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

Abstract

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.

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

“…homeotic genes from the bithorax complex (BX-C) in Drosophila is regulated by cis-regulatory elements in a 100-kb intergenic region (1)(2)(3). This region harbors an organized series of infraabdominal (iab) regions (Fig.…”

“…3I). This transformation of segmental identity is due to ectopic activation of AbdB in the fourth abdominal segment, anterior to its normal expression domain (1,3). We reasoned that deletion of the Mcp sequence may disrupt expression of the sense transcripts in the iab4 region.…”

Silencers in abdominal-B, a homeotic Drosophila gene.

“…Indeed, we were able to isolate ry+ transformants at the normal frequency, if we inserted into the same construct between the IAB5 fragment and the ry+ gene an 'insulation' sequence known to interfere with enhancer -promoter interactions (suppressor-of-Hairy-wing protein binding sites; Dorsett, 1990;Holdridge and Dorsett, 1991 (Lewis, 1978;Karch et al, 1985;Gyurkovics et al, 1990;see Figure 1). These are gain-of-function mutations: Mcp causes ectopic expression ofAbd-B protein in ps9 in a pattern and at a level similar to that normally found in pslO (Celniker et al, 1990;Sanchez-Herrero, 1991), whereas the phenotype of Fab implies that Abd-B protein is expressed more strongly in psl1, at the level normally found in ps12 (Gyurkovics et al, 1990 Figure 1) did not show any enhancer activity in transformed embryos (see above), we wondered whether they may have silencing activity. We linked either of these fragments to the BXD enhancer and the Ubx proximal promoter and tested the combination constructs (called MB and FB) for (-gal expression in transformed embryos.…”

“…Genetic analysis suggested that ps-specific patterns of Abd-B expression within ps0-12 are controlled by multiple cisregulatory elements residing in the large 3' flanking region of the Abd-B gene (Karch et al, 1985;Gyurkovics et al, 1990). Indeed, breakpoints and deletions within this region eliminate Abd-B m expression in individual parasegments from psIO to ps12 (Celniker et al, 1990;Sanchez-Herrero, 1991; these mutations have no effect on Abd-B expression in ps13 and ps14 which therefore must be controlled by upstream Abd-B sequences).…”

Complete sequence of the bithorax complex of Drosophila.

“…Genetic studies have identified cis-regulatory regions of the BX-C that are involved in controlling the development of specific organs and body structures; 12 such regions are now known (14)(15)(16). Thus, the anterobithorax (abx), bithorax (bx), and postbithorax (pbx) regions function in the wild type to promote development of the third thoracic segment into a haltere-bearing instead of a wing-bearing segment.…”