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We report here the coding sequence of the gene for a 56-kDa type II keratin (designated K6b). Using a subclone specific for a unique 3' noncoding region of the encoded mRNA, we have shown that this gene is one of at least two 56-kDa keratin genes expressed in abundance in human epidermis. Segmenting the coding portion of this gene are eight introns, six of which are identically positioned with those of a distantly related type III intermediate filament gene (vimentin), and five of which are identically positioned with those of a distantly related type I gene (50-kDa keratin). These results indicate a common ancestral origin for all three classes of intermediate filament genes. All of the highly conserved intron positions are located within, but do not demarcate, the four central alpha-helical domains common to all intermediate filament polypeptides, suggesting that these genes were probably not created piecemeal by recombination-mediated linkage of separate structural domains as they presently are known.
Tyner et al. (Mon,) studied this question.