Protein products of the bithorax complex in Drosophila

A sequence from the Ubx 5' exon in the bithorax complex of Drosophila melanogaster was expressed as a fusion protein in bacteria. This protein was used to raise rabbit antisera and monoclonal antibodies. These antibodies detect antigens that, on protein blots and by immunofluorescence on whole mounts of imaginal discs, show the predicted segmental distribution of Ubx products. These products are predominantly, if not totally, localized in the cell nucleus. In the embryonic nervous system nuclei are labeled from the second thoracic segment to the eighth abdominal segment. There is no labeling in homozygous Df bxd100 embryos.     

Why cells respond differently to DNA damage: A chromatin perspective

In response to DNA double-stranded breaks (DSBs) cells activate a signaling cascade known as the DNA damage response (DDR) whose main function is to promote the repair of the lesions while it delays cell cycle progression until repair is completed. Whereas most cells respond alike to an equivalent dose of DNA damage, certain degree of heterogeneity exists in the strength of the DDR that is assembled in each individual cell. This variability might be accounted for by erratic changes that aggregate into the inherent noise of biological systems. However, we have recently found that the overall degree of chromatin compaction impinges a direct constrain on the activation of the DDR, providing a simple chromatin-based model to explain the cell-to-cell variability observed in cell populations. We here provide an overview of the available data, including our own, that would be supportive of such a model and discuss how this perspective might be used to explain previous observations     

Studies on transvection at the bithorax complex in Drosophila melanogaster

Summary We have studied the influence of some mutations in the bithorax complex on the observed synapsis dependent phenotype of the genotypes Cbx ¹Ubx¹/+ and bx ^34e/Ubx¹. The effect of these mutations is similar to that introduced by disruption of pairing or by the z ^a mutation. Among the bx mutations, we find that bx ⁸ behaves differently from most other bx mutations in its influence on the synapsis dependent phenotype. This observation induced us to map the position of bx ⁸ with respect to other bx mutations; we find that it maps between bx ^34e and bx ³. We show how some of the observations reported here can be fitted into a model of activation of the bithorax complex proposed by one of us.     

Characterization of new regulatory elements within the Drosophila bithorax complex

The homeotic Abdominal-B (Abd-B) gene expression depends on a modular cis-regulatory region divided into discrete functional domains (iab) that control the expression of the gene in a particular segment of the fly. These domains contain regulatory elements implicated in both initiation and maintenance of homeotic gene expression and elements that separate the different domains. In this paper we have performed an extensive analysis of the iab-6 regulatory region, which regulates Abd-B expression at abdominal segment A6 (PS11), and we have characterized two new polycomb response elements (PREs) within this domain. We report that PREs at Abd-B cis-regulatory domains present a particular chromatin structure which is nuclease accessible all along Drosophila development and both in active and repressed states. We also show that one of these regions contains a dCTCF and CP190 dependent activity in transgenic enhancer-blocking assays, suggesting that it corresponds to the Fab-6 boundary element of the Drosophila bithorax complex.     

Mutants of the bithorax complex and determinative states in the thorax of Drosophila melanogaster.

Homoeotic mutations of the bithorax complex cause segmental transformations. The genes in which these mutations occur are good candidates for genes that are involved in determination. The determination system in imaginal discs must have at least two functions. One is a cell heredity function that is responsible for maintaining the determined state during growth and development. A second is the expression of the determined state (e.g., different imaginal discs have different morphologies). The homoeotic mutations of the bithorax complex could be affecting either of these two functions. I have found that when posterior haltere disc cells, that are transformed by the mutation postbithorax so that they form wing cuticle in situ, regenerate anterior structures, these structures are anterior wing. This is the same result as that seen when wild-type posterior-wing disc cells regenerate anterior structures. On the other hand, when anterior haltere disc cells transformed by the mutation bithorax³, so that they produce wing cuticle in situ, regenerate, they produce posterior haltere structures. This is unlike wild-type anterior-wing disc cells, which regenerate posterior-wing structures. From these results, I conclude that bithorax³ affects the expression of the determined state and postbithorax affects the cell heredity of determination.     

The M/SAR elements of the bithorax complex in Drosophila melanogaster

The bithorax (BX) complex of Drosophila is a complex polygenic region with a multifactorial system of regulation. One of the levels of the regulatory system of the BX complex is its association with the nuclear skeleton structures through a specific interaction of the M/SAR DNA with the nuclear matrix proteins. In the present work, M/SAR elements were mapped on the molecular-genetic map of the region. All of the elements examined were found to colocalize with regulatory elements and form clusters that restrict/bracket the genetically active domains. All M/SAR DNA revealed was shown to bins specifically to the purified Drosophila melanogaster lamin.     

Heat shock induced phenocopies of dominant mutants of the bithorax complex in Drosophila melanogaster.

Summary Heat shock of 37°C applied to Drosophila embryos at blastoderm stage induces phenocopies of some dominant mutants of the bithorax complex. Heat shock might shut down functions of cis regulator genes involved in embryonic pattern formation.     

Gonad formation and development requires the abd-A domain of the bithorax complex in Drosophila melanogaster.

The abdominal-A (abd-A) gene, a member of the bithorax complex, is required for the correct identity of parasegments (PS) 7 through 13. Mutations in iab-4, one of the cis-regulatory regions of abd-A, transform epidermal structures of PS 9 and also cause loss of gonads in adult flies. Here, we describe a developmental and molecular analysis of the role of iab-4 functions in gonadal development. In flies homozygous for a strong iab-4 allele, gonadogenesis is not initiated in the embryo because the mesodermal cells fail to encapsulate the pole cells. Flies homozygous for weaker iab-4 mutations sometimes form ovaries. The ovary-oviduct junctions are abnormal, however, and egg transfer from the ovary to the uterus is blocked in the adult. To localize the sites that require iab-4 function, we have analyzed animals chimeric for the mutant and wild-type cells. These chimeras were generated by three kinds of transplantation experiments: pole cells, embryonic somatic nuclei or larval ovaries. Our results suggest that iab-4 is required in the somatic cells of the gonadal primordia, but not the germ line. In addition, the formation of functional ovary-oviduct junctions and egg transfer also requires iab-4 functions in the somatic cells of the ovary and in at least one additional somatic tissue.     

The molecular genetics of the bithorax complex of Drosophila: cis-regulation in the Abdominal-B domain.

In Drosophila the Abdominal-B (Abd-B) domain of the bithorax complex (BX-C) spans over 100 kb and is responsible for specifying the identities of adult abdominal segments five (A5) to nine (A9), inclusive, and correspondingly, neuromeres 10-14 of the embryonic central nervous system. The domain consists of a region coding for two proteins, ABD-BI (54 kd) and ABD-BII (36 kd) and cis-regulatory regions extending from infra-abdominal-5 (iab-5) to iab-9, inclusive. We have used a monoclonal anti-ABD-B antibody to infer that mutants in iab-8 eliminate the expression of ABD-BI in neuromeres 10-13, inclusive, and that mutants in iab-9 eliminate expression of ABD-BII in neuromere 14. ABD-B expression is also analyzed in homozygotes for (i) loss-of-function mutants involving the iab-5, iab-6 and iab-7 regions, (ii) gain-of-function mutants Miscadastral pigmentation (Mcp) and Superabdominal (Sab), and (iii) a trans-regulator, Polycomb (Pc). ABD-B expression along the antero-posterior axis is colinear with the chromosomal order of the cis-regulatory regions. The behavior of rearrangement-associated iab-6 and iab-7 mutants suggests that the enhancer-like region, iab-5, and possibly also iab-6, may be shared between the abd-A and Abd-B domains. Such sharing is proposed as a factor that tends to keep gene complexes intact during evolution.