Emergent dynamics of adult stem cell lineages from single nucleus and single cell RNA-Seq of Drosophila testes

Proper differentiation of sperm from germline stem cells, essential for production of the next generation, requires dramatic changes in gene expression that drive remodeling of almost all cellular components, from chromatin to organelles to cell shape itself. Here, we provide a single nucleus and single cell RNA-seq resource covering all of spermatogenesis in Drosophila starting from in-depth analysis of adult testis single nucleus RNA-seq (snRNA-seq) data from the Fly Cell Atlas (FCA) study. With over 44,000 nuclei and 6000 cells analyzed, the data provide identification of rare cell types, mapping of intermediate steps in differentiation, and the potential to identify new factors impacting fertility or controlling differentiation of germline and supporting somatic cells. We justify assignment of key germline and somatic cell types using combinations of known markers, in situ hybridization, and analysis of extant protein traps. Comparison of single cell and single nucleus datasets proved particularly revealing of dynamic developmental transitions in germline differentiation. To complement the web-based portals for data analysis hosted by the FCA, we provide datasets compatible with commonly used software such as Seurat and Monocle. The foundation provided here will enable communities studying spermatogenesis to interrogate the datasets to identify candidate genes to test for function in vivo.

The adult Drosophila testis lacks a mechanism to replenish missing niche cells

The adult Drosophila testis contains a well-defined niche created by a cluster of hub cells, which secrete signals that maintain adjacent germline stem cells and somatic cyst stem cells (CySCs). Hub cells are normally quiescent in adult flies but can exit quiescence, delaminate from the hub and convert into CySCs after ablation of all CySCs. The opposite event, CySC conversion into hub cells, was proposed to occur under physiological conditions, but the frequency of this event is debated. Here, to probe further the question of whether or not hub cells can be regenerated, we developed methods to genetically ablate some or all hub cells. Surprisingly, when flies were allowed to recover from ablation, the missing hub cells were not replaced. Hub cells did not exit quiescence after partial ablation of hub cells, and labeled cells from outside the hub did not enter the hub during or after ablation. Despite its ability to exit quiescence in response to CySC ablation, we conclude that the hub in the adult Drosophila testis does not have a mechanism to replenish missing hub cells.

Live Imaging of the Drosophila Testis Stem Cell Niche

Live imaging of adult tissue stem cell niches provides key insights into the dynamic behavior of stem cells, their differentiating progeny, and their neighboring support cells, but few niches are amenable to this approach. Here, we discuss a technique for long-term live imaging of the Drosophila testis stem cell niche. Culturing whole testes ex vivo for up to 18 h allows for tracking of cell-type-specific behaviors under normal and various chemically or genetically modified conditions. Fixing and staining tissues after live imaging allows for the molecular confirmation of cell identity and behavior. By using live imaging in intact niches, we can better uncover the cellular and molecular mechanisms that regulate stem cell function in vivo.

Activation of the EGFR/MAPK pathway drives transdifferentiation of quiescent niche cells to stem cells in the Drosophila testis niche

Adult stem cells are maintained in niches, specialized microenvironments that regulate their self-renewal and differentiation. In the adult Drosophila testis stem cell niche, somatic hub cells produce signals that regulate adjacent germline stem cells (GSCs) and somatic cyst stem cells (CySCs). Hub cells are normally quiescent, but after complete genetic ablation of CySCs, they can proliferate and transdifferentiate into new CySCs. Here we find that Epidermal growth factor receptor (EGFR) signaling is upregulated in hub cells after CySC ablation and that the ability of testes to recover from ablation is inhibited by reduced EGFR signaling. In addition, activation of the EGFR pathway in hub cells is sufficient to induce their proliferation and transdifferentiation into CySCs. We propose that EGFR signaling, which is normally required in adult cyst cells, is actively inhibited in adult hub cells to maintain their fate but is repurposed to drive stem cell regeneration after CySC ablation.

Fly Cell Atlas: A single-nucleus transcriptomic atlas of the adult fruit fly

For more than 100 years, the fruit fly Drosophila melanogaster has been one of the most studied model organisms. Here, we present a single-cell atlas of the adult fly, Tabula Drosophilae, that includes 580,000 nuclei from 15 individually dissected sexed tissues as well as the entire head and body, annotated to >250 distinct cell types. We provide an in-depth analysis of cell type-related gene signatures and transcription factor markers, as well as sexual dimorphism, across the whole animal. Analysis of common cell types between tissues, such as blood and muscle cells, reveals rare cell types and tissue-specific subtypes. This atlas provides a valuable resource for the Drosophila community and serves as a reference to study genetic perturbations and disease models at single-cell resolution.

Retinoblastoma Intrinsically Regulates Niche Cell Quiescence, Identity, and Niche Number in the Adult Drosophila Testis

Homeostasis in adult tissues depends on the precise regulation of stem cells and their surrounding microenvironments, or niches. Here, we show that the cell cycle inhibitor and tumor suppressor Retinoblastoma (RB) is a critical regulator of niche cells in the Drosophila testis. The testis contains a single niche, composed of somatic hub cells, that signals to adjacent germline and somatic stem cells. Hub cells are normally quiescent, but knockdown of the RB homolog Rbf in these cells causes them to proliferate and convert to somatic stem cells. Over time, mutant hub cell clusters enlarge and split apart, forming ectopic hubs surrounded by active stem cells. Furthermore, we show that Rbf’s ability to restrict niche number depends on the transcription factors E2F and Escargot and the adhesion molecule E-cadherin. Together this work reveals how precise modulation of niche cells, not only the stem cells they support, can drive regeneration and disease.

Greenspan and Matunis find that the tumor suppressor Retinoblastoma is required in niche cells to maintain quiescence, cell fate, and niche number. Loss of Retinoblastoma causes niche cell divisions, conversion to somatic stem cells, and ectopic niche formation through niche fission, suggesting that mutations in niche cells may drive disease.

Live Imaging of the Drosophila Testis Stem Cell Niche

Live imaging of adult tissue stem cell niches provides key insights into the dynamic behavior of stem cells, their differentiating progeny, and their neighboring support cells, but few niches are amenable to this approach. Here we discuss a technique for long-term live imaging of the Drosophila testis stem cell niche. Culturing whole testes ex vivo for up to 12.5 h allows for tracking of cell-type specific behaviors under normal and various chemically or genetically modified conditions. Fixing and staining tissues after live imaging allows for the molecular confirmation of cell identity and behavior. Utilization of live imaging in intact niches will facilitate further understanding of the cellular and molecular mechanisms that regulate stem cell function in vivo.

Chinmo is sufficient to induce male fate in somatic cells of the adult Drosophila ovary

Sexual identity is continuously maintained in specific differentiated cell types long after sex determination occurs during development. In the adult Drosophila testis, the putative transcription factor Chronologically inappropriate morphogenesis (Chinmo) acts with the canonical male sex determinant DoublesexM (Dsx(M)) to maintain the male identity of somatic cyst stem cells and their progeny. Here we find that ectopic expression of chinmo is sufficient to induce a male identity in adult ovarian somatic cells, but it acts through a Dsx(M)-independent mechanism. Conversely, the feminization of the testis somatic stem cell lineage caused by loss of chinmo is enhanced by expression of the canonical female sex determinant Dsx(F), indicating that chinmo acts in parallel with the canonical sex determination pathway to maintain the male identity of testis somatic cells. Consistent with this finding, ectopic expression of female sex determinants in the adult testis disrupts tissue morphology. The miRNA let-7 downregulates chinmo in many contexts, and ectopic expression of let-7 in the adult testis is sufficient to recapitulate the chinmo loss-of-function phenotype, but we find no apparent phenotypes upon removal of let-7 in the adult ovary or testis. Our finding that chinmo is necessary and sufficient to promote a male identity in adult gonadal somatic cells suggests that the sexual identity of somatic cells can be reprogrammed in the adult Drosophila ovary as well as in the testis.

Coordinate regulation of stem cell competition by Slit-Robo and JAK-STAT signaling in the Drosophila testis

Stem cells in tissues reside in and receive signals from local microenvironments called niches. Understanding how multiple signals within niches integrate to control stem cell function is challenging. The Drosophila testis stem cell niche consists of somatic hub cells that maintain both germline stem cells and somatic cyst stem cells (CySCs). Here, we show a role for the axon guidance pathway Slit-Roundabout (Robo) in the testis niche. The ligand Slit is expressed specifically in hub cells while its receptor, Roundabout 2 (Robo2), is required in CySCs in order for them to compete for occupancy in the niche. CySCs also require the Slit-Robo effector Abelson tyrosine kinase (Abl) to prevent over-adhesion of CySCs to the niche, and CySCs mutant for Abl outcompete wild type CySCs for niche occupancy. Both Robo2 and Abl phenotypes can be rescued through modulation of adherens junction components, suggesting that the two work together to balance CySC adhesion levels. Interestingly, expression of Robo2 requires JAK-STAT signaling, an important maintenance pathway for both germline and cyst stem cells in the testis. Our work indicates that Slit-Robo signaling affects stem cell function downstream of the JAK-STAT pathway by controlling the ability of stem cells to compete for occupancy in their niche.

Stem cell competition: finding balance in the niche

Adult stem cells reside in local microenvironments (niches) that produce signals regulating the outcome of stem cell divisions and stem cell-niche interactions. Limited space and signals in the niche often force stem cells to compete with one another. Although previous studies have uncovered several examples of genetically distinct stem cells competing for niche access, recent studies demonstrate that genetically equivalent stem cells compete under normal conditions, resulting in dynamic stem cell behavior in the niche. New work in multiple vertebrate and invertebrate tissues shows that stem cell competition occurs continuously and mutations disrupting the balance between competing stem cells can cause diseases and defects in the niche. This review discusses recent insights into stem cell competition in mammals and Drosophila.

JAK-STAT signaling in stem cells

Adult stem cells are essential for the regeneration and repair of tissues in an organism. Signals from many different pathways converge to regulate stem cell maintenance and differentiation while preventing overproliferation. Although each population of adult stem cells is unique, common themes arise by comparing the regulation of various stem cell types in an organism or by comparing similar stem cell types across species. The JAK-STAT signaling pathway, identified nearly two decades ago, is now known to be involved in many biological processes including the regulation of stem cells. Studies in Drosophila first implicated JAK-STAT signaling in the control of stem cell maintenance in the male germline stem cell microenvironment, or niche; subsequently it has been shown play a role in other niches in both Drosophila and mammals. In this chapter, we will address the role of JAK-STAT signaling in stem cells in the germline, intestinal, hematopoietic and neuronal niches in Drosophila as well as the hematopoietic and neuronal niches in mammals. We will comment on how the study of JAK-STAT signaling in invertebrate systems has helped to advance our understanding of signaling in vertebrates. In addition to the role of JAK- STAT signaling in stem cell niche homeostasis, we will also discuss the diseases, including cancers, that can arise when this pathway is misregulated.

Recent advances in Drosophila male germline stem cell biology

The ability of stem cells to divide asymmetrically to produce both self-renewing and differentiating daughter cells sustains many adult tissues, but germline stem cells (GSCs) are unique among stem cells as they perpetuate the genome of the species. The cellular and molecular mechanisms regulating most mammalian stem cells in their endogenous local microenvironments, or niches, are quite challenging to study. However, studies of stem cell niches such as those found in the Drosophila gonads have proven very useful. In these tissues, GSCs are housed in a readily identifiable niche, and the ability to genetically manipulate these cells and their neighbors has uncovered several fundamental mechanisms that are relevant to stem cells more generally. Here, we summarize recent work on the regulation of GSCs in the Drosophila testis niche by intercellular signals, and on the intracellular mechanisms that cooperate with these signals to ensure the survival of the germline. This review focuses on GSCs within the adult Drosophila testis; somatic stem cells in this tissue are reviewed by Zoller and Schulz in this issue.(1) For a review of the testis niche as a whole, see de Cuevas and Matunis,(2) and for more comprehensive reviews of the Drosophila testis, refer to Fuller(3) and Davies and Fuller.(4).

The stem cell niche: lessons from the Drosophila testis

In metazoans, tissue maintenance and regeneration depend on adult stem cells, which are characterized by their ability to self-renew and generate differentiating progeny in response to the needs of the tissues in which they reside. In the Drosophila testis, germline and somatic stem cells are housed together in a common niche, where they are regulated by local signals, epigenetic mechanisms and systemic factors. These stem cell populations in the Drosophila testis have the unique advantage of being easy to identify and manipulate, and hence much progress has been made in understanding how this niche operates. Here, we summarize recent work on stem cells in the adult Drosophila testis and discuss the remarkable ability of these stem cells to respond to change within the niche.

Epigenetic regulation of stem cell maintenance in the Drosophila testis via the nucleosome-remodeling factor NURF

Regulation of stem cells depends on both tissue-specific transcriptional regulators and changes in chromatin organization, yet the coordination of these events in endogenous niches is poorly understood. In the Drosophila testis, local JAK-STAT signaling maintains germline and somatic stem cells (GSCs and cyst progenitor cells, or CPCs) in a single niche. Here we show that epigenetic regulation via the nucleosome-remodeling factor (NURF) complex ensures GSC and CPC maintenance by positively regulating JAK-STAT signaling, thereby preventing premature differentiation. Conversely, NURF is not required in early differentiating daughter cells of either lineage. Because three additional ATP-dependent chromatin remodelers (ACF, CHRAC, and dMi-2/NuRD) are dispensable for stem cell maintenance in the testis, epigenetic regulation of stem cells within this niche may rely primarily on NURF. Thus, local signals cooperate with specific chromatin-remodeling complexes in intact niches to coordinately regulate a common set of target genes to prevent premature stem cell differentiation.

Make room for dedifferentiation

The reversal of cellular differentiation, or dedifferentiation, has fascinated biologists for many decades. While cells can be re-programmed extensively in culture, examples of in vivo dedifferentiation have recently emerged in both vertebrate and invertebrate systems, allowing for analysis of this intriguing process under more physiologically relevant conditions. Studies suggest that dedifferentiation occurs not only during large-scale cellular regeneration, but also at low levels to replenish stem cells lost due to normal turnover. Our recent paper demonstrates a novel method to induce the dedifferentiation of lineage-committed stem cell daughters back into germline stem cells (GSCs) in the Drosophila testis. We also show a requirement for activation of Janus kinase-signal transducer and activator of transcription (Jak-STAT) signaling in this process, and suggest that normally non-motile germline cells gain mobility and out-compete resident somatic cells for occupancy in the stem cell-maintaining microenvironment (niche). Here, we discuss what our findings reveal about stem cell competition and the capacity of various cell types to dedifferentiate.

Dedifferentiating spermatogonia outcompete somatic stem cells for niche occupancy in the Drosophila testis

Differentiating cells can dedifferentiate to replace stem cells in aged or damaged tissues, but the underlying mechanisms are unknown. In the Drosophila testis, a cluster of stromal cells called the hub creates a niche by locally activating Janus kinase-signal transducer and activator of transcription (Jak-STAT) signaling in adjacent germline and somatic stem cells. Here, we establish a system to study spermatogonial dedifferentiation. Ectopically expressing the differentiation factor bag-of-marbles (Bam) removes germline stem cells from the niche. However, withdrawing ectopic Bam causes interconnected spermatogonia to fragment, move into the niche, exchange positions with resident somatic stem cells, and establish contact with the hub. Concomitantly, actin-based protrusions appear on subsets of spermatogonia, suggesting acquired motility. Furthermore, global downregulation of Jak-STAT signaling inhibits dedifferentiation, indicating that normal levels of pathway activation are required to promote movement of spermatogonia into the niche during dedifferentiation, where they outcompete somatic stem cells for niche occupancy.

Essential role for a heterogeneous nuclear ribonucleoprotein (hnRNP) in oogenesis: hrp40 is absent from the germ line in the dorsoventral mutant squid

The Drosophila melanogaster hrp40 proteins are abundant nuclear pre-mRNA-binding proteins that are similar to the heterogeneous nuclear ribonucleoprotein (hnRNP) A/B proteins of vertebrates. Recently, hrp40 has been shown to be encoded by the squid gene, which is required for dorsoventral axis formation during oogenesis. Eggs and embryos from homozygous squid mothers are severely dorsalized, and complete deletion of the squid gene results in lethality. Here we have examined the expression and localization of hrp40 in wild-type and squid mutant ovaries. Using a monoclonal antibody specific for hrp40, the same isoforms of hrp40 are detected in both wild-type and squid ovaries, but the amount of hrp40 is reduced in squid ovaries. Furthermore, immunolocalization of hrp40 in wild-type egg chambers shows that hrp40 is present in the nurse cells, oocyte, and follicle cells. In contrast, in squid mutant egg chambers, hrp40 is absent from the germ-line-derived nurse cells and oocyte, but it is detected in the somatic follicle cells. The absence of hrp40 from the germ-line-derived cells of developing egg chambers is likely to lead to the striking dorsalized phenotype of squid eggs. In addition, dramatic stage-specific changes in the cellular localization of hrp40 are seen; the protein found in the nurse cell nuclei during early stages of oogenesis migrates to the cytoplasm at later stages. These findings reveal dynamic patterns of expression and localization of hnRNP proteins during development and provide evidence for an essential role for hnRNP proteins.

PUB1: a major yeast poly(A)+ RNA-binding protein

The expression of RNA polymerase II transcripts can be regulated at the posttranscriptional level by RNA-binding proteins. Although extensively characterized in metazoans, relatively few RNA-binding proteins have been characterized in the yeast Saccharomyces cerevisiae. Three major proteins are cross-linked by UV light to poly(A)+ RNA in living S. cerevisiae cells. These are the 72-kDa poly(A)-binding protein and proteins of 60 and 50 kDa (S.A. Adam, T.Y. Nakagawa, M.S. Swanson, T. Woodruff, and G. Dreyfuss, Mol. Cell. Biol. 6:2932-2943, 1986). Here, we describe the 60-kDa protein, one of the major poly(A)+ RNA-binding proteins in S. cerevisiae. This protein, PUB1 [for poly(U)-binding protein 1], was purified by affinity chromatography on immobilized poly(rU), and specific monoclonal antibodies to it were produced. UV cross-linking demonstrated that PUB1 is bound to poly(A)+ RNA (mRNA or pre-mRNA) in living cells, and it was detected primarily in the cytoplasm by indirect immunofluorescence. The gene for PUB1 was cloned and sequenced, and the sequence was found to predict a 51-kDa protein with three ribonucleoprotein consensus RNA-binding domains and three glutamine- and asparagine-rich auxiliary domains. This overall structure is remarkably similar to the structures of the Drosophila melanogaster elav gene product, the human neuronal antigen HuD, and the cytolytic lymphocyte protein TIA-1. Each of these proteins has an important role in development and differentiation, potentially by affecting RNA processing. PUB1 was found to be nonessential in S. cerevisiae by gene replacement; however, further genetic analysis should reveal important features of this class of RNA-binding proteins.

A unique ribonucleoprotein complex assembles preferentially on ecdysone-responsive sites in Drosophila melanogaster

The protein on ecdysone puffs (PEP) is associated preferentially with active ecdysone-inducible puffs on Drosophila polytene chromosomes and contains sequence motifs characteristic of transcription factors and RNA-binding proteins (S. A. Amero, S. C. R. Elgin, and A. L. Beyer, Genes Dev. 5:188-200, 1991). PEP is associated with RNA in vivo, as demonstrated here by the sensitivity of PEP-specific chromosomal immunostaining in situ to RNase digestion and by the immunopurification of PEP in Drosophila cell extract with heterogeneous nuclear ribonucleoprotein (hnRNP) complexes. As revealed by sequential immunostaining, PEP is found on a subset of chromosomal sites bound by the HRB (heterogeneous nuclear RNA-binding) proteins, which are basic Drosophila hnRNPs. These observations lead us to suggest that a unique, PEP-containing hnRNP complex assembles preferentially on the transcripts of ecdysone-regulated genes in Drosophila melanogaster presumably to expedite the transcription and/or processing of these transcripts.

Association of individual hnRNP proteins and snRNPs with nascent transcripts

As they are transcribed, RNA polymerase II transcripts (hnRNAs or pre-mRNAs) associate with hnRNP proteins and snRNP particles, and the processing of pre-mRNA occurs within these ribonucleoprotein complexes. To better understand the relationship between hnRNP proteins and snRNP particles and their roles in mRNA formation, we have visualized them as they associate with nascent transcripts on the polytene chromosomes of Drosophila melanogaster salivary glands. Simultaneous pairwise detection of the abundant hnRNP proteins hrp36, hrp40, and hrp48 by direct double-label immunofluorescence microscopy reveals all of these proteins are bound to most transcripts, but their relative amounts on different transcripts are not fixed. Numerous differences in the relative amounts of snRNP particles and hnRNP proteins on nascent transcripts are also observed. These observations directly demonstrate that individual hnRNP proteins and snRNP particles are differentially associated with nascent transcripts and suggest that different pre-mRNAs bind different combinations of these factors to form transcript-specific, rather than a single type of, hnRNA-hnRNP-snRNP complexes. The distinct and specific constellation of hnRNP proteins and snRNP particles that assembles on different pre-mRNAs is likely to affect the fate and pathway of processing of these transcripts.

Characterization of the major hnRNP proteins from Drosophila melanogaster

To better understand the role(s) of hnRNP proteins in the process of mRNA formation, we have identified and characterized the major nuclear proteins that interact with hnRNAs in Drosophila melanogaster. cDNA clones of several D. melanogaster hnRNP proteins have been isolated and sequenced, and the genes encoding these proteins have been mapped cytologically on polytene chromosomes. These include the hnRNP proteins hrp36, hrp40, and hrp48, which together account for the major proteins of hnRNP complexes in D. melanogaster (Matunis et al., 1992, accompanying paper). All of the proteins described here contain two amino-terminal RNP consensus sequence RNA-binding domains and a carboxyl-terminal glycine-rich domain. We refer to this configuration, which is also found in the hnRNP A/B proteins of vertebrates, as 2 x RBD-Gly. The sequences of the D. melanogaster hnRNP proteins help define both highly conserved and variable amino acids within each RBD and support the suggestion that each RBD in multiple RBD-containing proteins has been conserved independently and has a different function. Although 2 x RBD-Gly proteins from evolutionarily distant organisms are conserved in their general structure, we find a surprising diversity among the members of this family of proteins. A mAb to the hrp40 proteins crossreacts with the human A/B and G hnRNP proteins and detects immunologically related proteins in divergent organisms from yeast to man. These data establish 2 x RBD-Gly as a prevalent hnRNP protein structure across eukaryotes. This information about the composition of hnRNP complexes and about the structure of hnRNA-binding proteins will facilitate studies of the functions of these proteins.

Isolation of hnRNP complexes from Drosophila melanogaster

Nascent RNA polymerase II transcripts, heterogeneous nuclear RNAs (hnRNAs), become associated with nuclear proteins (hnRNP Proteins), and their processing into mRNAs takes place in these hnRNP complexes. hnRNP complexes have previously been purified from vertebrate cells. Here we report the isolation of hnRNP complexes from an invertebrate organism, the fruitfly Drosophila melanogaster. Candidate hnRNP proteins were purified from D. melanogaster embryos by ssDNA affinity chromatography, and mAbs were produced to many of the major proteins. Genuine hnRNP proteins were identified by several criteria, including nucleoplasmic localization, association with nascent transcripts, crosslinking to poly(A)-containing RNA in living cells, and amino acid sequence. In addition, mAbs that cross-react between the fruitfly and human hnRNP proteins were obtained. Most importantly, using hnRNP-specific mAbs we have purified the hnRNP complexes from D. melanogaster cells. These RNAase-sensitive complexes contain at least 10 major proteins designated hrps, the most abundant proteins having apparent molecular masses of 36, 38, 39, 40, 44, 48, 54, 62, 70, and 75 kD. cDNAs and complete sequences for several of these proteins have been obtained and are presented in the accompanying paper (Matunis, E. L., M. J. Matunis, and G. Dreyfuss. 1992. J. Cell Biol. 116:257-269). The purification of D. melanogaster hnRNP complexes will facilitate genetic and cytological studies on the function of hnRNA-binding proteins and on the posttranscriptional regulation of gene expression.

The multiple RNA-binding domains of the mRNA poly(A)-binding protein have different RNA-binding activities

The poly(A)-binding protein (PABP) is the major mRNA-binding protein in eukaryotes, and it is essential for viability of the yeast Saccharomyces cerevisiae. The amino acid sequence of the protein indicates that it consists of four ribonucleoprotein consensus sequence-containing RNA-binding domains (RBDs I, II, III, and IV) and a proline-rich auxiliary domain at the carboxyl terminus. We produced different parts of the S. cerevisiae PABP and studied their binding to poly(A) and other ribohomopolymers in vitro. We found that none of the individual RBDs of the protein bind poly(A) specifically or efficiently. Contiguous two-domain combinations were required for efficient RNA binding, and each pairwise combination (I/II, II/III, and III/IV) had a distinct RNA-binding activity. Specific poly(A)-binding activity was found only in the two amino-terminal RBDs (I/II) which, interestingly, are dispensable for viability of yeast cells, whereas the activity that is sufficient to rescue lethality of a PABP-deleted strain is in the carboxyl-terminal RBDs (III/IV). We conclude that the PABP is a multifunctional RNA-binding protein that has at least two distinct and separable activities: RBDs I/II, which most likely function in binding the PABP to mRNA through the poly(A) tail, and RBDs III/IV, which may function through binding either to a different part of the same mRNA molecule or to other RNA(s).