Kharat A S, Mahadevan S
Department of Molecular Reproduction, Development and Genetics, Indian Institute of Science, Bangalore.
Acta Biochim Pol. 1999;46(4):853-61.
SSOR, a clinical isolate of Shigella sonnei which exhibits a Salicin-negative phenotype, is unable to mutate to give rise to Sal+ derivatives although a homolog of the Escherichia coli bgl operon is retained by the strain. This was correlated to the presence of an endogenous plasmid in the strain. A plasmid-cured derivative, AK711, could give rise to Sal+ mutants in two steps. Introduction of the plasmid DNA, extracted from SSOR, into various strains of E. coli and S. sonnei, resulted in ampicillin resistant transformants. Interestingly, the presence of the plasmid suppressed the mutational activation of the bgl operon in the transformants. This was further substantiated by the observation that, transformants that have lost the plasmid regained the ability for mutational activation of the bgl operon. Preliminary characterisation of the plasmid indicated a size of 3.8 kb with an origin of replication resembling that of ColE1 replicons and the bla gene homolog of Tn3. Observations of the mutation frequency at the srl and lac loci in the presence of the plasmid indicate that there is a reduction in the mutation frequency, suggesting an antimutator activity associated with the plasmid.
索氏志贺菌临床分离株SSOR表现出水杨苷阴性表型，尽管该菌株保留了大肠杆菌bgl操纵子的同源物，但它无法突变为产生Sal+衍生物。这与该菌株中存在内源性质粒有关。质粒消除衍生物AK711可分两步产生Sal+突变体。将从SSOR中提取的质粒DNA导入各种大肠杆菌和索氏志贺菌菌株，产生了氨苄青霉素抗性转化体。有趣的是，质粒的存在抑制了转化体中bgl操纵子的突变激活。这一现象通过以下观察得到进一步证实：丢失质粒的转化体恢复了bgl操纵子突变激活的能力。对该质粒的初步表征表明其大小为3.8 kb，复制起点类似于ColE1复制子，且含有Tn3的bla基因同源物。在存在质粒的情况下对srl和lac位点突变频率的观察表明，突变频率降低，这表明该质粒具有抗突变活性。
