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Two cDNAs derived from Xenopus laevis calmodulin mRNA have been cloned. Both cDNAs contain the complete protein-coding region and various lengths of untranslated segments. The two cDNAs encode an identical protein but differ from each other by 5% nucleotide substitutions. The 5' and 3' untranslated regions, to the extent available, are highly homologous between the two cDNAs. The predicted sequence of X. laevis calmodulin is identical to that of vertebrate calmodulins from mammals and chickens and shows one substitution compared with electric eel calmodulin. Genomic DNA sequences homologous to each of the two cDNA clones have been isolated and were shown to account for the major calmodulin-coding DNA sequences in X. laevis. These data suggest that X. laevis carries two active, nonallelic calmodulin genes. Although no complete analysis has been carried out, it appears that the X. laevis calmodulin genes are interrupted by at least four introns. The relative concentrations of calmodulin mRNA have been estimated in different embryonic stages and adult tissues and found to vary by up to a factor of 10. The highest levels of calmodulin mRNA were found in ovaries, testes, and brains. In these three tissues, the two calmodulin genes appear to be expressed at approximately equal levels.
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