The enhancer in an MHC class II gene, in vitro and in mouso.

The E alpha class II gene of the major histocompatibility complex is expressed in a variety of immunocompetent cells. Part of the control of tissue-specific expression is mediated by a block of sequences found far upstream of the transcriptional startsite; this stretch is necessary for expression in the B lymphocytes of transgenic mice, but largely dispensable elsewhere. We review the evidence for the role of this region in E alpha transcription in transgenic animals, as well as data from transfections into tissue-culture cells, which indicate that this region has non-specific enhancer activity. We discuss possible models to explain how a non-specific enhancer can participate in cell-specific control.