Studies on Mutant Phenylalanyl RNA Synthetases of Escherichia coli

The temperature conditional mutant NP 37 of Escherichia coli differs from the wild type K 10 in that cell‐free extracts exhibit a 20‐fold reduced specific activity of phenylalanyl RNA synthetase. The mutant enzyme activity is characterized by an increased K m value for phenylalanine. Molecular weight determinations by sucrose density gradient centrifugation yield an approximate value of 180,000 for the wild type enzyme and of 100–110,000 for that of the mutant. Immunological analysis of the mutant extracts in these experiments, by means of antiserum prepared against purified phenylalanyl RNA synthetase, shows no cross‐reacting material at the position where the native enzyme sediments. However, an increased amount of cross‐reacting material is present in the fractions containing the mutant enzyme activity. A phenotypic revertant of NP 37 was found to possess heat stability properties of the phenylalanyl RNA synthetase intermediate between the wild type and the NP 37 enzymes. Incubation of cell‐free extracts of such a results in preparations which show the catalytic properties of phenylalanyl RNA synthetase that are characteristic of mutant NP 37. The results indicate that temperature inactivation of phenylalanyl RNA synthetase in NP 37 is a consequence of the dissociation of the enzyme into subunits with impaired catalytic properties.