Homeotic genes determine the developmental fates of cells. Restriction of their expression along the body axis is of prime importance for normal development. We searched for cis‐regulatory sequences within Abdominal‐B (Abd‐B), a homeotic Drosophila gene, by testing genomic Abd‐B fragments for their ability to confer beta‐galactosidase (beta‐gal) expression in transformed embryos. One of the Abd‐B fragments, called IAB5, mediates a beta‐gal pattern restricted along the body axis to the Abd‐B expression domain. Alterations of the IAB5 pattern in gap mutants provide evidence that the protein products of the gap genes hunchback, Krüppel and knirps act as repressors through IAB5. The anterior Abd‐B expression limit is apparently determined by Krüppel repression, whereas the knirps repressor may be responsible for the graded Abd‐B expression within the Abd‐B domain. IAB5 and two other fragments called MCP and FAB show region‐specific silencing activity: they suppress at a distance beta‐gal expression mediated by a linked heterologous enhancer. Silencing requires hunchback as well as Polycomb function and evidently provides maintenance of Abd‐B expression limits throughout embryogenesis. We conclude that transcriptional repression is a key mechanism operating at multiple levels to control Abd‐B expression. The striking similarities between the control of Abd‐B and of Ultrabithorax, another homeotic Drosophila gene, may point to a universal principle underlying homeotic gene regulation.

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The Polycomb and trithorax groups of genes control the maintenance of homeotic gene expression in a variety of organisms. A putative participant in the regulation of this process is the murine RYBP (Ring and YY1 Binding Protein) gene. Sequence comparison between different species has identified the homologous gene in Drosophila, the dRYBP gene. We have investigated whether dRYBP participates in the mechanisms of silencing of homeotic genes expression. We first studied its expression by RNA in situ hybridisation and detected dRYBP expression ubiquitously and throughout development. Moreover, we generated a polyclonal anti-dRYBP antibody that recognises the dRYBP protein. dRYBP protein is nuclear and expressed maternally and ubiquitously throughout development. To study the transcriptional activity of dRYBP, we generated a fusion protein containing the entire dRYBP protein and the GAL4 DNA binding domain. This fusion protein functions, in vivo, as a transcriptional repressor throughout development. Importantly, this repression is dependent on the function of the Polycomb group genes. Furthermore, using the GAL4/UAS system, we have over expressed dRYBP in the haltere and the wing imaginal discs. In the haltere discs, high levels of dRYBP repress the expression of the homeotic Ultrabithorax gene. This repression is Polycomb dependent. In the wing discs, dRYBP over expression produces a variety of phenotypes suggesting the overall miss regulation of the many putative genes affected by high levels of dRYBP. Taking together, our results indicate that dRYBP is able to interact with PcG proteins to repress transcription suggesting that the dRYBP gene might belong to the Polycomb group of genes in Drosophila.