The mitotically active basal layers of most stratified squamous epithelia express 10 to 30% of their total protein as keratin. The two keratins specifically expressed in these cells are the type II keratin K5 (58 kilodaltons) and its corresponding partner, type I keratin K14 (50 kilodaltons), both of which are essential for the formation of 8-nm filaments. Dissecting the molecular mechanisms underlying the coordinate regulation of the two keratins is an important first step in understanding epidermal differentiation and in designing promoters that will enable delivery and expression of foreign gene products in stratified squamous epithelia, e.g., skin. Previously, we reported the sequence of the gene encoding human K14 (D. Marchuk, S. McCrohon, and E. Fuchs, Cell 39:491-498, 1984; Marchuk et al., Proc. Natl. Acad. Sci. USA 82:1609-1613, 1985. We have now isolated and characterized the gene encoding human K5. The sequence of the coding portion of this gene matched perfectly with that of a partial K5 cDNA sequence obtained from a cultured human epidermal library (R. Lersch and E. Fuchs, Mol. Cell. Biol. 8:486-493, 1988), and gene transfection studies indicated that the gene is functional. Nuclear runoff experiments demonstrated that the K5 and K14 genes were both transcribed at dramatically higher levels in cultured human epidermal cells than in fibroblasts, indicating that at least a part of the regulation of the expression of this keratin pair is at the transcriptional level. When the K5 gene was transfected transiently into NIH 3T3 fibroblasts, foreign expression of the gene caused the appearance of endogenous mouse K14 and the subsequent formation of a keratin filament array in the cells. In this case, transcriptional changes did not appear to be involved in the regulation, suggesting that there may be multiple control mechanisms underlying the pairwise expression of keratins.Only epithelial cells express keratins, a large family of proteins (molecular size, 40 to 67 kilodaltons) (kDa) that assemble into a cytoskeletal network of 8-nm intermediate filaments (IFs). There are two distinct classes of keratins: type I keratins (K9 to K19) are small (40 to 56.5 kDa) and relatively acidic (pKi, 4.5 to 5.5), whereas type II keratins (Kl to K8) are larger (53 to 67 kDa) and more basic (pKi, 5.5 to 7.5) (for reviews, see references 8, 16, 50, and 64). Both types of keratins are essential for filament assembly (12,28,41,63). Type I and type II keratins are frequently expressed as specific pairs, and different pairs are expressed in a tissue-specific, differentiation-specific, and developmentalspecific fashion (11, 66).The molecular mechanisms underlying the regulation of pairs of type I and type II keratins remain largely unknown. It is clear that self-propagating epithelial cells expressing a specific pair of keratins must necessarily cotranscribe these genes. However, coordinate expression does not restrict these genes to a common regulatory mechanism, nor does it rule out the possibility that the two genes may b...