Two mutations in the locus of , which abolish the process of cell lysis resulting from the combination of the R and non-allelic incompatibility genes, have previously been shown to produce complete suppression of a laccase exoenzyme. In the present study it was found that, in addition, the mutants show a marked increase in the activity of an amino-acid oxidase (an exo- and endoenzyme), most of the increase (some 9- or 15-fold) being in the extracellular fraction. The mutant strains are hypersensitive to cycloheximide in vivo, but biochemical and genetical investigations suggested that the hypersensitivity is not due to effects on the ribosome. The (l) strain also shows increased resistance to chlorate, a specific sensitivity to acid (6.5) and neutral (7.5) pH, and significant hypersensitivity to methylammonium and thiourea. cells accumulate a higher amount of the urea analogue [C]thiourea. The pH sensitivity associated with the (l) mutation is specifically suppressed by the combination of the R and V incompatibility genes, and the (l) strain is resistant to sorbose. The specific consequences of the (l) mutation and the suppression of some of its effects by the combination of the and incompatibility genes lead to the suggestion that protoplasmic incompatibility might be induced by action of the initiating complex in association with the plasma membrane.


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