David S. Bernstein

A conserved RNA-binding protein controls germline stem cells in Caenorhabditis elegans

Germline stem cells are defined by their unique ability to generate more of themselves as well as differentiated gametes. The molecular mechanisms controlling the decision between self-renewal and differentiation are central unsolved problems in developmental biology with potentially broad medical implications. In Caenorhabditis elegans, germline stem cells are controlled by the somatic distal tip cell. FBF-1 and FBF-2, two nearly identical proteins, which together are called FBF (‘fem-3 mRNA binding factor’), were originally discovered as regulators of germline sex determination. Here we report that FBF also controls germline stem cells: in an fbf-1 fbf-2 double mutant, germline proliferation is initially normal, but stem cells are not maintained. We suggest that FBF controls germline stem cells, at least in part, by repressing gld-1, which itself promotes commitment to the meiotic cell cycle. FBF belongs to the PUF family (‘Pumilio and FBF’) of RNA-binding proteins. Pumilio controls germline stem cells in Drosophila females, and, in lower eukaryotes, PUF proteins promote continued mitoses. We suggest that regulation by PUF proteins may be an ancient and widespread mechanism for control of stem cells.

Analyzing mRNA–protein complexes using a yeast three-hybrid system

RNA-protein interactions are essential for the proper execution and regulation of every step in the life of a eukaryotic mRNA. Here we describe a three-hybrid system in which RNA-protein interactions can be analyzed using simple phenotypic or enzymatic assays in Saccharomyces cerevisiae. The system can be used to detect or confirm an RNA-protein interaction, to analyze RNA-protein interactions genetically, and to discover new protein or RNA partners when only one is known. Multicomponent complexes containing more than one protein can be detected, identified, and analyzed. We describe the method and how to use it, and discuss applications that bear particularly on eukaryotic mRNAs.

Dose-dependent control of proliferation and sperm specification by FOG-1/CPEB.

RNA-binding proteins control germline development in metazoans. This work focuses on control of the C. elegans germline by two RNA-binding proteins: FOG-1, a CPEB homolog; and FBF, a PUF family member. Previous studies have shown that FOG-1 specifies the sperm fate and that FBF promotes proliferation. Here, we report that FOG-1 also promotes proliferation. Whereas fbf-1 fbf-2 double mutants make ∼120 germ cells, fog-1; fbf-1 fbf-2 triple mutants make only ∼10 germ cells. The triple mutant germline divides normally until early L2, when germ cells prematurely enter meiosis and begin oogenesis. Importantly, fog-1/+; fbf-1 fbf-2 animals make more germ cells than fbf-1 fbf-2 double mutants, demonstrating that one dose of wild-type fog-1 promotes proliferation more effectively than two doses – at least in the absence of FBF. FOG-1 protein is barely detectable in proliferating germ cells, but abundant in germ cells destined for spermatogenesis. Based on fog-1 dose effects, together with the gradient of FOG-1 protein abundance, we suggest that low FOG-1 promotes proliferation and high FOG-1 specifies spermatogenesis. FBF binds specifically to regulatory elements in the fog-1 3′UTR, and FOG-1 increases in animals lacking FBF. Therefore, FBF represses fog-1 expression. We suggest that FBF promotes continued proliferation, at least in part, by maintaining FOG-1 at a low level appropriate for proliferation. The dose-dependent control of proliferation and cell fate by FOG-1 has striking parallels with Xenopus CPEB, suggesting a conserved mechanism in animal development.

The future city on the Inland Sea: a history of imaginative geographies of Lake Superior

Eric Olmanson’s new book examines how an unbounded space on the south shore of Lake Superior became a place with a regional identity. The Future City on the Inland Sea traces how ‘maps, expedition narratives, geological surveys, railroad pamphlets, local and distant newspapers, periodicals, and advertisements’, coalesced in the late nineteenth and early twentieth century to ‘produce what might be called, for the first time, a region’ (pp. 14 18). Owing much to the work of geographers Yi-Fu Tuan and William Cronon, Olmanson takes a ‘narrative descriptive’ approach to reveal how larger cultural movements influenced environmental perception (p. 12). In other words, this is a story about stories. The book is broken into two sections. The first section, analyzes the work of four men as they begin the process of creating a regional identity. While well written, there is little surprise in the fact that both Henry Rowe Schoolcraft’s and Thomas L. Mckenney’s accounts of their expeditions in the 1820s were shaped by romantic thinkers. Similarly, Olmanson’s textual reading of the reports of geologists David Dale Owen and Charles Whittlesey offer good narrative, but little in the way of original analysis. It is in the book’s second section, where Olmanson focuses his attention on Chequamegon Bay, that readers of this journal will be particularly interested. While primarily interested in the settlement of northern Wisconsin, for Olmanson, this story is inexorably linked to a burgeoning tourist trade. In the first chapter of this section, for example, he uses pamphlets and posters to deftly show how boosters uniquely promoted burgeoning cities around the bay as both natural escapes and industrial centers. When successful, this tactic benefited speculators, newspaper editors, and railroad companies who could ‘increase the volume of both passengers and freight if tourism and industry rose’ (p. 136). Such was the case when Andrew Jackson Turner, father of historian Frederick Jackson Turner and editor of the Wisconsin State Register, became one of the first tourists to ride the rails to the city of Ashland. Based on this excursion he would later write, ‘no portion of Wisconsin offers as great inducements to actual settlers’ as the land bordering the railway (p. 143). Such boosterism leads Olmanson to conclude that ‘the frontier thesis may actually have emerged from rhetoric that was essentially advertisement for railroad lands’ (p. 166). Thus while Olmanson himself does not always explicitly connect tourism and settlement, such connections are easily made. Chapter 5 is both the analytical heart of the book and the site of Olmanson’s most interesting conclusions. In it he traces three narrative models used by newspaper editors to (re-)create the Chequamegon Bay region: the future city model which sought to convince readers that theirs would be the site of Lake Superior’s