We have shown that the murine c-rel protein can act as a transcriptional transactivator in both yeast and mammalian cells. Fusion proteins generated by linking rel sequences to the DNA-binding domain of the yeast trascriptional activator GAL4 activate transcription from a reporter gene linked in cis to a GAL4 binding site. The full-length mouse c-rel protein (588 amino acids long) is a poor transactivator; however, the C-terminal portion of the protein between amino acid residues 403 to 568 is a potent transcriptional transactivator. Deletion of the N-terminal half of the c-rel protein augments its transactivation function. We propose that c-rel protein has an N-terminal regulatory domain and a C-terminal transactivation domain which together modulate its function as a transcriptional transactivator.The immediate cellular response to external stimulus culminates in the induction of a large set of nuclear genes. Some of these early response genes are proto-oncogenes, and their products have been shown to be involved in the transcription of other genes (1,9,13,17,32,33,41,42). The theme of oncogenes as transcriptional factors or cofactors has been well illustrated by the Fos-Jun paradigm (13). The two nuclear oncoproteins associate to form heterodimers which bind to their cognate DNA motif to promote transactivation (12, 50). More recently, the product of the protooncogene myb has also been shown to act as a transcriptional activator (42, 62). Interestingly, the BAS1 protein, which is required for activation of GCN4-independent (basal) HIS4 transcription in yeast, shares homology to the N-terminal region of myb protein (58). The c-rel gene is a member of the immediate-early response gene family and is induced by serum and phorbol ester (TPA) (9). Its viral homolog, the v-rel protein, has been shown to function as a transcriptional activator in certain cell types (20,25).v-rel is the resident transforming gene of reticuloendotheliosis virus strain T, a highly oncogenic avian retrovirus that induces a rapidly fatal lymphoma in young birds (18,57). Reticuloendotheliosis virus strain transforms only avian lymphoid cells, and despite high levels of expression in chicken embryo fibroblasts, no transformed phenotype is evident (10,23,54). The product of v-rel is a 59-kilodalton phosphoprotein that is located in the cytoplasm of transformed spleen cells and in the nucleus of the nontransformed fibroblasts (10,19,22,54). However, subcellular localization appears to be irrelevant, because both cytoplasmic and nuclear forms of the v-rel protein can induce cellular transformation (23, 25). The v-rel protein has been shown to associate with a number of cellular proteins, including a closely associated serine-threonine protein kinase in both untransformed chicken embryo fibroblasts and transformed spleen cells (15,53,60 whereas in mouse and human cells it is 7.5 and 10 kilobases, respectively (8)(9)(10)26). Although c-rel expression is seen in many cell types, high levels are observed only in lymphoid cells (6,8).The c-rel prot...

The activity of the mouse mammary tumor virus promoter was assessed in various sequence contexts with a transient transfection assay in which promoter activity was determined by way of expression of a linked gene encoding chloramphenicol acetyltransferase, as well as by direct analysis of RNA transcripts. The results indicate that the proviral long terminal repeat contains a negative transcriptional control element in addition to the glucocorticoid-responsive transcriptional enhancer that has been described previously. The negative element is able to function in both orientations and, at least to some extent, at multiple positions with respect to the regulated transcription unit. The effects on gene expression cannot be explained by alterations in transfection efficiency. The element has been localized to a 91 base pair fragment located immediately 5' of binding sites for the glucocorticoid receptor protein that have been defined in vitro. The role of the negative element may be to repress the inherent activity of the proviral promoter in the absence of glucocorticoids, resulting in an increased ratio of gene expression in the presence and absence of hormone.

Sets of genes under a common regulatory control in a given cell type are often differentially transcribed. The possibility that this differential transcription can be modulated by the number or strength of cis-acting regulatory sequences associated with a given gene was tested by using the glucocorticoid-responsive enhancer element associated with the mouse mammary tumor virus promoter. Results indicate that differential levels of hormone-inducible gene expression can be modulated in an additive way by the number of glucocorticoidresponsive enhancers associated with this promoter. Realization of these effects shows little preference for position of the additional elements with respect to the promoter. When sequences that bind the glucocorticoid receptor in vitro with somewhat lower affinity than the enhancer were tested, these additive effects were not detected. The results support the view that differential transcription of genes subject to a common regulatory control can be mediated, at least in part, by the number or strength of their associated cis-acting regulatory sequences.The control of gene expression can, in principle, occur at a number of levels, the most basic of which is transcription initiation. For eucaryotic genes transcribed by RNA polymerase II, the DNA sequences required in cis for this kind of control have been extensively studied in several systems (for a review, see reference 12). One class of these sequences, the enhancer elements (for a review, see reference 22), has been shown to play a role in the determination of cell specificity of viral gene transcription (5, 17) as well as in the tissue-specific expression of endogenous cellular genes (10,63). These elements appear to act as binding sites for trans-acting factors (35,47,51) that are required for the enhancer-directed stimulation of transcription from a linked promoter (57,65). An emerging view of enhancer action is that different enhancers may act independently of one another to confer different controls on a gene by two or more distinct biochemical pathways (66). An extension of this view is that differential regulation of transcription for a set of genes under a common control could be conferred by the strength or number of enhancer elements associated with each gene (13,33,52).The regulated transcription of mouse mammary tumor virus (MMTV) provides a relevant biological system in which to test the proposal that differential levels of transcription can be mediated by multiple functionally related enhancer elements of different strength or number. The glucocorticoid-regulated transcription of the proviral genes of MMTV (Fig. 1A), a retrovirus, is mediated by cis-acting sequences that have been termed the glucocorticoid response element (GRE). These sequences are specifically bound by a partially purified hormone-receptor complex in vitro (9, 16, 40-42, 48, 49) and serve to increase the rate of transcription initiation from the MMTV promoter (60) in a manner reminiscent of enhancer elements (6, 43). Functional GRE sequences have be...

A sequence element within pBR322 DNA mediates a cis-acting negative effect on expression from eucaryotic genes in transient expression assays. The negative element overlaps with sequences that inhibit DNA replication, but its effect is observed in the absence of detectable replication of transfected DNA.

Sets of genes under a common regulatory control in a given cell type are often differentially transcribed. The possibility that this differential transcription can be modulated by the number or strength of cis-acting regulatory sequences associated with a given gene was tested by using the glucocorticoid-responsive enhancer element associated with the mouse mammary tumor virus promoter. Results indicate that differential levels of hormone-inducible gene expression can be modulated in an additive way by the number of glucocorticoid-responsive enhancers associated with this promoter. Realization of these effects shows little preference for position of the additional elements with respect to the promoter. When sequences that bind the glucocorticoid receptor in vitro with somewhat lower affinity than the enhancer were tested, these additive effects were not detected. The results support that differential transcription of genes subject to a common regulatory control can be mediated, at least in part, by the number or strength of their associated cis-acting regulatory sequences.