Structure and expression of the Arabidopsis CaM-3 calmodulin gene

Genomic and cDNA sequences encoding a calmodulin (CaM) gene from Arabidopsis (ACaM-3) have been isolated and characterized. ACaM-3 represents a sequence distinct from two previously isolated Arabidopsis CaM cDNA clones. A 2.3 kb Eco RI restriction fragment was sequenced and found to encode a complete CaM-coding sequence interrupted by a single 491 bp intron, together with 750 bp and 600 bp of 5′ and 3′ flanking sequences, respectively. The polypeptide encoded by ACaM-3 is identical to that encoded by ACaM-2 and it differs from the one encoded by ACaM-1 by four of 148 residues. The putative promoter of ACaM-3 was atypical of CaM genes previously isolated from animals in that it contained consensus TATA and CAAT box sequences and lacked GC-rich regions. Two DNA sequence elements closely resembling cyclic AMP regulatory elements, which have been identified in animal CaM genes, were located in the 5′ flanking region of ACaM-3. Northern blot and polymerase chain reaction amplification assays confirmed that each of the three ACaM mRNAs were expressed in similar but distinct patterns in different organs. ACaM-1 mRNA was the only species detectable in root RNA fractions, and ACaM-3 mRNA could not be detected in floral stalks. Accumulation of each of the three CaM mRNAs in leaves was induced by a touch stimulus, but the kinetics and extent of the induction varied among the three mRNA species. Run-on transcription assays indicated that a portion of the differences in accumulation of ACaM-1, 2, and 3 mRNAs in leaves and siliques was attributable to differences in their net rates of transcription.