Boundary Bypass Activity in theAbdominal-BRegion of theDrosophilaBithorax Complex is Position Dependent and Regulated

Abstract

ABSTRACTExpression ofAbdominal-B(Abd-B) in abdominal segments A5–A8 is controlled by four regulatory domains,iab-5 – iab-8. Each domain has an initiator element (which sets the activity state), elements that maintain this state and tissue-specific enhancers. To ensure their functional autonomy, each domain is bracketed by boundary elements (Mcp,Fab-7,Fab-7andFab-8). In addition to blocking crosstalk between adjacent regulatory domains, theFabboundaries must also have bypass activity so the relevant regulatory domains can “jump over” intervening boundaries and activate theAbd-Bpromoter. In the studies reported here we have investigated the parameters governing bypass activity. We find that the bypass elements in theFab-7andFab-8boundaries must be located in the regulatory domain that is responsible for drivingAbd-Bexpression. We suggest that bypass activity may also be subject to regulation.Summary StatementBoundaries separatingAbd-Bregulatory domains block crosstalk between domains and mediate their interactions withAbd-B. The latter function is location but not orientation dependent.