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A thermosensitive mutant of Escherichia coli has been isolated that is unable to replicate the bacteriophage MS2 at 42 degrees but permits phage production at 37 degrees . Thermal inactivation studies of the supernatant enzymes show that this mutant contains a factor essential for the polymerization of phenylalanine from phenylalanyl-tRNA that at 50 degrees is more rapidly inactivated than the corresponding wild-type factor. The elongation factor Tu (EF-Tu) was isolated and purified to apparent homogeneity as the EF-Tu.GDP complex, both from mutant and wild-type cells. Addition of purified wild-type EF-Tu.GDP to reaction mixtures fully restored the activity of thermally inactivated mutant supernatants. These experiments excluded EF-Ts as the thermolabile factor involved. Similar inactivation studies, dealing with the purified factors and performed in reaction mixtures that were not supplemented with GDP, revealed that the half-life of mutant EF-Tu.GDP at 50 degrees was 1.5 min, that of the wild-type factor 6 min. Addition of GDP (10muM) to the medium reduced the inactivation rate of both wild-type and mutant factor and also the difference in inactivation kinetics. Besides the altered elongation factor Tu, the mutant skill contains a second mutation affecting the glutaminyl-tRNA synthetase.
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F W de Rooij, and
A Rörsch, and
L Bosch
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J H Lupker, and
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