AbstractA contingent auxotrophic mutant of CHO‐Kl cell is described. This mutant grow in minimal medium. Its growth is inhibited by the exogenous addition of guanine at levels which do not affect the wild type parent. Adenine reverses the guanine effect. This mutant does not complement ade−H (defective in adenylosuccinate synthetase) and has been denoted as ade−HGbecause of its guanine sensitivity. Some partial revertants of ade−H are found to be also sensitive to guanine, suggesting a close relationship between the ade−H locus and the guanine sensitivity. Studies of14C‐hypoxanthine incorporation into nucleotides indicated that ade−HGhas some adenylosuccinate synthetase activity whether it is pre‐exposed to guanine or not. Earlyde novopurine synthesis in ade−HG, however, is greatly inhibited when pre‐exposed to guanine. This inhibition of purine synthesis by guanine is reversible and its recovery is fac
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