Cooperative regulation of cell proliferation by calcium and calmodulin in Aspergillus nidulans

Calcium and calmodulin have been widely implicated in the control of cell proliferation. We have created a strain of the genetically tractable filamentous fungus, Aspergillus nidulans, that is conditional for calmodulin expression. This was accomplished by replacing the unique endogenous calmodulin gene with one regulated by the inducible alcohol dehydrogenase (a/cA) gene promoter by homologous recombination. This strain cannot grow when the cells are incubated in medium containing a carbon source that represses the a/cA promoter. Characterization of the arrested cells shows that 83% are blocked in the G₂ phase of the cell cycle. The block is due to very low levels of calmodulin and is fully reversible upon changing to medium that contains an inducer of the a/cA promoter. The rate of cell proliferation in this strain is dependent upon both the intracellular calmodulin and extracellular Ca²⁺ concentrations. Raising the calmodulin concentration by inducing the a/cA promoter not only causes the cells to enter the proliferative cycle more quickly and to grow faster, but also decreases the concentration of extracellular Ca²⁺ required to support growth by 10-fold, as compared with cells grown in noninducing medium. Thus both the intracellular calmodulin and extracellular Ca²⁺ concentrations are important and interactive factors in regulating the nuclear division cycle of Aspergillus nidulans.