Calmodulin gene expression during sea urchin development: persistence of a prevalent maternal protein

Calmodulin gene expression during embryogenesis of the sea urchin Strongylocentrotus purpuratus was investigated. Several identical bacteriophages containing a cDNA insert encoding sea urchin calmodulin (CM-1) were identified by screening a lambda gt10 library of S. purpuratus gastrula-stage cDNAs with a chicken calmodulin cDNA sequence. A 1.2-kb cDNA fragment from CM-1 was subcloned into pUC-8 to give plasmid pCAL-8. pCAL-8 contains a single open reading frame encoding 79 amino acids, a termination codon, and 0.9 kb of 3'-untranslated message. This sea urchin amino acid sequence shows 95% homology to amino acid residues 69-148 of the predicted sequence of chicken calmodulin. Northern analysis showed that pCAL-8 hybridizes to a single size (3.2 kb) of mRNA in both embryonic and adult somatic tissues. Genome blots suggested that there is a single calmodulin gene in the S. purpuratus genome. We used pCAL-8 to study calmodulin mRNA accumulation in S. purpuratus embryos. Calmodulin mRNA is present in the unfertilized egg at the level of a typical rare-class mRNA (1000-2000 transcripts) and accumulates approximately 100-fold to levels representing about 1/10th of 1% of the total mRNA in pluteus-stage cells. Synthesis of calmodulin, identified by two-dimensional gel electrophoresis, shows a similar developmental pattern. However, in spite of the very active synthesis of calmodulin during embryogenesis, most of the calmodulin in the pluteus is apparently provided for by an enormous store of calmodulin in the egg, corresponding to about 2% of the mass of total protein.