Complementation and enzyme studies of revertants induced in an am mutant of N. crassa

A total of eighty-seven revertants were induced by ultra-violet light in an am3 strain. All of these revertants appear to be the result of mutation at sites in or close to the am locus. Fourteen of the eighty-seven revertants were partial revertants in that under some conditions of assay they possessed low glutamate dehydrogenase activity compared with the wild-type although their growth rate was similar to that of the wild-type. Enzyme extracts of thirteen of the partial revertants were assayed for glutamate dehydrogenase in various ways in order to establish qualitative distinctions between different kinds of mutant enzyme. On the basis of these tests six different groups were established, of which one contained six revertants, one three and the others one. All except one of the mutant enzyme types showed a marked activation when incubated with α-oxoglutarate plus NADPH2, and all of these had Michaelis constants for ammonium ion much higher than is found for the wild-type enzyme. The remaining group of three revertants gave, at first, no enzyme activity in any of the assay systems. Two of these (the third was not tested) were shown to produce an enzyme variety which becomes quite inactive in phosphate buffer at pH 8·0 but can be fully activated by the addition of ethylenediamine tetra-acetic acid. Forced heterocaryons between each of six partial revertants and eleven am mutants were made and the resultant sixty-six heterocaryons assayed for glutamate dehydrogenase activity. The partial revertants differed among themselves in their complementation characteristics. Some complemented with none of the am mutants, some with am1 only, and some with am1 or with am7. The complementation tests confirmed the differences established by the enzyme studies. The data presented here, together with previous work, demonstrate that ultra-violet light induced mutation in an am strain can result in at least eight types of revertant differing from each other in respect of the glutamate dehydrogenase variety which each can produce.