Jin Jianling, Gao Peiji, Mao Yumin
State key laboratory of microbial technology, Shandong University, 27 Shanda South Road, Jinan 250100, PR China.
BMC Genet. 2002 Apr 25;3:6. doi: 10.1186/1471-2156-3-6.
Many investigations have reported that advantageous mutations occurred more frequently under selective conditions than those under non-selective conditions. This phenomenon is referred to as adaptive mutation. Their characteristics are that adaptive mutations are directed and growth-independent. The idea of directed adaptive mutation had been objected by some reports, however, the idea of growth-independent adaptive mutation has been held till today.
In this paper, we have observed that under leucine starvation conditions, leu+ revertants accumulated as a function of time; leu- to leu+ reverse mutation rates and frequencies were higher than those under non starvation conditions; and no divided cells could be monitored by the penicillin method. These results were similar to the time-dependent manner of adaptive mutation from previous reports. However, leucine concentration determinate experiments revealed that certain traces of leucine, which leaked from the E. coli cells, was almost always present in the culture. More numbers of leu+ revertants appeared when the similar cultures were dropped in small areas on the selective plates than when spread on the whole selective plates. These results have shown that mutations under leucine starving conditions are growth-dependent. Fluctuation analysis of leu+ revertants indicated that leu-leu+ mutation occurred spontaneously and randomly. In addition, the spectra of leuB gene in the revertants proved that mutations under selective conditions were not specific or directed.
The above investigations led to the conclusion (1) that the occurrence of leu+ mutations under starvation conditions was growth-dependent. The occurrence mutations was also similar to that under non-starvation conditions (2). Under starvation conditions the mutation rates were higher, and was not constant during the long process.
许多研究报告称，有利突变在选择性条件下比在非选择性条件下更频繁地发生。这种现象被称为适应性突变。其特点是适应性突变是定向的且与生长无关。然而，定向适应性突变的观点受到了一些报告的反对，而与生长无关的适应性突变观点一直持续至今。
在本文中，我们观察到在亮氨酸饥饿条件下，亮氨酸营养缺陷型回复突变体随时间积累；亮氨酸营养缺陷型向亮氨酸营养型的反向突变率和频率高于非饥饿条件下的；并且通过青霉素法无法监测到分裂细胞。这些结果与先前报道的适应性突变的时间依赖性方式相似。然而，亮氨酸浓度测定实验表明，从大肠杆菌细胞中泄漏的某些微量亮氨酸几乎总是存在于培养物中。当将相似的培养物滴在选择性平板的小区域上时，出现的亮氨酸营养型回复突变体数量比将其铺在整个选择性平板上时更多。这些结果表明，亮氨酸饥饿条件下的突变是依赖生长的。亮氨酸营养型回复突变体的波动分析表明，亮氨酸营养缺陷型向亮氨酸营养型的突变是自发且随机发生的。此外，回复突变体中亮氨酸B基因的谱图证明，选择性条件下的突变不是特异性的或定向的。
上述研究得出以下结论：（1）饥饿条件下亮氨酸营养型突变的发生是依赖生长的。突变的发生也与非饥饿条件下的相似（2）。在饥饿条件下，突变率更高，并且在长时间过程中并非恒定不变。
