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Polyamines are ubiquitous polycationic molecules with multiple effects. Spermidine was present in all the life stages of Phycomyces blakesleeanus, fulfilled the physiological requirement for polyamines during germination, and became most abundant at the emergence of germinating tubes. Putrescine was not found in resting spores or in stationary cultures, but was synthesized during apical growth and greatly exceeded spermidine in fast-growing stages of the vegetative and sexual life cycles. Changes in the polyamines did not correlate with the various stages of sporulation. Ornithine decarboxylase was so strongly inhibited in vitro by its product, putrescine, that it would completely block the enzyme if not compartmentalized away. 1,4-Diamino-2-butanone inhibited mycelial growth throughout the vegetative cycle without killing the cells. The inhibition was counteracted very effectively by putrescine, which acts as a close analog of the inhibitor, and very little by spermidine. Four independent spe mutants were obtained by a procedure that selected for resistance to diaminobutanone among functionally-uninucleate spores that survived exposure to N-methyl-N'-nitro-N-nitrosoguanidine. The stability of the enzyme, in vivo and in vitro, and its inhibition by diaminobutanone in vitro were the same in the wild type and in the mutants. Two of these were hypomorph mutants, with lower affinity of their ornithine decarboxylase for its substrate, ornithine, and lower maximal velocity. The other two were hypermorph transport mutants; we propose that they are affected in a protein that binds putrescine and its analogs for transport across the plasmalemma and sequestration away from the active enzyme. The transport mutants concentrated the exogenous diaminobutanone and the endogenous putrescine in inactive compartments; the highest enzyme activity was reached when the plasmalemma of the mutants was permeabilized with diethylaminoethyl dextran. |
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