Two distinct classes of keratin genes and their evolutionary significance

Bacterial plasmids containing cDNA sequences specific for keratins were constructed from mRNA of cultured human epidermal cells. Two separate classes of cloned cDNAs were identified by positive hybrid selection: one class removed from total human epidermal mRNA a fraction that was translated into 56 and 58 kilodalton (kd) keratins, and the other class selected mRNAs that translated into a mixture of 50 kd and 46 kd keratins. When probes specific for these two keratin classes were hybridized with human DNA digested with a restriction endonuclease that does not cleave within the probe, two distinct patterns of about ten fragments each were observed. Most of the hybridizing genomic fragments corresponded to complete cDNA sequences, and it is estimated that each of the two classes is encoded by about 10 genes. When the probes were hybridized with DNA from different species, all vertebrates were found to contain discrete sequences homologous to both human keratin probes. Within each vertebrate species, the two probes always hybridized with approximately equal intensities to nonoverlapping sets of genomic sequences, suggesting a coordinate evolution between the two subfamilies of keratin genes. This finding has important functional implications for keratin filament assembly.