PRIMARY STRUCTURES OF ARABIDOPSIS CALMODULIN ISOFORMS DEDUCED FROM THE SEQUENCES OF CDNA CLONES

Complementary DNA (cDNA) clones encoding calmodulin isoforms were isolated from an Arabidopsis leaf lambda-gt10 library by screening with cloned barley calmodulin cDNA probes. Two cDNAs, one a 626-base pair partial-length clone (ACaM-1) and one a 1400-base pair full-length clone (ACaM-2), encode calmodulin polypeptides that differ by four conservative amino acid substitutions. None of the amino acid sequence differences occur within the four Ca2+-binding domains of the proteins. Whereas the deduced amino acid sequences of the two Arabidopsis calmodulin isoforms share 97% identity, the nucleotide sequences encoding the two isoforms share 87% sequence identity. Most of these nucleotide sequence differences (80%) occur in codon wobble positions. ACaM-1 and ACaM-2 both hybridize with a distinct set of restriction fragments of Arabidopsis total DNA, indicating that they were derived from transcripts of separate genes; these genes are single- or very low-copy in the Arabidopsis genome. Both cDNAs hybridize to messenger RNA (mRNA) species of 0.8 kilobases that are expressed to a greater extent in developing siliques compared with leaves, flowers, and stems. Northern blot and polymerase chain reaction assays both indicate that ACaM-1 mRNA is more highly expressed than ACaM-2 mRNA in developing siliques. The steady-state levels of both isoform mRNAs increase as a result of touch stimulation; the kinetics and extent of increase are comparable for the two mRNAs.