Fab-7 functions as a chromatin domain boundary to ensure proper segment specification by the Drosophila bithorax complex.
Fab-7 deletions in the bithorax complex have a novel gain-of-function phenotype, typically transforming parasegment 11 (PS11) into PS12 identity. Genetic analysis indicates that removal of the Fab-7 element results in the fusion of the iab-6 (PS11) and iab-7 (PS12) cis-regulatory domains into a single regulatory domain that inappropriately regulates Abdominal-B in PS11. This has led to the hypothesis that Fab-7 is a chromatin domain boundary that normally functions to ensure the autonomous activity of the iab-6 and iab-7 cis-regulatory domains. We use several different enhancer blocking assays to demonstrate that Fab-7 has the insulating properties expected of a domain boundary. We define a minimal fragment of Fab-7 sufficient for enhancer blocking, and demonstrate that it is completely distinct from an adjacent Polycomb-dependent silencer. We compare Fab-7 to the su(Hw) insulator element, and show that Fab-7 enhancer blocking activity is intermediate between that of five and twelve reiterated binding sites for the Su(Hw) protein. These results support the model that Fab-7 functions as a domain boundary within the context of the bithorax complex, making Fab-7 one of the first boundary elements that is known to have an essential function in vivo.
Hagstrom
K
K
Department of Molecular Biology, Princeton University, New Jersey 08544, USA.
Muller
M
M
Schedl
P
P
D000818:Animals; D002843:Chromatin / Q000648:ultrastructure; D004251:DNA Transposable Elements; D004268:DNA-Binding Proteins / Q000235:genetics; D029721:Drosophila Proteins; D004331:Drosophila melanogaster / Q000196:embryology* / Q000235:genetics; D004742:Enhancer Elements, Genetic; D018507:Gene Expression Regulation, Developmental; D018840:HSP70 Heat-Shock Proteins / Q000235:genetics; D009687:Nuclear Proteins / Q000235:genetics; D011401:Promoter Regions, Genetic; D012045:Regulatory Sequences, Nucleic Acid; D012097:Repressor Proteins
D002843:Chromatin; D004251:DNA Transposable Elements; D004268:DNA-Binding Proteins; D029721:Drosophila Proteins; D018840:HSP70 Heat-Shock Proteins; D009687:Nuclear Proteins; D012097:Repressor Proteins; C083738:su(Hw) protein, Drosophila