Bockrath R, Harper D, Kristoff S
Mutat Res. 1980 Nov;73(1):43-58. doi: 10.1016/0027-5107(80)90134-7.
Strains of E. coli B/r were exposed to UV radiation and assayed for reversion mutation, using a standard selection medium. If more irradiated bacteria were assayed per petri dish, a proportional increase in the number of indicated reversion mutants was found only up to a limiting plating density. Beyond a density of about 10(8) viable bacteria per petri dish, the number of indicated revertants per viable bacteria assayed (the mutation frequency) decreased as the plating density was increased. The crowding depression of mutagenesis was more severe for de novo and converted suppressor mutations, the mutation frequency being reduced 100-fold at a plating density of about 6 x 10(9) viable bacteria per plate. The effect on backmutation was 10 times less. Crowding depression of mutagenesis occurred in excision-proficient and -deficient strains, with identical effects in the 2 strains on de novo and converted suppressor mutation, but different effects on backmutations. There were no accompanying effects on viability. Irreversible loss of potential mutants during crowded growth was indicated in wash-off experiments. The kinetics suggested a half-life of approximately 1 h. Kinetics for accumulation by the bacteria of the limiting metabolite (tyrosine) on the assay plate indicated a short period of time for protein synthesis, but direct examination of the proteins synthesized during early growth on a crowded plate demonstrated successful induction of recA protein. The results suggested a possible disruption in the rec/lex respondency system somewhere between induction of recA protein and the various end points, including mutational repair, in cells plated close to one another.
将大肠杆菌B/r菌株暴露于紫外线辐射下，使用标准选择培养基测定回复突变。如果每个培养皿中检测的受辐照细菌更多，那么仅在达到极限接种密度之前，所指示的回复突变体数量会成比例增加。超过每个培养皿约10⁸个活细菌的密度后，随着接种密度增加，每个检测的活细菌所指示的回复体数量（突变频率）会下降。对于从头突变和转换抑制突变，突变的拥挤抑制更为严重，在每个平板约6×10⁹个活细菌的接种密度下，突变频率降低100倍。对回复突变的影响则小10倍。在切除 proficient 和 -deficient 菌株中均发生了突变的拥挤抑制，这两种菌株对从头突变和转换抑制突变的影响相同，但对回复突变的影响不同。对活力没有伴随影响。洗脱实验表明在拥挤生长过程中潜在突变体存在不可逆的损失。动力学表明半衰期约为1小时。在测定平板上细菌积累极限代谢物（酪氨酸）的动力学表明蛋白质合成时间较短，但直接检查在拥挤平板上早期生长过程中合成的蛋白质显示recA蛋白成功诱导。结果表明，在用含有recA蛋白的细胞进行mutational修饰实验时，在recA蛋白诱导和包括突变修复在内的各个终点之间的rec/lex响应系统可能存在干扰。
