Embryonic simple epithelial keratins 8 and 18: chromosomal location emphasizes difference from other keratin pairs

The keratins 8 and 18 of simple epithelia differ from stratified epithelial keratins in tissue expression and regulation. To examine the specific properties of human keratin 8, we cloned and sequenced the cDNA from a placental mRNA expression library and defined the optimum state of such clones for expression in bacterial plasmid vectors. Using the polymerase chain reaction we identified and sequenced three introns and located the single active gene for keratin 8, out of a background of 9 to 24 pseudogenes, on chromosome 12. This chromosome contains several genes for type II keratins and also the gene for keratin 18, the type I keratin that is coexpressed with keratin 8. This location of both members of a keratin pair on a single chromosome is thus far unique among the keratin genes; it is consistent with the hypothesis that keratins 8 and 18 may be closer to an ancestral keratin gene than the keratins of more highly differentiated epithelia.