Isolation of multiple classes of mutants of CHO cells resistant to cyclic AMP

From mutagenized Chinese hamster ovary (CHO) cells we have isolated, in a single step, 11 independent mutants resistant to the growth-inhibitory effects of 8-Br-cyclic AMP, cholera toxin, and methylisobutylxanthine. Two major classes and several subclasses of mutants were obtained. Mutants from all classes have a normal doubling time. None of the mutants respond to cyclic AMP treatment with increased flattening and elongation as do the parental cells. Members of the first class have an altered protein kinase activity which has either an increased Ka for cyclic AMP or an absent response to cyclic AMP. Most of those mutations which result in a protein kinase with increased Ka for cyclic AMP (6/11) are dominant in somatic cell hybrids. Those mutations which result in a protein kinase with little or no response to cyclic AMP (3/11) are recessive. Members of the second major class (2/11) have normal levels of basal and cyclic AMP-dependent protein kinase activity. One is recessive and one is dominant by genetic tests. The basis for the defect in this second class of mutants has not been determined.