Ghelardini P, Liebart J C, Paolozzi L, Pedrini A M
Dipartimento di Genetica e Biologia Molecolare, Università di Roma La Sapienza, Italy.
Mol Gen Genet. 1989 Mar;216(1):31-6. doi: 10.1007/BF00332227.
We have previously shown that Mu can sustain the growth at non-permissive temperature of an Escherichia coli strain harbouring a thermosensitive mutation in the DNA ligase structural gene. This "complementation" reaches a maximal level with the Mu lig3 mutant which restores the viability of a ligts7 strain to the level of the wild type (Ghelardini et al. 1980; Paolozzi et al. 1980). In this study we analysed the characteristics of this phenotypic suppression in order to clarify its molecular mechanism. We found that an E. coli ligts7 strain lysogenic for the Mu lig3 mutant shows: (i) an increment in the host DNA ligase activity; (ii) an increase in the specific mRNA of the host lig gene; (iii) an increase (towards the relaxed state) in the average linking number of a resident plasmid; and (iv) a reduction in DNA gyrase activity. These results are compatible with the hypothesis that the Mu lig gene product by interfering with the host enzymatic apparatus controlling DNA topology leads to a reduction in chromosomal supercoiling. The relaxation of the chromosome could affect the transcription of the DNA ligase gene, amongst others. Thus, through this mechanism, the Mu lig gene product is able to modulate gene expression and hence suppress the effects of the E. coli ligts7 mutation. On the basis of the identification of this mechanism of action, we propose to change the name of the Mu lig gene (thought originally to be the structural gene for a bacteriophage ligase) to gem (gene expression modulation).
我们先前已经表明，Mu能够维持在DNA连接酶结构基因中携带温度敏感突变的大肠杆菌菌株在非允许温度下的生长。这种“互补作用”在Mu lig3突变体中达到最大水平，该突变体可将ligts7菌株的活力恢复到野生型水平（Ghelardini等人，1980年；Paolozzi等人，1980年）。在本研究中，我们分析了这种表型抑制的特征，以阐明其分子机制。我们发现，携带Mu lig3突变体的溶原性大肠杆菌ligts7菌株表现出：（i）宿主DNA连接酶活性增加；（ii）宿主lig基因的特异性mRNA增加；（iii）常驻质粒的平均连环数增加（趋向于松弛状态）；以及（iv）DNA促旋酶活性降低。这些结果与以下假设相符，即Mu lig基因产物通过干扰控制DNA拓扑结构的宿主酶装置导致染色体超螺旋减少。染色体的松弛可能会影响DNA连接酶基因等的转录。因此，通过这种机制，Mu lig基因产物能够调节基因表达，从而抑制大肠杆菌ligts7突变的影响。基于对这种作用机制的鉴定，我们建议将Mu lig基因（最初被认为是噬菌体连接酶的结构基因）的名称更改为gem（基因表达调节）。
