2 EXONS ENCODE THE CALMODULIN-BINDING DOMAIN IN THE MOUSE PHOSPHORYLASE-KINASE CATALYTIC SUBUNIT GENE

The catalytic subunit, gamma, of phosphorylase kinase contains two cal modulin-binding sequences that define a domain in gamma that is homolo gous to the troponin-C-binding domain in troponin I. The homology is b ased on both sequence and functional similarities. To account for this homology, it has been proposed that the calmodulin-binding sequences in gamma and the troponin-C-binding domain in troponin I have evolved from a common ancestor. We investigated this possibility by comparing the exon structure of the gamma gene with that of the troponin-I gene over their homologous domains. In the quail troponin-I gene, it is kno wn that the entire troponin-C-binding domain is encoded by a single ex on. However, two exons are found to encode the calmodulin-binding doma in in the gamma gene from mouse. This result indicates that convergent evolution may be responsible for the sequence and functional similari ties between the homologous domains in troponin I and gamma.