Mechanisms of μ enhancer regulation in B lymphocytes

Previous work has demonstrated that a B-cell-specific multiprotein complex that included members of the ets family of transcription factors activates the μ enhancer. Here we provide two examples of combinatorial mechanisms that operate on the μ enhancer. Interactions between proximal elements are exemplified by the need for ETS proteins to mediate transcriptional synergy between bHLH proteins; cooperation between distal elements is exemplified by the ability of IRF proteins to substitute for μE2/μE3 elements from a location more than 100 bp away. Finally, we show that the ETS protein PU.1 increases accessibility of chromatin-assembled μ enhancer DNA in vitro and in vivo. Because increased chromatin accessibility is thought to be necessary but not sufficient for transcriptional activation, our studies elucidate one step in the multistage pathway leading to tissue- specific μ gene expression. Whether the PU.1-containing targe-some complex activates recombination and transcription in conjunction with bHLH and IRF proteins is currently being investigated. The relationships between chromatin accessibility, recombination, and transcription are currently based on correlative studies. A true understanding of the similarities and differences of these three processes is key to understanding gene expression in vivo and remains our long-term goal. Obvious questions stemming from this work include: (1) Can transcription factors that alter chromatin accessibility to restriction endonucleases activate transcription through altering accessibility to the multimeric RNA polymerase machinery? (2) How are chromatin changes during transcriptional regulation related to chromatin disruption and re-formation during DNA replication? (3) Do enhancers regulate recombination only by altering chromatin structure or do enhancer-binding proteins participate in the recombination process? (4) What are the biochemical parameters that define recombinatorial accessibility versus transcriptional activation? (5) Does experimentally perceived functional redundancy exist in the context of chromatin? We believe that study of ETS proteins in chromatin regulation both in vitro and in vivo will be valuable for understanding and altering tissue-specific gene expression patterns in the future.