Summary: Guanine auxotrophs of were isolated following mutagenesis by -methyl--nitro--nitrosoguanidine or ethyl methanesulphonate. The mutants were classified according to growth properties and absence of IMP dehydrogenase or GMP synthetase activity. Mutations in (IMP dehydrogenase-less) were analysed by reversion and suppression tests; all were of the base substitution missense type except for one possible frameshift and one polar nonsense mutation. mutants were examined for protein (CRM) that cross-reacts with monospecific antibodies to IMP dehydrogenase; approximately half were CRM. Enzyme complementation was detected in mixed denatured and renatured cell-free extracts of any CRM mutant and PL1138 , CRM); CRM mutants did not complement. maps distal to all other mutations except (CRM). Two hybrid enzymes produced by complementation were less stable to heat than native IMP dehydrogenase, although kinetic constants were similar. These observations indicate interallelic complementation between mutants and are consistent with the demonstration of identical subunits for IMP dehydrogenase (Gilbert , 1979). Only the subunits supplied by PL1138 are catalytically active in the hybrid enzymes suggesting that this mutant may produce a repairable polypeptide whereas the enzymes of complementing mutants may be defective at the active site.


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