CALMODULIN ISOFORMS IN ARABIDOPSIS ENCODED BY MULTIPLE DIVERGENT MESSENGER-RNAS

Three new, unique cDNA sequences encoding isoforms of calmodulin (CaM) were isolated from an Arabidopsis cDNA library cloned in lambdagt10. These sequences (ACaM-4,-5, and -6) represent members of the Arabidops is CaM gene family distinct from the three DNA sequences previously re ported. ACaM-4 and -6 encode full-length copies of CaM mRNAs of ca. 0. 75 kb. The ACaM-5 sequence encodes a partial length copy of CaM mRNA t hat is lacking sequences encoding the amino-terminal 10 amino acids of mature CaM and the initiator methionine. The derived amino acid seque nce of ACaM-5 is identical to the sequences encoded by two of the prev iously characterized ACaM cDNAs, and is identical to TCH-1 mRNA, whose accumulation was increased by touch stimulation. The polypeptides enc oded by ACaM-4 and -6 differ from that encoded by ACaM-5 by six and tw o amino acid substititions, respectively. Most of the deduced amino ac id sequence substitutions in the Arabidopsis CaM isoforms occurred in the fourth Ca2+-binding domain. Polymerase chain reaction amplificatio n assays of ACaM-4, -5 and -6 mRNA sequences indicated that each accum ulated in Arabidopsis leaf RNA fractions, but only ACaM-4 and -5 mRNAs were detected in silique total RNA. The six different CaM cDNA sequen ces each hybridize with unique Eco RI restriction fragments in genomic Southern blots of Arabidopsis DNA, indicating that these sequences we re derived from distinct structural genes. Our results suggest that Ca M isoforms in Arabidopsis may have evolved to optimize the interaction of this Ca2+-receptor protein with specific subsets of response eleme nts.