Much of the patterning of early C. elegans embryos involves a series of Notch interactions that occur in rapid succession and have distinct outcomes; however, none of the targets for these interactions have been identified. We show that the REF-1 family of bHLH transcription factors is a major target of Notch signaling in all these interactions and that most examples of Notch-mediated transcriptional repression can be attributed to REF-1 activities. The REF-1 family is expressed and has similar functions in both Notch-dependent and Notch-independent pathways, and this dual mode of deployment is used repeatedly to pattern the embryo. REF-1 proteins are unusual in that they contain two different bHLH domains and lack the distinguishing characteristics of Hairy/Enhancer of Split (HES) bHLH proteins that are Notch targets in other systems. Our results show that the highly divergent REF-1 proteins are nonetheless HES-like bHLH effectors of Notch signaling.

The four-cell C. elegans embryo contains two sister cells called ABa and ABp that initially have equivalent abilities to produce ectodermal cell types. Multiple Notch-mediated interactions occur during the early cell divisions that diversify the ABa and ABp descendants. The first interaction determines the pattern of ectodermal cell types produced by ABp. The second interaction induces two ABa granddaughters to become mesodermal precursors. We show that T-box transcription factors called TBX-37 and TBX-38 are essential for mesodermal induction, and that these factors are expressed in ABa, but not ABp, descendants. We provide evidence that the first Notch interaction functions largely, if not entirely, to prevent TBX-37, TBX-38 expression in ABp descendants. Neither the second Notch interaction nor TBX-37, TBX-38 alone are sufficient for mesodermal induction, indicating that both must function together. We conclude that TBX-37, TBX-38 play a key role in distinguishing the outcomes of two sequential Notch-mediated interactions.

RNA polymerase sigma factor F initiates the prespore-specific program of gene expression during Bacillus subtilis sporulation. F governs transcription of spoIIIG, encoding the late prespore-specific regulator G . However, transcription of spoIIIG is delayed relative to other genes under the control of F , and after synthesis, G is initially kept in an inactive form. Activation of G requires the complete engulfment of the prespore by the mother cell and expression of the spoIIIA and spoIIIJ loci. We screened for random mutations in spoIIIG that bypassed the requirement for spoIIIA for the activation of G . We found a mutation (spoIIIGE156K) that resulted in an amino acid substitution at position 156, which is adjacent to the position of a mutation (E155K) previously shown to prevent interaction of SpoIIAB with G . Comparative modelling techniques and in vivo studies suggested that the spoIIIGE156K mutation interferes with the interaction of SpoIIAB with G . The GE156K isoform restored G -directed gene expression to spoIIIA mutant cells. However, expression of sspElacZ in the spoIIIA spoIIIGE156K double mutant was delayed relative to completion of the engulfment process and was not confined to the prespore. Rather, ␤-galactosidase accumulated throughout the entire cell at late times in development. This suggests that the activity of GE156K is still regulated in the prespore of a spoIIIA mutant, but not by SpoIIAB. In agreement with this suggestion, we also found that expression of spoIIIGE156K from the promoter for the early prespore-specific gene spoIIQ still resulted in sspE-lacZ induction at the normal time during sporulation, coincidently with completion of the engulfment process. In contrast, transcription of spoIIIGE156K, but not of the wild-type spoIIIG gene, from the mother cell-specific spoIID promoter permitted the rapid induction of sspE-lacZ expression. Together, the results suggest that SpoIIAB is either redundant or has no role in the regulation of G in the prespore.Gene expression in the prespore and mother cell chambers of sporulating Bacillus subtilis is controlled by RNA polymerase sigma subunits whose activity is restricted to a specific cell type (22,31,37,46). The activation of the sporulation-specific sigma factors is tightly coupled to the completion of key morphological intermediates in the process and also relies on signaling pathways that operate between the two cell types and that keep the prespore and mother cell lines of gene expression in close register (22,31,37,46). Soon after the asymmetric division of the sporangial cell, an event that creates the prespore and the much larger mother cell, the first compartmentspecific sigma factor F becomes active in the prespore (22,31,37,46

The Notch signaling pathway is involved in a wide variety of cell-fate decisions during development. The diverse behavior of Notchactivated cells is thought to depend on tissue-or cell-type-specific transcription factors, yet the identities of such factors and the mechanism of cooperation with the Notch pathway are largely unknown. We identify here an enhancer in the promoter of ref-1, a C. elegans Notch target, which promotes Notch-dependent expression in mesodermal and endodermal cells. The enhancer contains predicted binding sites for the Notch transcriptional effector LAG-1/CSL that are essential for expression, a non-CSL site required for mesodermal expression, and four predicted binding sites for GATA transcription factors that are required for endodermal expression. We show that endodermal expression involves the GATA transcription factor ELT-2, and that ELT-2 can bind LAG-1/CSL in vitro. In many types of Notch-activated embryonic cells, ectopic ELT-2 is sufficient to drive expression of reporters containing the enhancer.

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The transcriptional mechanisms that allow neural stem cells (NSC) to balance self-renewal with differentiation are not well understood. Employing an in vivo RNAi screen we identify here NSC-TAFs, a subset of nine TATA-binding protein associated factors (TAFs), as NSC identity genes in Drosophila . We found that depletion of NSC-TAFs results in decreased NSC clone size, reduced proliferation, defective cell polarity and increased hypersensitivity to cell cycle perturbation, without affecting NSC survival. Integrated gene expression and genomic binding analyses revealed that NSC-TAFs function with both TBP and TRF2, and that NSC-TAF-TBP and NSC-TAF-TRF2 shared target genes encode different subsets of transcription factors and RNA-binding proteins with established or emerging roles in NSC identity and brain development. Taken together, our results demonstrate that core promoter factors are selectively required for NSC identity in vivo by promoting cell cycle progression and NSC cell polarity. Because pathogenic variants in a subset of TAFs have all been linked to human neurological disorders, this work may stimulate and inform future animal models of TAF-linked neurological disorders.

The four-terminal magnetoresistance and quantised Hall effect through a quantum point contact are investigated in a two-dimensional electron gas ( Z D E G ) based on an n-type (AlGa)As/GaAs single heterostructure. Depending on the choice of current and voltage contacts we measure three different magnetoresistances in a quantising magnetic field. The results agree with a simple model based on conduction via edge states and also with a more conventional analysis based on the properties of a bulk ZDEG.

The resistance of two quantum point contacts (QPCs) in series is investigated experimentally. The voltage is measured both across and in between the series pair. The normalised transmission coefficient, T, for ballistic transport is determined from the data using simple theory. T varies between 0.5 and 1.0 depending on the number of conducting 1D channels through each point contact. T is always found to be a minimum value when the number of 1D channels in each QPC is the same.