Recent whole-genome analyses in Drosophila suggest that critical developmental control genes sometimes contain “shadow” enhancers [1]. These can be located in remote positions, including the introns of neighboring genes. They nonetheless produce patterns of gene expression that are the same or similar as those produced by more proximal primary enhancers. It was suggested that shadow enhancers help foster robustness in gene expression in response to environmental or genetic perturbations [2,3]. Here, we critically test this hypothesis by employing a combination of BAC recombineering and quantitative confocal imaging methods [2,4]. Evidence is presented that the snail gene is regulated by a distal shadow enhancer located within the neighboring Tim17b2 locus. snail encodes a zinc finger transcription factor that has been implicated in epithelial/mesenchyme transitions (EMT) in a broad spectrum of developmental processes and cancers [5-7]. Removal of the proximal primary enhancer does not significantly perturb snail function, including the repression of neurogenic genes and formation of the ventral furrow during gastrulation at normal temperatures of development. However, at elevated temperatures there is sporadic loss of snail expression and coincident disruptions in gastrulation. Similar defects are observed at normal temperatures upon reductions in the levels of Dorsal, a key activator of snail expression [reviewed in 8]. Altogether, these results suggest that shadow enhancers represent a novel mechanism of canalization, whereby complex developmental processes “bring about one definite end-result regardless of minor variations in conditions…” [9].

No abstract

We present the use of recently developed live imaging methods to examine the dynamic regulation of even-skipped (eve) stripe 2 expression in the precellular Drosophila embryo. Nascent transcripts were visualized via MS2 RNA stem loops. The eve stripe 2 transgene exhibits a highly dynamic pattern of de novo transcription, beginning with a broad domain of expression during nuclear cycle 12 (nc12), and progressive refinement during nc13 and nc14. The mature stripe 2 pattern is surprisingly transient, constituting just ∼15 min of the ∼90-min period of expression. Nonetheless, this dynamic transcription profile faithfully predicts the limits of the mature stripe visualized by conventional in situ detection methods. Analysis of individual transcription foci reveals intermittent bursts of de novo transcription, with duration cycles of 4-10 min. We discuss a multistate model of transcription regulation and speculate on its role in the dynamic repression of the eve stripe 2 expression pattern during development.

Metazoan genes are embedded in a rich milieu of regulatory information that often includes multiple enhancers possessing overlapping activities. In this study, we employ quantitative live imaging methods to assess the function of pairs of primary and shadow enhancers in the regulation of key patterning genes-knirps, hunchback, and snail-in developing Drosophila embryos. The knirps enhancers exhibit additive, sometimes even super-additive activities, consistent with classical gene fusion studies. In contrast, the hunchback enhancers function sub-additively in anterior regions containing saturating levels of the Bicoid activator, but function additively in regions where there are diminishing levels of the Bicoid gradient. Strikingly sub-additive behavior is also observed for snail, whereby removal of the proximal enhancer causes a significant increase in gene expression. Quantitative modeling of enhancer–promoter interactions suggests that weakly active enhancers function additively while strong enhancers behave sub-additively due to competition with the target promoter.DOI: http://dx.doi.org/10.7554/eLife.07956.001

Thin films based on melanin are prepared and characterized. The films display electrical conductivities comparable with amorphous silicon as well as a host of other interesting and potentially useful optoelectronic properties. The results may be useful for melanin‐based applications such as chemi‐sensors (in a variety of architectures including OFETS with chemi‐sensitive channels) and bolometric photon detectors.