Interest in the Ets proteins has grown enormously over the last decade. The v‐ets oncogene was originally discovered as part of a fusion protein expressed by a transforming retrovirus (avian E26), and later shown to be transduced from a ceilular gene. About 30 related proteins have now been found in species ranging from flies to humans, that resemble the vEts protein in the so‐called ‘ets domain’. The ets domain has been shown to be a DNA‐binding domain, that specifically interacts with sequences containing the common core trinucleotide GGA. Furthermore, it is involved in protein–protein interactions with co‐factors that help determine its biological activity. Many of the Ets‐related proteins have been shown to be transcription activators, like other nuclear oncoproteins and anti‐oncoproteins (Jun, Fos, Myb, Myc, Rel, p53, etc.). However, Ets‐like proteins may have other functions, such as in DNA replication and a general role in transcription activation. Ets proteins have been implicated in regulation of gene expression during a variety of biological processes, including growth control, transformation, T‐cell activation, and developmental programs in many organisms. Signals regulating cell growth are transmitted from outside the cell to the nucleus by growth factors and their receptors, G‐proteins, kinases and transcription factors. We will discuss how several Ets‐related proteins fit into this scheme, and how their activity is regulated both post‐and pre‐translationally. Loss of normal control is often associated with conversion to an onco‐protein. vEts has been shown to have different properties from its progenitor, which might explain how it has become oncogenic. Oncogene‐related products have been implicated in the control of various developmental processes. Evidence is accumulating for a role for Ets family members in Drosophila development, Xenopus oocyte maturation, lymphocyte differentiation, and viral infectious cycles. An ultimate hope in studying transformation by oncoproteins is to understand how cells become cancerous in humans, which would lead to more effective treatments. vEts induces erythroblastosis in chicken. Cellular Ets‐family proteins can be activated by proviral insertion in mice and, most interestingly, by chromosome translocation in humans. We are at the beginning of understanding the multiple facets of regulation of Ets activity. Future work on the Ets family promises to provide important insights into both normal control of growth and differentiation, and deregulation in illness.

Ras signaling appears to be mediated in part by transcription factors that belong to the ets gene family. To identify downstream targets for the Ras signal transduction pathway, we have used Ras-transformed mouse fibroblasts to isolate a new member of the ets gene family, net. Net has sequence similarity in three regions with the ets factors Elkl and SAP1, which have been implicated in the serum response of the fos promoter. Net shares various properties with these proteins, including the ability to bind to ets DNA motifs through the Ets domain of the protein and form ternary complexes with the serum response factor SRF on the fos serum response element, SRE. However, Net differs from Elkl and SAP1 in a number of ways. The pattern of net RNA expression in adult mouse tissues is different. Net has negative effects on transcription in a number of assays, unlike Elkl. Strikingly, Ras, Src, and Mos expression switch Net activity to positive. The study of Net should help in understanding the interplay between Net and other members of the Elk subfamily and their contribution to signal transduction through Ras to the nucleus.

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We analyzed the 5' transcription control sequences of the human CD4 gene. We located the transcription initiation site and showed that the CD4 core promoter (positions -40 to +16) lacks a classical "TATA" or initiator positioning consensus sequence but directs precise and efficient transcription when coupled to the ubiquitously active simian virus 40 enhancer. The transcriptional activity of the CD4 gene promoter correlated with CD4 expression in various cell types. Interestingly, the CD4 core promoter also displayed a tissuespecific transcriptional activity. Within this fragment, three nucleic acid sequences are completely conserved in the murine CD4 gene. One of these sequences contains a perfect ETS consensus sequence. Another ETS consensus sequence is located 1060 nt upstream. Electrophoretic-mobility-shift assays showed that the core promoter ETS motif binds an Ets-related protein specifically expressed at high levels in CD4+ cells. Moreover, in CD4-cells, overexpression of Ets-1 or Ets-2 efficiently and specifically activated transcription from the CD4 promoter and core promoter. These data indicate that Ets transcription factors play a central role in controlling CD4 gene expression, by binding to both a classical remote site and an unusual proximal activator sequence.The expression of the T-cell antigen receptor (TCR), and the CD4 and CD8 coreceptor molecules, is tightly regulated during T-lymphocyte differentiation in the thymus. Early thymocytes express a low level of CD4 and no CD8 and have not yet rearranged their genes coding for the TCR (for review, see ref. 1). Later, they express high levels of both CD4 and CD8, as well as a rearranged TCR. At this stage, most of the thymocytes undergo apoptosis, presumably because of the expression of an inappropriate set of receptor/coreceptor molecules. The surviving thymocytes continue differentiation, losing the expression of either CD4 or CD8, depending on whether their TCR is specific for antigen presented in the context of major histocompatibility complex class I or II molecules, respectively. Experiments with transgenic mice (2, 3), suggest that the switch from CD4+/CD8+-doublepositive thymocytes to single-positive lymphocytes is likely to be an instructive rather than a stochastic process. Thus, the CD4 and CD8 molecules should control their mutual expression during the thymocyte maturation process. Since the expression of CD4 and CD8 molecules is regulated at the transcriptional level (4, 5), the analysis of the CD4 and CD8 genes transcription control elements is essential to understand the complex regulation of their expression.We mapped the human CD4 gene transcription start site and showed that the surrounding core promoter (positions -40 to + 16) is sufficient for precise initiation oftranscription,

The Ras signalling pathway targets transcription factors such as the ternary complex factors that are recruited by the serum response factor to form complexes on the serum response element (SRE) of the fos promoter. We have identified a new ternary complex factor, Net-b. We report the features of the net gene and show that it produces several splice variants, net-b and net-c. net-b RNA and protein are expressed in a variety of tissues and cell lines. net-c RNA is expressed at low levels, and the protein was not detected, raising the possibility that it is a cryptic splice variant. We have studied the composition of ternary complexes that form on the SRE of the fos promoter with extracts from fibroblasts (NIH 3T3) cultured under various conditions and pre-B cells (70Z/3) before and after differentiation with lipopolysaccharide (LPS). The fibroblast complexes contain mainly Net-b followed by Sap1 and Elk1. Net-b complexes, as well as Sap1 and Elk1, are induced by epidermal growth factor (EGF) stimulation of cells cultured in low serum. Pre-B-cell complexes contain mainly Sap1, with less of Net-b and little of Elk1. There is little change upon LPS-induced differentiation compared to the increase with EGF in fibroblasts. We have also found that Net-b is a nuclear protein that constitutively represses transcription. Net-b is not activated by Ras signalling, in contrast to Net, Sap1a, and Elk1. We have previously reported that down-regulation of Net proteins with antisense RNA increases SRE activity. The increase in SRE activity is observed at low serum levels and is even greater after serum stimulation, showing that the SRE is under negative regulation by Net proteins and the level of repression increases during induction. Net-b, the predominant factor in ternary complexes in fibroblasts, may both keep the activity of the SRE low in the absence of strong inducing conditions and rapidly shut the activity off after stimulation.

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