extramacrochaetae, a negative regulator of sensory organ development in Drosophila, defines a new class of helix-loop-helix proteins.

The function of the extramacrochaetae (emc) gene is required to establish the normal spatial pattern of adult sensory organs in Drosophila. emc acts to suppress sensory organ development in certain regions of the body surface, apparently by antagonizing the function of the achaete and scute genes of the achaetescute complex (AS.C). We have found that emc encodes a novel member of the helix-loop-helix (HLH) family of proteins. The emc protein shares the dimerization domain of other HLH proteins but lacks their DNA binding motif. We propose a model in which the emc protein negatively regulates sensory organ determination by forming heterodimers with the HLH proteins encoded by the AS-C and/or daughterless, thereby altering or interfering with their activity.     

Model investigation of central regulatory contour of gene net of D. melanogaster macrochaete morphogenesis

Morphogenesis of drosophila macrochaete functioning as mechanoreceptors includes several steps, each of which has their own genetic support described in terms of gene nets. Mechanoreceptor develops from one parental cell (Parental Cell of Sensor Organ-PCSO), the determination of which has a critical role in macrochaete development. The highest content of AS-C proneural proteins with respect to surrounding cells that initiate a neural way of cellular development and by means of it mechanoreceptor morphogenesis is typical for PCSO. The key object of gene net providing PCSO determination consists of gene complex achaete-scute (AS-C). This complex activity is controlled by central regulatory contour (CRC). Besides AS-C, CRC includes the following genes: hairy, senseless (sens), charlatan (chn), scratch (scrt), daughterless (da), extramacrochaete (emc), and groucho (gro). The system of direct relation and feedback and induction and repression relations between CRC components are realized via the coding by these genes proteins. A mathematical model of CRC functioning as a regulator of proneural AS-C protein content in PCSO determining successful passing of the main phase of morphogenesis of D. melanogaster mechanoreceptor is discussed.     

asense, a member of the Drosophila achaete-scute complex, is a proneural and neural differentiation gene.

The asense (ase) gene of the achaete-scute complex (AS-C) is expressed in the precursors of all adult sensory organs (SOs), the sensory mother cells (SMCs) and in their immediate progeny. Its deletion causes the loss of some SOs and the abnormal differentiation of part of the remaining ones. These defects, which include malformations of the external part of the SOs, duplication of the innervating neuron etc, are enhanced by the haploid condition for the other AS-C genes and are corrected by an ase transgene. We conclude that ase participates, in combination with other members of the AS-C, in implementing the neural program of differentiation of the SMCs. ase also has a proneural function that participates in the singling out of the SMCs that give rise to the recurved bristles of the anterior wing margin. The proneural potential of ase is shown, in addition, by the generation of SOs induced by the generalized expression of an ase gene driven by a hsp70 promoter.     

The extramacrochaetae gene provides information for sensory organ patterning.

The Drosophila adult epidermis displays a stereotyped pattern of bristles and other types of sensory organs (SOs). Its generation requires the proneural achaete (ac) and scute (sc) genes. In the imaginal wing disc, the anlage for most of the thoracic and wing epidermis, their products accumulate in groups of cells, the proneural clusters, whose distribution prefigures the adult pattern of SOs. These proteins then induce the emergence of SO mother cells (SMCs). Here, we show that the extramacrochaetae (emc) gene, an antagonist of the proneural function, is another agent that contributes to SO positioning. In the wing disc, emc is expressed in a complex and evolving pattern. SMCs appear not only within proneural clusters but also within minima of emc expression. When one of these spatial restrictions is eliminated, by ubiquitously expressing ac-sc, SMCs still emerge within minima of emc. When in addition, the other spatial restriction is reduced by decreasing emc expression, many ectopic SMCs emerge in a relatively even spaced and less constant pattern. Thus, the heterogeneous distribution of the emc product is one of the elements that define the positions where SMCs arise. emc probably refines SMC (and SO) positioning by reducing both the size of proneural clusters and the number of cells within clusters that can become SMCs.     

Gene-dose titration analysis in the search of trans-regulatory genes in Drosophila.

We have searched for trans-regulatory genes in two genetic systems in Drosophila, the bithorax complex (BX-C) and the achaete-scute complex (AS-C). Previous genetic evidence suggests that the activation of both BX-C and AS-C, depends on trans-regulatory genes (Polycomb, Pc, in the former and hairy, h, in the latter) acting in a negative type of control. Mutants of these regulatory genes in heterozygous condition have dominant derepression phenotypes in flies with extra doses of the corresponding gene complexes. We have searched for new loci, with similar gene-dose relationships. We have isolated only new alleles (six) of Pc in the BX-C experiment. In the AS-C experiment four h alleles, and 13 alleles of a new locus (extramacrochaetae, emc) have been discovered. Whereas the h locus shows specific interactions upon achaete, the new locus, emc, is specific for the scute part of the AS-C. Statistical analysis suggests that these are the only loci in the genome with those dose-dependent properties in the two systems.     

Model investigation of central regulatory contour of gene net of <Emphasis Type="Italic">D. melanogaster</Emphasis> macrochaete morphogenesis

Morphogenesis of drosophila macrochaete functioning as mechanoreceptors includes several steps, each of which has their own genetic support described in terms of gene nets. Mechanoreceptor develops from one parental cell (Sensory Organ Precursor cell—SOP cell), the determination of which has a critical role in macrochaete development. The highest content of AS-C proneural proteins with respect to surrounding cells that initiate a neural way of cellular development and by means of it mechanoreceptor morphogenesis is typical for SOP cell. The key object of gene net providing parental cell determination consists of gene complex achaete-scute (AS-C). This complex activity is controlled by central regulatory contour (CRC). Besides AS-C, CRC includes the following genes: hairy, senseless (sens), charlatan (chn), scratch (scrt), daughterless (da), extramacrochaete (emc), and groucho (gro). The system of direct relation and feedback and induction and repression relations between CRC components are realized via the coding by these genes proteins. A mathematical model of CRC functioning as a regulator of proneural AS-C protein content in SOP cell determining successful passing of the main phase of morphogenesis of D. melanogaster mechanoreceptor is discussed.     

The EGF receptor and N signalling pathways act antagonistically in Drosophila mesothorax bristle patterning

An early step in the development of the large mesothoracic bristles (macrochaetae) of Drosophila is the expression of the proneural genes of the achaete-scute complex (AS-C) in small groups of cells (proneural clusters) of the wing imaginal disc. This is followed by a much increased accumulation of AS-C proneural proteins in the cell that will give rise to the sensory organ, the SMC (sensory organ mother cell). This accumulation is driven by cis-regulatory sequences, SMC-specific enhancers, that permit self-stimulation of the achaete, scute and asense proneural genes. Negative interactions among the cells of the cluster, triggered by the proneural proteins and mediated by the Notch receptor (lateral inhibition), block this accumulation in most cluster cells, thereby limiting the number of SMCs. Here we show that the proneural proteins trigger, in addition, positive interactions among cells of the cluster that are mediated by the Epidermal growth factor receptor (EGFR) and the Ras/Raf pathway. These interactions, which we denominate 'lateral co-operation', are essential for macrochaetae SMC emergence. Activation of the EGFR/Ras pathway appears to promote proneural gene self-stimulation mediated by the SMC-specific enhancers. Excess EGFR signalling can overrule lateral inhibition and allow adjacent cells to become SMCs and sensory organs. Thus, the EGFR and Notch pathways act antagonistically in notum macrochaetae determination.     

Yeast centromere binding protein CBF1, of the helix-loop-helix protein family, is required for chromosome stability and methionine prototrophy

The centromere and its binding proteins constitute the kinetochore structure of metaphase chromosomes, which is crucial for the high accuracy of the chromosome segregation process. Isolation and analysis of the gene encoding a centromere binding protein from the yeast S. cerevisiae, CBF1, are described in this paper. DNA sequence analysis of the CBF1 gene reveals homology with the transforming protein myc and a family of regulatory proteins known as the helix-loop-helix (HLH) proteins. Disruption of the CBF1 gene caused a decrease in the growth rate, an increase in the rate of chromosome loss/nondisjunction, and hypersensitivity to the antimitotic drug thiabendazole. Unexpectedly, the cbf1 null mutation concomitantly resulted in a methionine auxotrophic phenotype, which suggests that CBF1, like other HLH proteins in higher eukaryotic cells, participates in the regulation of gene expression.     

Mutually exclusive expression of a helix-loop-helix gene and N-myc in human neuroblastomas and in normal development.

We have isolated a novel human gene encoding a helix-loop-helix (HLH) protein by molecularly cloning chromosome 1p36-specific CpG islands. The gene termed heir-1 was localized to the neuroblastoma consensus deletion at 1p36.2-p36.12. Its predicted protein is 95.8% identical to the mouse HLH462 protein and has clear homology to the mouse Id and Drosophila emc proteins. Heir-1 does not encode a basic DNA binding domain as found in basic HLH proteins. The gene is expressed specifically at high abundance in adult lung, kidney and adrenal medulla, but not in adult brain. Despite prominent heir-1 expression in adrenal medulla, which is a prime target for neuroblastomas, 10 out of 12 neuroblastoma-derived cell lines revealed very low levels of heir-1 mRNA. Low heir-1 expression was generally found in tumor cell lines with N-myc overexpression, whereas the two cell lines displaying high heir-1 levels did not overexpress N-myc. Mutually exclusive expression of both genes was also found by in situ hybridization in developing mouse tissues, particularly in the forebrain neuroectoderm. We conclude that heir-1 expression is reduced specifically in the majority of neuroblastomas and suggest an inverse correlation between heir-1 and N-myc expression in neuroblastoma tumors and in embryonic development.     

The protein Id: a negative regulator of helix-loop-helix DNA binding proteins

We have isolated a cDNA clone encoding a novel helix-loop-helix (HLH) protein, Id. Id is missing the basic region adjacent to the HLH domain that is essential for specific DNA binding in another HLH protein, MyoD. An in vitro translation product of Id can associate specifically with at least three HLH proteins (MyoD, E12, and E47) and attenuate their ability to bind DNA as homodimeric or heterodimeric complexes. Id is expressed at varying levels in all cell lines tested. In three cell lines that can be induced to undergo terminal differentiation, Id RNA levels decrease upon induction. Transfection experiments indicate that over-expression of Id inhibits the trans-activation of the muscle creatine kinase enhancer by MyoD. Based on these findings, we propose that HLH proteins lacking a basic region may negatively regulate other HLH proteins through the formation of nonfunctional heterodimeric complexes.