Characterization and expression of the unique calmodulin gene of Aspergillus nidulans

Complete cDNA and genomic clones for the unique calmodulin (CaM) gene of the filamentous fungus Aspergillus nidulans have been isolated and characterized. The gene contains five introns, of which three are at unique positions relative to other CaM genes. The A. nidulans CaM gene is transcribed as a single, 0.85-kilobase mRNA species that encodes a predicted protein 84% identical (93% similar if conservative changes are considered) to vertebrate CaM. The complete cDNA was ligated into a λ_PL promoter-regulated bacterial expression vector to allow expression of A. nidulans CaM in Escherichia coli. The expressed protein was purified from bacterial lysates by phenyl-Sepharose chromatography and migrated as a single species on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In the presence of Ca²⁺, A. nidulans CaM exhibited a shift in apparent M_r identical to vertebrate CaM. The bacterially synthesized protein activated vertebrate CaM-dependent phosphodiesterase, CaM-dependent protein kinase II, and myosin light chain kinase with kinetics similar to vertebrate CaM. Isolated conidia (G₀ spores) were germinated to induce synchronous cell cycle re-entry and the levels of CaM mRNA and protein determined. Both CaM and its mRNA were regulated during cell cycle re-entry. Calmodulin mRNA levels increased 20-fold as germlings progressed through the G₁ phase, while CaM levels increased 2-fold prior to the initiation of DNA synthesis. Messenger RNA levels decreased during S-phase while protein levels increased an additional 2-fold, peaking at the onset of mitosis followed by a subsequent decrease as cells completed mitosis. Disruption of the CaM gene by site-specific homologous recombination was lethal, indicating that CaM is essential for cell cycle progression.