A new homeotic mutation in the Drosophila bithorax complex removes a boundary separating two domains of regulation

EMBO J. 1990 Aug;9(8):2579-85.


The bithorax complex specifies the identity of parasegments 5-14 of Drosophila. Although nine parasegment-specific functions, abx/bx, bxd/pbx and iab-2 to iab-8,9 have been identified, the whole bithorax complex appears to encode only three classes of proteins, Ubx, abd-A and Abd-B. Many observations suggest that the parasegment-specific functions act as positive cis-regulatory elements of Ubx, abd-A and Abd-B. We report the molecular genetics of a new gain-of-function mutation, Fab-7, which transforms parasegment 11 into parasegment 12. Induction of Abd-B mutations in cis (one of which removes the Abd-B homeobox) causes reversion of the dominant phenotype, demonstrating that Fab-7 misregulates Abd-B. A 4 kb deletion, 30 kb downstream from the Abd-B transcription unit, is solely responsible for the Fab-7 phenotype. We consider that the parasegment-specific functions lie in DNA domains that are sequentially and independently 'opened' along the chromosome. Once a domain is opened, the cis-regulatory sequences within it can carry out their function. We propose that the Fab-7 deletion removes a boundary separating the iab-6 and iab-7 cis-regulatory regions (the functions specific for parasegments 11 and 12) allowing the open configuration of iab-6 to invade iab-7 in parasegment 11. This is strongly supported by our finding that Fab-7 can be caused to revert by lesions not only in iab-7 but also in iab-6.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Blotting, Southern
  • Chromosome Mapping
  • Crosses, Genetic
  • DNA / analysis
  • DNA / genetics
  • Drosophila / anatomy & histology
  • Drosophila / genetics*
  • Drosophila / radiation effects
  • Female
  • Gene Expression Regulation*
  • Genes, Homeobox*
  • Genotype
  • Larva
  • Male
  • Mutation


  • DNA