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Our knowledge of the mechanisms that regulate transcription in higher eukaryotic cells has increased enormously during the past 2 years. Earlier studies, using a combination of in vitro mutagenesis and DNA-mediated gene transfer, identified two distinct types of cis-acting regulatory sequences: promoters, which are located close to the initiation site and act in a position-dependent fashion, and enhancers, which can be located far from the initiation site and act in a position- and orientation-independent fashion. Promoters can be subdivided into proximal elements, including the cap site itself and the TATA box, which is involved in fixing the site of initiation, and distal elements, which can be spread over several hundred base pairs. It is now clear that many promoters, particularly those of 'housekeeping' genes, lack TATA boxes and are instead composed of GC-rich elements that are often located within methylation-free islands (Bird 1986). Transcription controlled by this latter class of promoters often initiates at multiple sites. Many enhancer elements, for example, those of the immunoglobulin, insulin, and elastase genes, impose tissue-specific expression on adjacent promoters. These cis-acting elements operate by interacting with protein factors, many of which have now been identified by gel retardation and footprinting assays and some of which have been purified to homogeneity. In a few cases the corresponding genes have been cloned. In many of these studies well-characterized cis-acting elements of viruses, particularly of the DNA tumor viruses, have played a major role. At the recent ICRF-sponsored

Trans-acting protein factors and the regulation of eukaryotic transcription: lessons from studies on DNA tumor viruses.