Boundary Bypass Activity in the Abdominal-B Region of the Drosophila Bithorax Complex is Position Dependent and Regulated

Expression of Abdominal-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-7 and Fab-8). In addition to blocking crosstalk between adjacent regulatory domains, the Fab boundaries must also have bypass activity so the relevant regulatory domains can "jump over" intervening boundaries and activate the Abd-B promoter. In the studies reported here we have investigated the parameters governing bypass activity. We find that the bypass elements in the Fab-7 and Fab-8 boundaries must be located in the regulatory domain that is responsible for driving Abd-B expression. We suggest that bypass activity may also be subject to regulation. Summary StatementBoundaries separating Abd-B regulatory domains block crosstalk between domains and mediate their interactions with Abd-B. The latter function is location but not orientation dependent.