The bluetail transposon: evidence for independent cis-regulatory domains and domain boundaries in the bithorax complex.

Abstract

An extremely large cis‐regulatory region generates the parasegment‐specific expression patterns of the homeotic genes in the bithorax complex. We present evidence supporting the idea that this cis‐regulatory region is subdivided into independent cis‐regulatory domains. We describe a Ubx‐lacZ transposon which is inserted into one of these domains, iab‐7. The PS12‐specific pattern of LacZ expression from this reporter indicates that it is subject to the control of the iab‐7 cis‐regulatory domain, but is protected from the effects of adjacent regulatory domains. Protection on the proximal side appears to be provided by the Fab‐7 boundary element. Deletion of this boundary results in the ectopic activation of iab‐7 in PS11 (where the iab‐6 cis‐regulatory domain normally functions). We show that the Fab‐7 boundary, like other boundaries, has an unusual chromatin structure.

Transcription defines the embryonic domains of cis-regulatory activity at the Drosophila bithorax complex

“…If the initial activation of the cis-regulatory elements in the iab regions is dependent on the intergenic transcripts, then it would be necessary not only to have unique transcription patterns within each region but also to ensure that transcription does not spread from one region to the next. Our study provides evidence that the insulators that organize the different iab regions into discrete chromatin domains (8,14,15) act to regulate the intergenic transcription program in this way.…”

“…Genetic, transgenic, and molecular studies have identified a number of classes of distinct cis-regulatory elements within the iab regions capable of directing abdA and AbdB expression. Insulator DNAs have been identified at the boundaries of the iab8-iab7 domains (Frontabdominal-8, Fab8) (4), iab7-iab6 domains (Fab7) (5)(6)(7)(8)(9)(10), and iab5-iab4 domains (Miscadastral pigmentation, Mcp) ( Fig. 1 A) (7,11).…”

“…Although not all insulators are thought to act in the same way (12,13), it has been suggested that the BX-C insulators organize the different iab regions into discrete chromatin domains ( Fig. 1 A) (8,14,15). In this way, cis-regulatory elements within the separate domains are able to direct expression of abdA and AbdB in specific abdominal segments of the Drosophila embryo.…”

Enhancer trap infidelity inDrosophila optomotor-blind

“…Only insertions close to the promoter of a BXC gene report the entire expression pattern of this gene. [29][30][31][32] Regional reporting may not be restricted to the exceptional HOX gene clusters. In decapentaplegic (dpp), a hobo enhancer trap insertion into the "disc region" rendered predominantly the activity of this regulatory region.…”

Hox cluster duplications and the opportunity for evolutionary novelties

“…But, with our current knowledge, it is not possible to say exactly why. For Drosophila Hox clusters the presence of boundary elements that make the function of cis-regulatory elements more modular could be a factor contributing to their structural flexibility (43,44). The nature of the differences, however, is obvious, with vertebrate Hox clusters being more constrained than invertebrate Hox clusters.…”