Cooper V S, Schneider D, Blot M, Lenski R E
Center for Microbial Ecology, Michigan State University, East Lansing, Michigan 48824, USA.
J Bacteriol. 2001 May;183(9):2834-41. doi: 10.1128/JB.183.9.2834-2841.2001.
Twelve populations of Escherichia coli B all lost D-ribose catabolic function during 2,000 generations of evolution in glucose minimal medium. We sought to identify the population genetic processes and molecular genetic events that caused these rapid and parallel losses. Seven independent Rbs(-) mutants were isolated, and their competitive fitnesses were measured relative to that of their Rbs(+) progenitor. These Rbs(-) mutants were all about 1 to 2% more fit than the progenitor. A fluctuation test revealed an unusually high rate, about 5 x 10(-5) per cell generation, of mutation from Rbs(+) to Rbs(-), which contributed to rapid fixation. At the molecular level, the loss of ribose catabolic function involved the deletion of part or all of the ribose operon (rbs genes). The physical extent of the deletion varied between mutants, but each deletion was associated with an IS150 element located immediately upstream of the rbs operon. The deletions apparently involved transposition into various locations within the rbs operon; recombination between the new IS150 copy and the one upstream of the rbs operon then led to the deletion of the intervening sequence. To confirm that the beneficial fitness effect was caused by deletion of the rbs operon (and not some undetected mutation elsewhere), we used P1 transduction to restore the functional rbs operon to two Rbs(-) mutants, and we constructed another Rbs(-) strain by gene replacement with a deletion not involving IS150. All three of these new constructs confirmed that Rbs(-) mutants have a competitive advantage relative to their Rbs(+) counterparts in glucose minimal medium. The rapid and parallel evolutionary losses of ribose catabolic function thus involved both (i) an unusually high mutation rate, such that Rbs(-) mutants appeared repeatedly in all populations, and (ii) a selective advantage in glucose minimal medium that drove these mutants to fixation.
在葡萄糖基本培养基中经过2000代进化后，大肠杆菌B的12个群体均丧失了D - 核糖分解代谢功能。我们试图确定导致这些快速且平行丧失的群体遗传过程和分子遗传事件。分离出了7个独立的Rbs(-)突变体，并相对于其Rbs(+)祖细胞测量了它们的竞争适应性。这些Rbs(-)突变体的适应性均比祖细胞高约1%至2%。波动测试显示，从Rbs(+)突变为Rbs(-)的突变率异常高，约为每细胞世代5×10(-5)，这有助于快速固定。在分子水平上，核糖分解代谢功能的丧失涉及核糖操纵子（rbs基因）部分或全部的缺失。缺失的物理范围在突变体之间有所不同，但每个缺失都与位于rbs操纵子上游紧邻的一个IS150元件相关。这些缺失显然涉及转座到rbs操纵子内的不同位置；新的IS150拷贝与rbs操纵子上游的那个拷贝之间的重组随后导致中间序列的缺失。为了证实有益的适应性效应是由rbs操纵子的缺失引起的（而不是其他未检测到的突变），我们使用P1转导将功能性rbs操纵子恢复到两个Rbs(-)突变体中，并通过不涉及IS150的缺失进行基因替换构建了另一个Rbs(-)菌株。所有这三个新构建体都证实，在葡萄糖基本培养基中，Rbs(-)突变体相对于其Rbs(+)对应物具有竞争优势。因此，核糖分解代谢功能的快速且平行的进化丧失涉及（i）异常高的突变率，使得Rbs(-)突变体在所有群体中反复出现，以及（ii）在葡萄糖基本培养基中的选择优势，促使这些突变体固定下来。
