Molecular analysis of human and rat calmodulin complementary DNA clones. Evidence for additional active genes in these species.
A cDNA clone, lambda rCB1, encoding calmodulin was isolated from a rat brain expression library. The sequence was determined and compared to the structures of the previously described rat genes, lambda SC4 and lambda SC8 (Nojima, H., and Sokabe, H. (1986) J. Mol. Biol. 190, 391-400). Faithful sequence conservation is observed in the coding regions of lambda rCB1 and lambda SC4, the bona fide gene. Both cDNAs encode identical amino acid sequence. Very limited sequence homology, however, is noted in the 3'-untranslated segments of these clones. Surprisingly, when the lambda rCB1 nucleotide structure is compared to the processed intronless gene, lambda SC8, extensive sequence homology is found both in the coding and noncoding regions. The inferred protein sequences of lambda SC8 and lambda rCB1, however, are divergent. Using a fragment of lambda rCB1 to screen an expression library derived from a human embryonic cell line, a calmodulin cDNA clone, lambda hCE1, was isolated and characterized. Comparison of the sequence of lambda hCE1 to the cDNA from human liver, hCWP (Wawrzynczak, E. J., and Perham, R. N. (1984) Biochem. Int. 9, 177-185), reveals substantial structural divergence. Strikingly poor homology is seen in the 5'- and 3'-noncoding segments, but the coding regions were 85% homologous. Both lambda hCE1 and hCWP encode proteins of identical primary structure which is equivalent to the protein sequence deduced from lambda rCB1 and lambda SC4. Taken together these results suggest the existence of an additional actively transcribed calmodulin gene, not previously identified, in each of the human and rat genomes. Transcripts of lambda rCB1 and lambda hCE1 were observed in all tissues examined indicating the absence of tissue-specific expression. Calmodulin gene polymorphisms were detected using TaqI, HindIII, and MspI.