Figueroa N, Wills N, Bossi L
Centre de Génétique Moléculaire du CNRS, Université P. et M. Curie, Gif-sur-Yvette, France.
EMBO J. 1991 Apr;10(4):941-9. doi: 10.1002/j.1460-2075.1991.tb08028.x.
Inhibiting the activity of DNA gyrase by mutation or by drugs in S. typhimurium causes the loss of transcription attenuation in the histidine operon. We show that gyrase activity is needed to maintain high-level expression of the tRNA(His) gene (hisR), a prerequisite for proper functioning of the attenuation mechanism. A point mutation in the promoter of the tRNA gene cluster which includes the hisR gene, specifically relieves the promoter response to negative supercoiling thereby restoring full hisR transcription (and, in turn, his attenuation) in the presence of a defective gyrase. The very same mutation, a single base-pair substitution between the -10 box and the transcription start site (-7), was found independently among suppressors of the transcriptional deficiency caused by disruption of DNA curvature upstream from the hisR promoter. We show that the -7 change does not lead to a generalized increase of promoter strength; the effects of the mutation are seen only when gyrase is inhibited or the upstream curvature is altered. This suggests that the upstream curvature intervenes in the same initiation step which is sensitive to superhelical tension. Three additional promoter mutations correcting (or alleviating) the effects of the upstream alteration (including a change at -8) are described and discussed.
在鼠伤寒沙门氏菌中，通过突变或药物抑制DNA促旋酶的活性会导致组氨酸操纵子中转录衰减的丧失。我们发现，促旋酶活性对于维持tRNA（His）基因（hisR）的高水平表达是必需的，而这是衰减机制正常运作的一个前提条件。在包含hisR基因的tRNA基因簇启动子中的一个点突变，可特异性地缓解启动子对负超螺旋的反应，从而在促旋酶有缺陷的情况下恢复hisR的完全转录（进而恢复his衰减）。在hisR启动子上游DNA曲率破坏导致的转录缺陷的抑制子中，独立发现了完全相同的突变，即在-10框与转录起始位点（-7）之间的单个碱基对替换。我们发现，-7处的变化不会导致启动子强度普遍增加；只有在促旋酶受到抑制或上游曲率改变时，才会看到该突变的影响。这表明上游曲率干预了对超螺旋张力敏感的同一起始步骤。还描述并讨论了另外三个校正（或减轻）上游改变影响的启动子突变（包括-8处的一个变化）。
