Cellular compartmentation of aromatic amino acids in Neurospora crassa. I. Occupation of a protein synthesis pool by phenylalanine in Tyr-1 mutants

Abstract

The p-fluorophenylalanine (FPA) resistance of accphe, which has previously been shown (Brooks et al., 1972) to be a try-1 mutant, has been further investigated. When incubated in the absence of tyrosine, accphe and also tyr-1 auxotrophs show a gradual increase in free phenylalanine in the cell but a sharp decrease in FPA incorporation into protein. The decrease in FPA incorporation is apparently due to the excess phenylalanine in the mutants, since the normal endogenous pool component in wild type and also in the mutants incubated on tyrosine does not appear to compete with FPA for incorporation. The rate of FPA incorporation into protein in accphe remains at 10–15% of the wild-type rate even when the ratio of free FPA to excess phenylalanine in the cell is high as 8:1. If wild type is supplied with exogenous phenylalanine and FPA simultaneously, phenylalanine is preferentially incorporated into protein but, in contrast to the mutant, the rate of FPA incorporation increases as the ratio of free FPA to phenylalanine increases. On the basis of differences in competition with FPA and in susceptibilities to mild extraction procedures, it is proposed that phenylalanine can be located in at least three compartments in Neurospora: a small constant-size endogenous pool always seen in wild type; an expandable exogenous pool; and a protein synthesis pool which is preferentially populated by endogenous phenylalanine but can be entered by exogenous molecules when biosynthesis is regulated. In accphe, where phenylalanine biosynthesis is not regulated, the excess phenylalanine is located primarily in the protein synthesis pool where it only has to compete with a small FPA component and is thereby preferentially incorporated into protein in this mutant.