Cloning, characterization and expression of three calmodulin genes of Solanum commersonii

Calmodulin (CaM) is modulated by calcium, which is an important factor of plant signal transduction. Three different CaM cDNAs were isolated from a cDNA library of the cold tolerant potato wild species S. commersonii, by screennig with a RT-PCR fragment (350bp) from S. tuberosum gene CaM1. Clones ScCaM1, ScCaM3, ScCaM5 are 918, 899, 881 bp long respectively and contain a complete ORF coding for 149 amino acid residues. While nucleotide identity inside the coding region of three clones is high (81%), the 3' untranslated regions are divergent (only 47 % identity) and different also in lenght (330bp ScCaM1, 290bp ScCaM3, 416bp ScCaM5). The three clones share 93% identity and are highly homologus to other cloned plant and animal calmodulin genes (80-90%). Highly conserved motives were identified in the coding region, including four Ca2+-binding motives (EF-hands), one N-glycosylation site, two N-myristoylation sites, 7 protein C kinase and 1 casein kinase II putative phosphorylation sites. To ascertain whether or not the three CaM clones were differentially induced by specific environmental stresses, the 3' untraslated divergent regions were used as probes in Northern blot analysis. Though transcripts corresponding to the three clones were rapidly induced (within 30 min) by NaCl, high and low temperature and water deficit, differences in the kinetics of induction and the amount of transcript were uncovered by each probe. Interestingly, the same stress treatments did not affect the steady-state level of CaM mRNAs in a potato cell population, adapted to growth in a medium containing 20%PEG. This finding suggests that one or more components involved in the perception and intracellular transduction of the stress signal might have been altered during the adaptation process.