restricted source, and the rate of its production may Weizmann Institute of Science vary due to genetic alterations or fluctuations in temper-Rehovot 76100 ature or nutrients. Yet, a reliable pattern is established Israel despite such fluctuations. Temporal averaging could provide one mechanism for buffering fluctuations in gene expression. However, such a mechanism would not apply to cases of persistent changes, such as alter-Summary ation in gene dosage. In the Drosophila wing imaginal disc, for example, it was shown that intermediate in-Morphogen gradients provide long-range positional crease in Dpp expression has little effect on wing and information by extending across a developing field. To thorax patterning (Morimura et al., 1996). While similar ensure reproducible patterning, their profile is invariexperiments were not yet done for Wg and Hh morphoable despite genetic or environmental fluctuations. gens, it is known that wing patterning is precise in het-Common models assume a morphogen profile that erozygous mutants that have only one functional allele decays exponentially. Here, we show that exponential of Hh or Wg. profiles cannot, at the same time, buffer fluctuations An emerging theme is that feedback mechanisms play in morphogen production rate and define long-range a prominent role in shaping morphogen gradients (Freegradients. To comply with both requirements, morphoman, 2000; Perrimon and McMahon, 1999). Regulatory gens should decay rapidly close to their source but mechanisms were identified at all levels of morphogen at a significantly slower rate over most of the field. function, including movement away from the source Numerical search revealed two network designs that support robustness to fluctuations in morphogen pro-(Bellaiche et al., 1998; Burke et al., 1999; Chen and duction rate. In both cases, morphogens enhance their Struhl, 1996), stability (Cadigan et al., 1998; Gerlitz and own degradation, leading to a higher degradation rate Basler, 2002; Giraldez et al., 2002), and the sensitivity close to their source. This is achieved through reciproof the receiving cells to morphogen signaling (Campbell cal interactions between the morphogen and its recepand Tomlinson, 1999; Jazwinska et al., 1999). Recently, tor. The two robust networks are consistent with propthe roles of receptors in shaping morphogen gradients erties of the Wg and Hh morphogens in the Drosophila received much interest. Theoretical analysis demonwing disc and provide novel insights into their function. strated that high binding affinities may hinder ligand diffusion, but biologically relevant gradients can still be formed by diffusion, when receptor-mediated ligand Introduction degradation is taken into account (Kerszberg and Wolpert, 1998; Lander et al., 2002). Feedback regulation Morphogens are signaling molecules that induce disof receptor expression was identified for all three mortinct cell fates at different concentrations. During develphogens patterning the Drosophila wing disc (Cadigan, opment, gradients of morphoge...

Gene expression depends on the frequency of transcription events (burst frequency) and on the number of mRNA molecules made per event (burst size). Both processes are encoded in promoter sequence, yet their dependence on mutations is poorly understood. Theory suggests that burst size and frequency can be distinguished by monitoring the stochastic variation (noise) in gene expression: Increasing burst size will increase mean expression without changing noise, while increasing burst frequency will increase mean expression and decrease noise. To reveal principles by which promoter sequence regulates burst size and frequency, we randomly mutated 22 yeast promoters chosen to span a range of expression and noise levels, generating libraries of hundreds of sequence variants. In each library, mean expression (m) and noise (coefficient of variation, h) varied together, defining a scaling curve: h 2 = b/m + h ext 2 . This relation is expected if sequence mutations modulate burst frequency primarily. The estimated burst size (b) differed between promoters, being higher in promoter containing a TATA box and lacking a nucleosome-free region. The rare variants that significantly decreased b were explained by mutations in TATA, or by an insertion of an out-of-frame translation start site. The decrease in burst size due to mutations in TATA was promoter-dependent, but independent of other mutations. These TATA box mutations also modulated the responsiveness of gene expression to changing conditions. Our results suggest that burst size is a promoter-specific property that is relatively robust to sequence mutations but is strongly dependent on the interaction between the TATA box and promoter nucleosomes.

The pattern of the sensory bristles in the fruit fly Drosophila is remarkably reproducible. Each bristle arises from a sensory organ precursor (SOP) cell that is selected, through a lateral inhibition process, from a cluster of proneural cells. Although this process is well characterized, the mechanism ensuring its robustness remains obscure. Using probabilistic modeling, we defined the sources of error in SOP selection and examined how they depend on the underlying molecular circuit. We found that rapid inhibition of the neural differentiation of nonselected cells, coupled with high cell-to-cell variability in the timing of selection, is crucial for accurate SOP selection. Cell-autonomous interactions (cis interactions) between the Notch receptor and its ligands Delta or Serrate facilitate accurate SOP selection by shortening the effective delay between the time when the inhibitory signal is initiated in one cell and the time when it acts on neighboring cells, suggesting that selection relies on competition between cis and trans interactions of Notch with its ligands. The cis interaction model predicts that the increase in ectopic SOP selections observed with reduced Notch abundance can be compensated for by reducing the abundance of the Notch ligands Delta and Serrate. We validated this prediction experimentally by quantifying the frequency of ectopic bristles in flies carrying heterozygous null mutations of Notch, Delta, or Serrate or combinations of these alleles. We propose that susceptibility to errors distinguishes seemingly equivalent designs of developmental circuits regulating pattern formation.

Epithelial tissue functions depend largely on a polarized organization of the individual cells. We examined the roles of the DrosophilaPDGF/VEGF receptor (PVR) in polarized epithelial cells, with specific emphasis on the wing disc epithelium. Although the receptor is broadly distributed in this tissue, two of its ligands, PVF1 and PVF3 are specifically deposited within the apical extracellular space, implying that polarized apical activation of the receptor takes place. The apical localization of the ligands involves a specialized secretion pathway. Clones for null alleles of Pvr or expression of RNAi constructs showed no phenotypes in the wing disc or pupal wing, suggesting that Pvr plays a redundant role in this tissue. However, when uniform expression of a constitutively dimerizing receptor was induced, loss of epithelial polarity, formation of multiple adherens and septate junctions, and tumorous growth were observed in the wing disc. Elevation of the level of full-length PVR also gave rise to prominent phenotypes, characterized by higher levels of actin microfilaments at the basolateral areas of the cells and irregular folding of the tissue. Together,these results suggest that polarized PVR activation is necessary for the proper organization of the wing disc epithelium, by regulating the apical assembly of the actin cytoskeleton.

Cells adapt to environmental changes through genetic mutations that stabilize novel phenotypes. Often, this adaptation involves regulatory changes which modulate gene expression. In the budding yeast, ribosomal-related gene expression correlates with cell growth rate across different environments. To examine whether the same relationship between gene expression and growth rate is observed also across natural populations, we measured gene expression, growth rate and ethanol production of twenty-four wild type yeast strains originating from diverse habitats, grown on the pentose sugar xylulose. We found that expression of ribosome-related genes did not correlate with growth rate. Rather, growth rate was correlated with the expression of amino acid biosynthesis genes. Searching other databases, we observed a similar correlation between growth rate and amino-acid biosyntehsis genes in a library of gene deletions. We discuss the implications of our results for understanding how cells coordinate their translation capacity with available nutrient resources.