Biological and biochemical effects of phenylalanine analogs

Differential effects of amino acid analogs on growth and heterocyst differentiation in two nitrogen-fixing blue-green algae

Effects of a few amino acid analogs on growth and heterocyst differentiation have been studied in two nitrogen-fixing species ofAnabaena. All the analogs except α-methyl-DL-aspartic acid inhibited growth. Exposure ofAnabaena doliolum, toDL-5-fluorotryptophan andDL-p-fluorophenylalanine caused pronounced fragmentation of filaments into single cells. At low concentrations (0.01 mM), α-methyl-DL-aspartic acid stimulated growth of the strain ofA. doliolum as well as the strain of the second (unidentified)Anabaena species. Ethionine,DL-p-fluorophenylalanine,DL-5-fluorotryptophan, and canavanine blocked heterocyst differentiation, whereas α-methyl-DL-aspartic acid, α-methyl-DL-methionine,N-o-nitrophenylsulfenyl-L-tryptophan, norleucine, andS-2-aminoethyl-L-cysteine did not show any significant effect. Treatment with 7-azatryptophan,DL-β-hydroxynorvaline,L-methionine-DL-sulfoximine,L-methionine sulfone, and β-2-thienyl-DL-alanine led to a twofold increase in heterocyst frequency. Possible modes of action of the analogs in growth inhibition and changes in heterocyst frequency are discussed.

Demonstration of an altered phenylalanyl-tRNA synthetase in an analogue-resistant mutant of Aspergillus nidulans

We have isolated and characterized a new class of p-fluorophenylalanine (FPA)-resistant mutant in Aspergillus nidulans using a phenA strain as the wild type, by optimizing the conditions of growth. All four spontaneous mutants selected on a medium containing FPA were found to be recessive to their wild-type alleles in heterozygous diploids. Complementation analyses and linkage data showed that they were allelic and mapped at a single locus (fpaU) in the facA-riboD interval on the right arm of linkage group V. Partial purification and characterization of Phe-tRNA synthetase from wild-type and mutant strains revealed that the mutant enzyme had a greatly reduced ability to activate the analogue. It is suggested that mutation in the fpaU gene brings about a structural alteration in Phe-tRNA synthetase.

Evolutionary aspects of accuracy of phenylalanyl-tRNA synthetase. A comparative study with enzymes from Escherichia coli, Saccharomyces cerevisiae, Neurospora crassa, and turkey liver using phenylalanine analogues

The phenylalanyl-tRNA synthetases from Escherichia coli, Saccharomyces cerevisiae, Neurospora crassa, and turkey liver activate a number of phenylalanine analogues (tyrosine, leucine, methionine, p-fluorophenylalanine, beta-phenylserine, beta-thien-2-ylalanine, 2-amino-4-methylhex-4-enoic acid, mimosine, N-benzyl-L- or N-benzyl-D-phenylalanine, and ochratoxin A), as demonstrated by Km and kcat of the ATP/PPi pyrophosphate exchange. Upon complexation with tRNA, the enzyme-tRNAPhe complexes show a significantly increased initial discrimination of these amino acid analogues expressed in higher Km and lower kcat values, as determined by amino-acylation of tRNAPhe-C-C-A(3'NH2). The overall accuracy is further enhanced by a second discrimination, a proofreading step. The strategies employed by the enzymes with respect to accuracy differ. Better initial discrimination in the aminoacylation and less elaborated proofreading for the E. coli enzyme can be compared to a more efficient proofreading by other synthetases. In this way the comparatively poor initial amino acid recognition in the case of the S. cerevisiae and N. crassa enzymes is balanced. The extent of initial discrimination is therefore inversely coupled to the hydrolytic capacity of the proofreading. A striking difference can be noted for the proofreading mechanisms. Whereas the enzymes from E. coli, S. cerevisiae, and N. crassa follow the pathway of posttransfer proofreading, namely, enzymatic hydrolysis of the misaminoacylated tRNA, the turkey liver enzyme uses tRNA-dependent pretransfer proofreading in the case of natural amino acids. In spite of the same subunit structure and similar molecular weight, the phenylalanyl-tRNA synthetases from a prokaryotic and lower and higher eukaryotic organisms show obvious mechanistic differences in their strategy to achieve the necessary fidelity.

The effect of canavanine on protein synthesis and protein degradation in IMR-90 fibroblasts

Proteins of IMR-90 fibroblasts incorporating [35S]methionine during a 1 h labelling period in the presence of the arginine analogue canavanine were degraded twice as rapidly in the cells as were proteins similarly made in the presence of arginine. Using both isoelectric focusing and SDS-polyacrylamide gel electrophoretic analyses, the banding patterns of proteins labelled in the presence of canavanine and arginine were found to differ. This banding difference was detected as early as 15 min after canavanine treatment. With the exception of one minor band in isoelectric focusing gel, the relative intensity of labelled protein bands for the control samples remained unchanged during the 2 h period of protein degradation being investigated. This was also true for the proteins labelled in the presence of canavanine, despite the increase in their rate of degradation. Banding difference between canavanine and arginine treatment was also detected in an in vitro reticulocyte lysate translation system dependent on fibroblast mRNA. Proteins labelled in the presence of a different analogue, p-fluorophenylalanine instead of phenylalanine, however, had similar banding patterns as the control both in the lysate system and in intact cells.

Potentiation by p-fluorophenylalanine of the division-delaying effects of high hydrostatic pressure in synchronized Tetrahymena pyriformis

Cultures of heat-synchronized Tetrahymena pyriformis, growing in a proteose peptone medium, were subjected to short pulses of the amino acid analogue, p-fluorophenylalanine, and high hydrostatic pressure. The pulses of these agents were chosen so that, when applied individually, they did not appreciably delay cell division. However, combined treatments, analogue pulse followed by pressure pulse, produced a pronounced synergism. The results are interpreted as further evidence to support the inclusion of analogue division proteins in pressure labile assemblies during the progression of Tetrahymena into division.