNAi. We show that STAU-1 is expressed as mRNA and protein at all stages of C. elegans development. The wild-type, full-length protein, purified from bacteria, binds duplex RNA with high affinity in vitro. Purified, mutant proteins lacking single dsRBDs still bind RNA efficiently, demonstrating that no single domain is required for binding to duplex RNA (although dsRBD2 could not be tested). STAU-1 mRNA targets were identified via immunoprecipitation with specific anti-STAU-1 antibodies, followed by microarray analysis (RIP-Chip). These studies define a set of 418 likely STAU-1 mRNA targets. Finally, we demonstrate that stau-1 mutants enhance exogenous RNAi and that stau-1;eri-1 double mutants exhibit sterility and synthetic germ line defects.

Cyclin E and Cdk2 control GLD-1, the mitosis/meiosis decision, and germline stem cells in Caenorhabditis elegans.

Coordination of the cell cycle with developmental events is crucial for generation of tissues during development and their maintenance in adults. Defects in that coordination can shift the balance of cell fates with devastating clinical effects. Yet our understanding of the molecular mechanisms integrating core cell cycle regulators with developmental regulators remains in its infancy. This work focuses on the interplay between cell cycle and developmental regulators in the Caenorhabditis elegans germline. Key developmental regulators control germline stem cells (GSCs) to self-renew or begin differentiation: FBF RNA-binding proteins promote self-renewal, while GLD RNA regulatory proteins promote meiotic entry. We first discovered that many but not all germ cells switch from the mitotic into the meiotic cell cycle after RNAi depletion of CYE-1 (C. elegans cyclin E) or CDK-2 (C. elegans Cdk2) in wild-type adults. Therefore, CYE-1/CDK-2 influences the mitosis/meiosis balance. We next found that GLD-1 is expressed ectopically in GSCs after CYE-1 or CDK-2 depletion and that GLD-1 removal can rescue cye-1/cdk-2 defects. Therefore, GLD-1 is crucial for the CYE-1/CDK-2 mitosis/meiosis control. Indeed, GLD-1 appears to be a direct substrate of CYE-1/CDK-2: GLD-1 is a phosphoprotein; CYE-1/CDK-2 regulates its phosphorylation in vivo; and human cyclin E/Cdk2 phosphorylates GLD-1 in vitro. Transgenic GLD-1(AAA) harbors alanine substitutions at three consensus CDK phosphorylation sites. GLD-1(AAA) is expressed ectopically in GSCs, and GLD-1(AAA) transgenic germlines have a smaller than normal mitotic zone. Together these findings forge a regulatory link between CYE-1/CDK-2 and GLD-1. Finally, we find that CYE-1/CDK-2 works with FBF-1 to maintain GSCs and prevent their meiotic entry, at least in part, by lowering GLD-1 abundance. Therefore, CYE-1/CDK-2 emerges as a critical regulator of stem cell maintenance. We suggest that cyclin E and Cdk-2 may be used broadly to control developmental regulators.

Phosphorylation state of a Tob/BTG protein, FOG-3, regulates initiation and maintenance of the Caenorhabditis elegans sperm fate program.

FOG-3, the single Caenorhabditis elegans Tob/BTG protein, directs germ cells to adopt the sperm fate at the expense of oogenesis. Importantly, FOG-3 activity must be maintained for the continued production of sperm that is typical of the male sex. Vertebrate Tob proteins have antiproliferative activity and ERK phosphorylation of Tob proteins has been proposed to abrogate "antiproliferative" activity. Here we investigate FOG-3 phosphorylation and its effect on sperm fate specification. We found both phosphorylated and unphosphorylated forms of FOG-3 in nematodes. We then interrogated the role of FOG-3 phosphorylation in sperm fate specification. Specifically, we assayed FOG-3 transgenes for rescue of a fog-3 null mutant. Wild-type FOG-3 rescued both initiation and maintenance of sperm fate specification. A FOG-3 mutant with its four consensus ERK phosphorylation sites substituted to alanines, called FOG-3(4A), rescued partially: sperm were made transiently but not continuously in both sexes. A different FOG-3 mutant with its sites substituted to glutamates, called FOG-3(4E), had no rescuing activity on its own, but together with FOG-3(4A) rescue was complete. Thus, when FOG-3(4A) and FOG-3(4E) were both introduced into the same animals, sperm fate specification was not only initiated but also maintained, resulting in continuous spermatogenesis in males. Our findings suggest that unphosphorylated FOG-3 initiates the sperm fate program and that phosphorylated FOG-3 maintains that program for continued sperm production typical of males. We discuss implications of our results for Tob/BTG proteins in vertebrates.

The PUF RNA-binding protein, FBF-2, maintains stem cells without binding to RNA.

Like all canonical PUF proteins, C. elegans FBF-2 binds to specific RNAs via tripartite recognition motifs (TRMs). Here we report that an FBF-2 mutant protein that cannot bind to RNA, is nonetheless biologically active and maintains stem cells. This unexpected result challenges the conventional wisdom that RBPs must bind to RNAs to achieve biological activity. Also unexpectedly, FBF-2 interactions with partner proteins can compensate for loss of RNA-binding. FBF-2 only loses biological activity when its RNA-binding and partner interactions are both defective. These findings highlight the complementary contributions of RNA-binding and protein partner interactions to activity of an RNA-binding protein.