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 ...

One way animals may cope with nutrient deprivation is to broadly repress translation by inhibiting 5′-cap initiation. However, under these conditions specific proteins remain essential to survival during fasting. Such peptides may be translated through initiation at 5′UTR Internal Ribosome Entry Sites (IRES). Here we show that the Drosophila melanogaster Forkhead box type O (dFoxO) transcription factor is required for adult survival during fasting, and that the 5′UTR of dfoxO has the ability to initiate IRES-mediated translation in cell culture. Previous work has shown that insulin negatively regulates dFoxO through AKT-mediated phosphorylation while dFoxO itself induces transcription of the insulin receptor dInR, which also harbors IRES. Here we report that IRES-mediated translation of both dFoxO and dInR is activated in fasted Drosophila S2 cells at a time when cap-dependent translation is reduced. IRES mediated translation of dFoxO and dInR may be essential to ensure function and sensitivity of the insulin signaling pathway during fasting.

The brown Dominant (bw D ) allele of Drosophila contains a heterochromatic block that causes the locus to interact with centric heterochromatin. This association silences bw 1 in heterozygotes (trans-inactivation) and is dependent on nuclear organizational changes later in development, suggesting that transinactivation may not be possible until later in development. To study this, a P element containing an upstream activating sequence (UAS)-GFP reporter was inserted 5 kb from the bw D insertion site. Seven different GAL4 driver lines were used and GFP fluorescence was compared in the presence or the absence of bw D . We measured silencing in different tissues and stages of development and found variable silencing of GFP expression driven by the same driver. When UAS-GFP was not expressed until differentiation in the eye imaginal disc it was more easily trans-inactivated than when it was expressed earlier in undifferentiated cells. In contrast to some studies by other workers on silencing in cis, we did not find consistent correlation of silencing with level of expression or evidence of relaxation of silencing with terminal differentiation. We suggest that such contrasting results may be attributed to a potentially different role played by nuclear organization in cis and trans position-effect variegation. P OSITION-EFFECT variegation (PEV) is the silencing of gene expression by nearby heterochromatin. The phenomenon was first described in Drosophila and has long served as a model for the downregulation of expression by chromatin compaction and changes in higher-order chromatin organization. Classic cis-acting PEV results from a chromosomal break and rejoining that juxtaposes a euchromatic gene and a block of constitutive heterochromatin. In contrast to earlier ideas concerning the continuous, straightforward linear spreading of the silent state, a collection of recent studies have found that the phenomenon is more complex. Many factors influence the susceptibility or resistance of a gene to silencing by heterochromatin in cis, such as the size, composition, and distance of the heterochromatic block and, most importantly, the specifics of the expression properties of the gene being silenced (for review see Talbert and Henikoff 2006).While these studies addressed cis-acting PEV, there is a similar, related phenomenon in flies of silencing by heterochromatin in trans. In trans-acting PEV, or ''transinactivation,'' the block of heterochromatin and the silenced gene are not necessarily on the same DNA molecule, but are brought close to each other within the space of the interphase nucleus. It is probable that this type of PEV is similar to downregulation of a number of normal genes in higher eukaryotes, as increasing evidence finds developmentally silenced loci that move into association with large blocks of heterochromatin (for example, see Su et al. 2004). A well-studied example of trans-inactivation is the brown Dominant (bw D

Chromosomes of higher eukaryotes contain blocks of heterochromatin that can associate with each other in the interphase nucleus. A well-studied example of heterochromatic interaction is the brownDominant (bwD) chromosome of D. melanogaster, which contains an ∼1.6-Mbp insertion of AAGAG repeats near the distal tip of chromosome 2. This insertion causes association of the tip with the centric heterochromatin of chromosome 2 (2h), which contains megabases of AAGAG repeats. Here we describe an example, other than bwD, in which distally translocated heterochromatin associates with centric heterochromatin. Additionally, we show that when a translocation places bwD on a different chromosome, bwD tends to associate with the centric heterochromatin of this chromosome, even when the chromosome contains a small fraction of the sequence homology present elsewhere. To further test the importance of sequence homology in these interactions, we used interspecific mating to introgress the bwD allele from D. melanogaster into D. simulans, which lacks the AAGAG on the autosomes. We find that D. simulans bwD associates with 2h, which lacks the AAGAG sequence, while it does not associate with the AAGAG containing X chromosome heterochromatin. Our results show that intranuclear association of separate heterochromatic blocks does not require that they contain the same sequence.