Intracellular Localization of Ornithine and Arginine Pools in Neurospora

The intracellular localization of the large pools of ornithine and arginine in Neurospora has been investigated. Over 90% of both amino acids were found to be associated with an osmotically sensitive organelle. This organelle, termed the vesicle, is readily distinguishable from mitochondria on sucrose gradients. The association of ornithine and arginine with this vesicle is not an artifact of cell fractionation, and appears to reflect their intracellular localization within a membrane-enclosed structure. Other amino acids also appear to be localized within the vesicle. These results provide evidence for a compartmentation mechanism which withholds endogenous ornithine and arginine from catabolic fates.