Protein kinase CK2 is essential for the growth of Saccharomyces cerevisiae. Yeast cells that lack the functional genes coding for both the catalytic subunits of protein kinase CK2 can grow only if they are complemented by exogenous cDNAs coding for this subunit. A series of deletion mutants of CK2alpha from Xenopus laevis was constructed. These mutants that have carboxyl end deletions yield a CK2alpha product that varies over four orders of magnitude in its capacity to phosphorylate casein in vitro. Complementation of yeast RPG41-1a, a mutant defective in CKA1 and CKA2 genes, with wild-type X. laevis CK2alpha and with cDNAs coding for truncated CK2alpha having amino acids 1-328 and 1-327 resulted in cells that grew in gal-minimal media at 30 degrees C as well as the cells harboring the yeast CKA2 gene. However, the growth was significantly diminished when cells were complemented with X. laevis CK2alpha containing 1-326 amido acids. This mutant has 0.6% of the catalytic activity of the wild-type enzyme. Yeast cells that expressed CK2alpha 1-324 and 1-323 which have 10-and 100-fold less activity, respectively, were not able to grow. The growth of cells containing the CK2alpha 1-326 mutant was very sensitive to temperature, and minimal growth was observed at 37 degrees C. This mutant was also more sensitive to UV radiation but was not significantly affected by 0.4 M NaCl.