Complementation in vitro between guaB mutants of Escherichia coli K12

Guanine auxotrophs of Escherichia coli were isolated following mutagenesis by N-methyl-N'-nitro-N-nitrosoguanidine or ethyl methanesulphonate. The mutants were classified according to growth properties and absence of IMP dehydrogenase or GMP synthetase activity. Mutations in guaB (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. GuaB mutants were examined for protein (CRM) that cross-reacts with monospecific antibodies to IMP dehydrogenase; approximately half were CRM+. Enzyme complementation in vitro was detected in mixed denatured and renatured cell-free extracts of any CRM+ guaB mutant and PL1138 (guaB105, CRM+); CRM- mutants did not complement. GuaB105 maps distal to all other guaB mutations except guaB86 (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 guaB mutants and are consistent with the demonstration of identical subunits for IMP dehydrogenase (Gilbert et al., 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.