Department of Biology, University of Massachusetts, MA, USA.
Genome Biol Evol. 2009 Oct 20;1:400-8. doi: 10.1093/gbe/evp042.
Bacteria engage in a never-ending arms race in which they compete for limited resources and niche space. The outcome of this intense interaction is the evolution of a powerful arsenal of biological weapons. Perhaps the most studied of these are colicins, plasmid-based toxins produced by and active against Escherichia coli. The present study was designed to explore the molecular responses of a colicin-producing strain during serial transfer evolution. What evolutionary changes occur when colicins are produced with no target present? Can killing ability be maintained in the absence of a target? To address these, and other, questions, colicinogenic strains and a noncolicinogenic ancestor were evolved for 253 generations. Samples were taken throughout the experiment and tested for killing ability. By the 38th transfer, a decreased killing ability and an increase in fitness were observed in the colicin-producing strains. Surprisingly, DNA sequence determination of the colicin plasmids revealed no changes in plasmid sequences. However, a set of chromosomally encoded loci experienced changes in gene expression that were positively associated with the reduction in killing. The most significant expression changes were observed in DNA repair genes (which were downregulated in the evolved strains), Mg ion uptake genes (which were upregulated), and late prophage genes (which were upregulated). These results indicate a fine-tuned response to the evolutionary pressures of colicin production, with far more genes involved than had been anticipated.
细菌之间进行着一场永无止境的军备竞赛，它们争夺有限的资源和生态位空间。这种激烈的相互作用的结果是进化出了强大的生物武器库。也许这些武器中研究最多的是肠毒素(colicins)，这是一种基于质粒的毒素，由大肠杆菌产生并针对其起作用。本研究旨在探索产肠毒素菌株在连续传代进化过程中的分子反应。当没有靶标存在时，产生肠毒素会发生哪些进化变化？在没有靶标的情况下，能否保持杀伤能力？为了解决这些问题和其他问题，我们对产肠毒素菌株和非产肠毒素祖先进行了 253 代的进化。在整个实验过程中取样并测试其杀伤能力。在第 38 次转移时，产肠毒素菌株的杀伤能力下降，适应性增加。令人惊讶的是，对肠毒素质粒的 DNA 序列测定显示质粒序列没有变化。然而，一组染色体编码的基因座经历了基因表达的变化，这些变化与杀伤能力的降低呈正相关。最显著的表达变化发生在 DNA 修复基因（在进化菌株中下调）、Mg 离子摄取基因（上调）和晚期噬菌体基因（上调）中。这些结果表明，对肠毒素产生的进化压力有一个精细的调控，涉及的基因远比预期的要多。
