A gene-enzyme relationship in neurospora : (neurospora, complementation, amino acid substitutions, chain termination, suppression, osmotic repair)
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The results of work on amino acid replacements in NADP-specific glutamatede hydrogenase of Neurospora crassa, due to mutation in the am gene, are reviewed: amino acid residues which are important in coenzyme binding and in connection with allosteric and complementation properties have been identified. One mutant, which has a-leucine-to-histidine replacement near the N-terminal end of the chain, appears to have drastically destabilized quaternary enzyme structure and is partly repaired by high molalities of non-toxic solutes. A chain-terminating mutant may be either 'ochre' or 'amber', and the amino acid inserted by a suppressor of this mutant is identified as tyrosine. The fine-structure genetic map, constructed on the basis of conversion polarity, is collinear with the polypeptide chain. Certain marker effects on intragenic recombination frequency may be interpretable on the basis of the molecular nature of the mutant sites concerned. The possibility is discussed that some enzyme-negative mutants may map outside the structural gene.