SEQUENCES 5' OF THE BOVINE KERATIN-5 GENE DIRECT TISSUE-TYPE AND CELL-TYPE SPECIFIC EXPRESSION OF A LACZ GENE IN THE ADULT AND DURING DEVELOPMENT

Expression of keratin K5 (and K14) in multilayered epithelia occurs pr edominantly in the basal layer of proliferating keratinocytes. When a keratinocyte becomes committed to terminal differentiation, it moves o ut of the basal layer towards the epithelial surface. As part of this program of terminal differentiation, the expression of K5 (and K14) is downregulated in suprabasal cells, and new pairs of differentiation-s pecific keratins are expressed. To define the cis-acting DNA sequences required for K5 cell-type- and differentiation-specific expression, c himeric gene fusions between portions of the bovine keratin K5 locus a nd the Escherichin coli lacZ gene were used to generate transgenic mic e. In the genomic fragment consisting of 5.3 kb of 5' flanking sequenc es, 6.1 kb corresponding to the body of the gene and 4.5 kb of 3' flan king sequences, the subfragment extending from -5300 bp to +138 bp was the smaller region that directed lacZ expression to stratified epithe lia in a manner analogous to the endogenous keratin K5. Proximal seque nces from -1300 bp to +138 bp were inactive. We also determined the ex pression pattern of keratin K5 during mouse development using an antis erum specific for mouse keratin K5. Expression was first detected in e ctodermal cells of 11.5 days postcoitum embryos, and from day 13.5 pos tcoitum onwards K5 was detected in the precursors of most epithelia an d organs which express K5 at adult stages. This pattern was reproduced , with few differences, by the construct with sequences from -5300 bp to +138 bp fused to the lacZ gene. These findings identify sequences b etween -5.3 kb and -1.3 kb of the bovine K5 gene as being important fo r cell-type- and differentiatian-specific gene expression both during mouse development and in the adult.