Compartmentation of spermidine in Neurospora crassa

The polyamines putrescine, spermidine, and spermine are multivalent cations that bind to anionic cell constituents such as nucleic acids. Their distribution between free and bound states within the cell is not known. Such knowledge would be important in relation to the negative control of polyamine synthesis. We report a tracer experiment in which [14C]ornithine was added to logarithmically growing Neurospora crassa mycelia. The amount and the specific radioactivity of the three polyamines thereafter suggested that new molecules of spermidine were made preferentially from new molecules of putrescine, and that new molecules of spermine were made from new molecules of spermidine. The extent of mixing of new [14C]- and resident [12C]spermidine indicated that 70% or more of the resident spermidine was sequestered, and not immediately accessible to spermine synthase. Cell fractionation revealed that about 28% of the cellular spermidine was vacuolar, and nonexchangeable with [14C] spermidine added at the time of cell breakage. We suggest that the remainder of sequestered spermidine is bound strongly to anionic sites in the cell, and is relatively inactive in the control and synthesis of polyamines.