The brown Dominant (bw D ) allele contains a large insertion of heterochromatin, which causes the locus to aberrantly associate with heterochromatin in interphase nuclei and silences the wild-type allele in heterozygotes. Transgenes placed near the bw ؉ locus, in trans to bw D , can also be silenced. The strength of silencing (called trans inactivation) varies with the regulatory sequences of the transgene and its distance away from the bw D insertion site in trans. In this study, we examine endogenous sequences in cis that influence susceptibility of a reporter gene to trans inactivation. Flanking deletions were induced in two parental lines containing P-element transgenes showing trans inactivation of the mini-white reporter. These new lines, which have mini-white under the influence of different endogenous sequence elements, now show varied ability to be silenced by bw D . Determination of the deleted regions and the levels of mini-white expression and trans inactivation has allowed us to explore the correlation between cis sequence elements and susceptibility to trans inactivation and to identify a 301-bp sequence that acts as an enhancer of trans inactivation. Intriguingly, this region encompasses the upstream regions of two divergently transcribed genes and contains a sequence motif that may bind BEAF, a protein involved in delimiting chromatin boundaries.Transcription of the eukaryotic genome is controlled at a number of levels. Not only must sequences near the coding region bind the basal transcription machinery and appropriate transcription factors, but also the surrounding chromatin must be arranged to promote access of these proteins to their binding sites. Recent evidence suggests yet another level of control: the positioning of a locus within the three-dimensional space of the interphase nucleus. In certain circumstances, it has been shown that loci that are being silenced move to a different region of the nucleus, where they associate with masses of highly condensed constitutive heterochromatin (for review, see reference 36). As the consequences of positioning in the nucleus have only recently begun to be explored, it is not surprising that the interplay among these levels of transcriptional regulation is poorly understood. This paper presents results that begin to address the interaction between cis-acting regulatory sequences, chromatin structure, and nuclear positioning.One of the first systems that identified a role for nuclear positioning in gene silencing involves an unusual allele of an eye-color locus in Drosophila melanogaster. The brown Dominant (bw D ) allele contains an insertion of approximately 1.6 Mbp of heterochromatin into the brown (bw) coding sequence near the distal end of the right arm of the second chromosome (27). Eyes of bw D /bw ϩ flies almost totally lack red eye pigment, except in a few small distinct spots. Silencing of the bw ϩ gene on the homologous chromosome, called trans inactivation, has similarities to classic cis-acting position effect variegation (PEV), where a ...
Previous studies demonstrated that Fasciclin II and Beaten path are necessary for regulating cell adhesion events that are important for motoneuron development in Drosophila. We observe that the cell adhesion molecule Fasciclin II and the secreted anti-adhesion molecule Beaten path have additional critical roles in the development of at least one set of sensory organs, the larval visual organs. Taken together, phenotypic analysis, genetic interactions, expression studies and rescue experiments suggest that, in normal development, secretion of Beaten path by cells of the optic lobes allows the Fasciclin II-expressing larval visual organ cells to detach from the optic lobes as a cohesive cell cluster. Our results also demonstrate that mechanisms guiding neuronal development may be shared between motoneurons and sensory organs, and provide evidence that titration of adhesion and anti-adhesion is critical for early steps in development of the larval visual system.
To identify genes necessary for establishing connections in the Drosophila sensory nervous system, we designed a screen for mutations affecting development of the larval visual system. The larval visual system has a simple and stereotypic morphology, can be recognized histologically by a variety of techniques, and is unnecessary for viability. Therefore, it provides an opportunity to identify genes involved in all stages of development of a simple, specific neuronal connection. By direct observation of the larval visual system in mutant embryos, we identified 24 mutations affecting its development; 13 of these are larval visual system-specific. These 13 mutations can be grouped phenotypically into five classes based on their effects on location, path or morphology of the larval visual system nerves and organs. These mutants and phenotypic classifications provide a context for further analysis of neuronal development, pathfinding and target recognition.
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