| Abstrakt: |
The utlraviolet light (UV) survival curve of Escherichia coliWP10 recA trpis almost biphasic, with a greatly reduced shoulder but demonstrating a transition to a decreased slope with increasing fluences, indicating the presenc ein the culture of a low freqeuency of resistant cells. Treatment of the culture with chloramphenicol before UV exposure brought almost all the cells to a high degree of UV resistance, by bringing them to the end of their DNA replication cycle. The survival curves of the repair-proficient E. colWP2 trpshowed a similar pattern with chloramphenicol treatment of tryptophan starvation before UV exposure, but only if protein synthesis were blocked by chloramphenicol for 60 min after UV exposure. The result suggests that when recA/lexA-regulon induction is prevented, either by the recAmutation or by inhibition of protein synthesis after UV exposure, death occurs unless the cells are in the resitant state characteristic of bacteria at the end of their DNA replication cycle. With repair-proficient bacteria treated before UV exposure with chloramphenicol, when protein synthesis is not blocked after UV exposure, a marked expansion of the shoulder occures because of the function of another resistance-conferring mechanism. This mechanism also depends on the recA+gene expansion of the shoulder does not occur in recAbacteria when protein synthesis is inhibited before UV exposure. |
