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The growth of thermosensitiveBacillus subtilis lysyl- and tryptophanyl-transfer ribonucleic acid synthetase mutants (lysS1 andtrypS1 ) (l -lysine:transfer ribonucleic acid [tRNA] ligase [AMP], EC 6.1.1.6; andl -tryptophan:tRNA ligase [AMP], EC 6.1.1.2) was terminated when exponential phase cells were shifted from 30 to 43 C in a rich medium. Under these conditions, the temperature-inhibited cells undergo thermal death; they rapidly lose their ability to form colonies at 30 C. Another lysyl-tRNA synthetase mutant (lysS2 ) is refractory to thermal death even though its growth is inhibited at 43 C. The thermal death response of thelysS1 mutant is affected by the stage of cell development. At periods in spore outgrowth and sporogenesis these cells become refractory to thermal death. The refractory state does not result from the production of an inhibitor, or from the degradation of an activator of thermal death. However, culture medium composition does modify the thermal death response. Rich media enhance the effect, and no thermal death occurs in thelysS1 strain grown in a minimal medium. Temperature-sensitive cells can grow in a lysine- (0.25 mM) or tryptophan- (0.25 mM) supplemented minimal medium at 43 C, but amino acid concentrations of 25 mM only transiently protecttrypS1 andlysS1 cells from thermal death in a rich medium. Osmotic agents such as sucrose (0.5 M) and NaCl (0.34 M) completely prevent thermal death in thelysS1 strain, although growth is still arrested. On solid media, sucrose stabilizedlysS1 cells can form colonies at the restrictive temperature, but neither sucrose (0.5 M) nor NaCl (0.34 M) stabilized thelysS1 enzyme in vitro. Chloramiphenicol increased the rate of thermal death of thelysS1 strain but decreased the thermal death response of thetrypS1 mutant. Considering the nature of the enzyme defect in thelysS1 strain, the common genetic origin of the spore and vegetative lysyl-tRNA synthetase, and the protective effects exerted by lysine and osmotic agents, it is tentatively concluded that thermal death results from irreversible inactivation of the mutant gene product. According to this hypothesis, either thelysS1 enzyme is altered during sporogenesis or some physiological or structural aspect of this developmental phase can stabilize the mutant phenotype and thereby rescue cells from thermal death.

journal of bacteriology

Thermal Death of Temperature-Sensitive Lysyl- and Tryptophanyl-Transfer Ribonucleic Acid Synthetase Mutants of <i>Bacillus subtilis</i> : Effect of Culture Medium and Developmental Stage

Thermal Death of Temperature-Sensitive Lysyl- and Tryptophanyl-Transfer Ribonucleic Acid Synthetase Mutants of Bacillus subtilis : Effect of Culture Medium and Developmental Stage