Most cell types start expressing vimentin when brought into tissue culture. Using both vimentinexpressing (HeLa) and vimcntin-negative (MCF-7) epithelial cell lines, we have identified the cisregulatory DNA elements involved in this process. Sequenccs located 1 . I -0.6 kb upstream of the vimentin transcription-initiation site strongly enhance expression in HeLa cells, but are silenced in MCF-7 cells. Other regulatory elements in the vimentin promoter (an enhancer 3.2. -2.6 kb upstream and a minimal promoter region including the CAAT-box) are potentially active in both cell types, but are silenced by the 0.5-kb fragment in MCF-7 cells. Deletion of this fragment restores transcriptional activity of a transfected vimentin promoter. Our data indicate that a double AP lljun-binding site present in the 0.5-kb fragment mediates the induction of vimentin expression in cultured epithelial cells, while silencing sequences located within the same fragment are responsible for the absence of vimentin expression in MCF-7 cells.In contrast to MCF-7 cells, a transfected vimentin promoter and gene arc transcriptionally active in the vimentin-negative epithelial cell line T24. Transfection studies show that type-111-intermediatefilament expression is not impaired at any level in these cells.Upon traasfection and expression of a desmin construct in T24 cells not only desmin, but also vimentin was detected. Both proteins assembled into intermediate filaments. This induction of vimentin expression appeared to be regulated at the post-transcriptional level.Intermediate filaments (IF) represent a group of cytoskelctal structures of approximately 10 nm in diameter which, along with microtubules and microfilamcnts, occur in the cytoplasm of virtually all mammalian cells. The IF protcins are encoded by a large multigene family and can be divided into six different types on the basis of gene structure and homology [l-41. Apart from the lamins, the different types of IF subunits are expressed in a developmentally regulated and tissuespecific fashion. This highly conserved specificity of expression suggests that each type or combination of subunits plays an important role in ccllular differentiation [l -61.The vimentin-expression pattern is more complex than that of the other IF subunits. In adult tissues, vimentin is mainly expressed in cells of mcsenchymal origin. Moreover, during differentiation of a variety of cell types vimentin expression is both positively and negatively regulated [l -3, 61. Depending on cell type, vimentin expression can precede the expression of the IF subunit specific for a particular tissue, it can be transiently coexpressed during development, or it can