Regulation of the Drosophila segmentation gene hunchback by two maternal morphogenetic centres

Segmentation in the inset embryo is initiated by maternally provided information, which is stored in the developing oocyte. In Drosophila, the genes necessary for this process have been genetically characterized. The anterior segmented region is organized by the bicoid (bcd) gene product. The posterior segmented region is organized by several interacting gene products, among them the oskar (osk) gene product. The first zygotic group of genes, which are thought to respond to the spatial cues provided by the maternal genes, are the gap genes, whose members include hunchback (hb), Krüppel (Kr) and knirps (kni). To elucidate the role played by the maternal genes in expression of the gap gene hb, antibodies were raised against a fusion protein and were used for the cytological localization of the hb gene product in wild-type and mutant embryos. The hb protein is predominantly located in the nucleus. Its spatial expression includes the formation of an anterior-posterior gradient during the early cleavage stages and a strong zygotic expression in the anterior half of the embryo. Analysis of embryos mutant for the maternal genes affecting the anterior-posterior segmentation pattern shows that the formation of the early gradient is controlled by the osk group of genes, whereas efficient activation of the zygotic anterior expression domain is dependent on bcd activity.     

Direct homeodomain-DNA interaction in the autoregulation of the fushi tarazu gene.

A major problem in the elucidation of the molecular mechanisms governing development is the distinction between direct and indirect regulatory interactions among developmental control genes. In vivo studies have indicated that the Drosophila segmentation gene fushi tarazu (ftz) directly or indirectly autoregulates its expression. Here we describe a generally applicable experimental approach which establishes a direct in vivo interaction of the homeodomain protein ftz with the ftz cis-autoregulatory control region. In vitro studies have shown that the DNA-binding specificity of the ftz homeodomain can be changed by a single amino-acid substitution in the recognition helix (Gln 50----Lys). Whereas wild-type ftz homeodomain binds preferentially to a CCATTA motif, the mutant homeodomain (ftzQ50K) recognizes a GGATTA motif. We now find that the in vivo activity of an ftz autoregulatory enhancer element is reduced by mutations of putative ftz-binding sites to GGATTA. This down-regulatory effect is specifically suppressed in vivo by the DNA-binding specificity mutant ftzQ50K. These results establish a direct positive autoregulatory feedback mechanism in the regulation of this homeobox gene.     

Borders of parasegments in Drosophila embryos are delimited by the fushi tarazu and even-skipped genes

One of the earliest molecular signs of segmentation in Drosophila embryos is the striped expression of some pair-rule genes during the blastoderm stage. Two of these genes, fushi-tarazu (ftz) and even-skipped (eve) are expressed during this stage in complementary patterns of seven stripes which develop and disappear in concert. Here, we map the cells expressing each of these two pair-rule genes with respect to the 14 stripes of cells expressing the engrailed gene. We find that both ftz and eve generate stripes which have sharp boundaries at the anterior margin, but fade away posteriorly. The anterior boundaries correspond cell by cell with the anterior boundaries of expression of the engrailed gene. We therefore suggest that a key function of early ftz and eve gene activity is the formation of a sharp stable boundary at the anterior margin of each stripe. These boundary lines, rather than the narrowing zonal stripes, would delimit the anterior boundaries of engrailed and other homoeotic genes and thereby subdivide the embryo into parasegments.     

Function of torso in determining the terminal anlagen of the Drosophila embryo

The formation of the unsegmented terminal regions of the Drosophila larva, acron and telson requires the function of at least five maternal genes (terminal genes class). In their absence, the telson and acron are not formed. One of them, torso (tor), has gain-of-function alleles which have an opposite phenotype to the lack-of-function (tor-) alleles: the segmented regions of the larval body, thorax and abdomen, are missing, whereas the acron is not affected and the telson is enlarged. In strong gain-of-function mutants, the pair-rule gene fushi tarazu (ftz) is not expressed, demonstrating the suppression of the segmentation process in an early stage of development. The tor gain-of-function effect is neutralized, and segmentation is restored in double mutants with the zygotic gene tailless (tll), which has a phenotype similar (but not identical) to that of tor-. This suggests that tor acts through tll, and that in the gain-of-function alleles of tor, the tll gene product is ectopically expressed at middle positions of the embryo, where it inhibits the expression of segmentation genes like ftz.     

Spatial and temporal patterns of Krüppel gene expression in early Drosophila embryos

The Krüppel (Kr) locus is a member of the 'gap' class of segmentation genes of Drosophila melanogaster. Mutations at the Kr locus cause the deletion of contiguous segments from the embryonic body pattern. We have elucidated the spatial and temporal characteristics of Kr gene expression during early embryo development, the localization of cytoplasmic Kr+ activity and its spatial requirement for normal segmentation.     

Sequence of a Drosophila segmentation gene: protein structure homology with DNA-binding proteins

Mutations in the fushi tarazu (ftz) locus of Drosophila result in embryos with half the usual number of body segments. The sequences of the wild-type gene, a temperature-sensitive allele and a dominant mutant allele are presented. A portion of the conserved protein domain present in ftz and several homoeotic genes resembles the DNA-binding region of prokaryotic DNA-binding proteins, and is also similar to products of the yeast mating-type locus.     

Control of neuronal fate by the Drosophila segmentation gene even-skipped

The central nervous system (CNS) contains a remarkable diversity of cell types. The molecular basis for generating this neuronal diversity is poorly understood. Much is known, however, about the regulatory genes which control segmentation and segment identity during early Drosophila embryogenesis. Interestingly, most of the segmentation and homoeotic genes in Drosophila, as well as many of their vertebrate homologues, are expressed during the development of the nervous system (for example, ref. 3). Are these genes involved in specifying the identity of individual neurons during neurogenesis, just as they specify the identity of cells during segmentation? We previously described the CNS expression of the segmentation gene fushi tarazu (ftz) and showed that ftz CNS expression is involved in the determination of an identified neuron. Here we show that another segmentation gene, even-skipped (eve), is expressed in a different but overlapping subset of neurons. Temperature-sensitive inactivation of the eve protein during neurogenesis alters the fate of two of these neurons. Our results indicate that the nuclear protein products of the eve and ftz segmentation genes are components of the mechanism controlling cell fate during neuronal development.     

Isolation of the dorsal locus of Drosophila

The establishment of embryonic polarity is a crucial step in pattern formation and morphogenesis. In the fruitfly Drosophila melanogaster, embryonic polarity depends primarily on genes expressed in the female during oogenesis. Mutations in these 'maternal effect' genes can lead to major disruptions in normal pattern formation. Two classes of maternal genes essential for the establishment of polarity in the embryo have been identified. Lesions in one class, the 'bicaudal' genes, disrupt the anterior-posterior axis; lesions in the other class disrupt dorsal-ventral polarity, and in the most extreme cases embryos fail to form any ventral or lateral structures. Genetic studies suggest that the anterior-posterior and dorsal-ventral axes may be independent as the defects observed in mutants from each class seem to be restricted to one axis only. The dorsal (dl) locus is one of the maternal effect genes involved in the establishment of dorsal-ventral polarity. Homozygous dl females produce embryos exhibiting the mutant phenotype--complete lack of dorsal-ventral polarity in the strongest alleles--irrespective of the genotype of the father. Although dl is a maternal effect locus and must be expressed during oogenesis, the gene product, or a substance depending on the normal function of the dl gene, seems to be active early in embryogenesis, as the dl phenotype can be partially rescued by injection of cytoplasm from wild-type cleavage-stage embryos. Here we report the molecular cloning of the dorsal locus and a study of its expression.     

Isolation of a homoeo box-containing gene from the engrailed region of Drosophila and the spatial distribution of its transcripts

The engrailed locus of Drosophila melanogaster has the characteristics of both a homoeotic gene and a segmentation gene: like a homoeotic gene, it specifies the development of specific compartments of the Drosophila embryo (the posterior compartments of each segment), and, like mutations of segmentation genes, lethal alleles of engrailed affect also the pattern of segmentation of the embryo. Here we report that like many of the homoeotic genes of the bithorax and Antennapedia complexes, engrailed has a 'homoeo box' sequence: also, like the segmentation gene fushi tarazu, the engrailed gene displays a periodic pattern of expression in Drosophila embryos.     

Regulation of segment polarity genes in the Drosophila blastoderm by fushi tarazu and even skipped

During the late cellular blastoderm stage of Drosophila embryo-genesis the segmentation genes engrailed, en, and wingless, wg, become expressed in two series of 14 stripes which will subsequently coincide with the anterior and posterior limits of each parasegment. Previous studies have shown that the establishment of the pattern of en stripes depends upon the activity of the homoeobox-containing pair-rule genes fushi tarazu, ftz and even skipped, eve. Here we show that these two genes also control the spatial expression of wg. Whereas ftz and eve behave as activators of en we find that both genes are required to repress wg expression, so that wg becomes expressed only in the narrow stripes of cells which come to separate the ftz and eve bands at the end of the blastoderm stage. In contrast, we propose that the precise positioning of the en stripes depends upon signals generated in a combinatorial manner by the overlaps between the ftz or eve domains and those of other pair rule genes, specifically odd paired, opa and paired, prd.     

Information for the dorsal--ventral pattern of the Drosophila embryo is stored as maternal mRNA

Maternal-effect mutations in 10 loci in Drosophila produce totally 'dorsalized' embryos. Injection of RNA isolated from wild-type embryos into mutants at six loci partially restores dorsal-ventral polarity. For the mutant snake, injection of poly(A)+ RNA restores a complete dorsal-ventral pattern.     

Mutations in the p53 gene occur in diverse human tumour types

The p53 gene has been a constant source of fascination since its discovery nearly a decade ago. Originally considered to be an oncogene, several convergent lines of research have indicated that the wild-type gene product actually functions as a tumour suppressor gene. For example, expression of the neoplastic phenotype is inhibited, rather than promoted, when rat cells are transfected with the murine wild-type p53 gene together with mutant p53 genes and/or other oncogenes. Moreover, in human tumours, the short arm of chromosome 17 is often deleted. In colorectal cancers, the smallest common region of deletion is centred at 17p13.1; this region harbours the p53 gene, and in two tumours examined in detail, the remaining (non-deleted) p53 alleles were found to contain mutations. This result was provocative because allelic deletion coupled with mutation of the remaining allele is a theoretical hallmark of tumour-suppressor genes. In the present report, we have attempted to determine the generality of this observation; that is, whether tumours with allelic deletions of chromosome 17p contain mutant p53 genes in the allele that is retained. Our results suggest that (1) most tumours with such allelic deletions contain p53 point mutations resulting in amino-acid substitutions, (2) such mutations are not confined to tumours with allelic deletion, but also occur in at least some tumours that have retained both parental 17p alleles, and (3) p53 gene mutations are clustered in four 'hot-spots' which exactly coincide with the four most highly conserved regions of the gene. These results suggest that p53 mutations play a role in the development of many common human malignancies.