Use of External, Biosynthetic, and Organellar Arginine by Neurospora

The fate of very low amounts of14 C-arginine derived from the medium or from biosynthesis was studied inNeurospora cells grown in minimal medium. In both cases, the label enters the cytoplasm, where it is very briefly used with high efficiency for protein synthesis without mixing with the bulk of the large, endogenous pool of12 C-arginine. The soluble14 C-arginine which is not used for protein synthesis is sequestered in a vesicle with the bulk of the endogenous arginine pool. After this time, it is selectively excluded from use in protein synthesis except by exchange with cytoplasmic arginine. The data suggest that in vivo, the non-organellar cytoplasm contains less than 5% of the soluble, cellular arginine. The cellular organization ofNeurospora described here also prevents the catabolism of arginine. Our results are discussed in relation to previous work on amino acid pools of other eukaryotic systems.