Strict Distance Requirement for Transcriptional Activation by Two Regulatory Elements of the Glucagon Gene

Abstract

The glucagon gene is specifically expressed in A cells of the pancreatic islets. We have previously identified three functional DNA control elements within the 5′-flanking sequence of the glucagon gene: an upstream promoter element (G₁), responsible for the A-cell-specific expression, and two enhancer-like elements (G₂ and G₃). Mutations within G₁ completely abolished the enhancer activities of G₂ and G₃. We show here that the loss of transcriptional activity is not due to an absence of binding of nuclear proteins to G₂ or G₃, but rather to an essential role of G₁ to enable G₂ and G₃ to exert their effects at a distance. When juxtaposed to the TATA box, both enhancers are capable of inducing transcription as efficiently as when an intact G₁ is present. However, the tandem arrangement of G₂ plus G₃ does not result in additional transcriptional activity compared with a single element. We conclude that G₂ and G₃ are capable of directly activating transcription but in a highly distance-dependent fashion and without cooperatively interacting. G₁ may thus serve only as a tissue-specific relay, bringing together factors that bind to enhancers and the TATA box.