A 300 BP 5'-UPSTREAM SEQUENCE OF A DIFFERENTIATION-DEPENDENT RABBIT K3 KERATIN GENE CAN SERVE AS A KERATINOCYTE-SPECIFIC PROMOTER

Keratinocytes of the suprabasal compartment of many stratified epithel ia synthesize as a major differentiation product a keratin pair, consi sting of an acidic and a basic keratin, which accounts for 10-20% of t he newly synthesized proteins. While genes of several differentiation- related keratins have been cloned and studied, relatively little is kn own about the molecular basis underlying their tissue-specific and dif ferentiation-dependent expression. We have chosen to study, as a proto type of these genes, the gene of K3 keratin, which has the unique prop erty of being expressed in the majority of corneal epithelial basal ce lls but suprabasally in peripheral cornea, the site of corneal epithel ial stem cells. Using a monoclonal antibody, AE5, specific for K3 kera tin, and a fragment of human K3 gene as probes, we have isolated sever al cDNA and genomic clones of rabbit K3 keratin. One genomic clone has been sequenced and characterized, and the identity of its coding sequ ence with that of cDNAs indicates that it corresponds to the single, f unctional rabbit K3 gene. Transfection assays showed that its 3.6 kb 5 '-upstream sequence can drive a chloramphenicol acetyl transferase (CA T) reporter gene to express in cultured corneal and esophageal epithel ial cells, but not in mesothelial and kidney epithelial cells or fibro blasts, all of rabbit origin. Serial deletion experiments narrowed thi s keratinocyte-specific promoter to within -300 bp upstream of the tra nscription initiation site. Its activity is not regulated by the codin g or 3'-noncoding sequences that have been tested so far. This 300 bp 5'-upstream sequence of K3 keratin gene, which can function in vitro a s a keratinocyte-specific promoter, contains two clusters of partially overlapping motifs, one with an NFkB consensus sequence and another w ith a GC box. The combinatorial effects of these multiple motifs and t heir cognate binding proteins may play an important role in regulating the expression of this tissue-restricted and differentiation-dependen t keratin gene.