Combinatorial determinants of tissue-specific transcription in B cells and macrophages

A tripartite domain of the immunoglobulin μ heavy-chain gene enhancer that activates transcription in B cells contains binding sites for PU.1, Ets- 1, and a leucine zipper-containing basic helix-loop-helix factor. Because PU.1 is expressed only in B cells and macrophages, we tested the activity of a minimal μ enhancer fragment in macrophages by transient transfections. The minimal μ enhancer activated transcription in macrophages, and the activity was dependent on all three sites. Analysis of mutated enhancers, in which spacing and orientation of the ETS protein binding sites had been changed, suggested that the mechanisms of enhancer activation were different in B cells and macrophages. Thus, ETS protein binding sites may be combined in different ways to generate tissue-specific transcription activators. Despite the activity of the minimal enhancer in macrophages, a larger μ enhancer fragment was inactive in these cells. We propose that formation of the nucleoprotein complex that is formed on the minimal enhancer in macrophages cannot be helped by the neighboring μE elements that are essential for activity of the monomeric enhancer.