Hall B G, Betts P W, Kricker M
Molecular and Cell Biology Department, University of Connecticut, Storrs 06268.
Mol Biol Evol. 1986 Sep;3(5):389-402. doi: 10.1093/oxfordjournals.molbev.a040406.
The genes for cellobiose utilization are normally cryptic in Escherichia coli. The cellobiose system was used as a model to understand the process by which silent genes are maintained in microbial populations. Previously reported was (1) the isolation of a mutant strain that expresses the cellobiose-utilization (Cel) genes and (2) that expression of those genes allows utilization of three beta-glucoside sugars: cellobiose, arbutin, and salicin. The Cel gene cluster has now been cloned from that mutant strain. In the course of locating the Cel genes within the cloned DNA segment, it was discovered that inactivation of the Cel-encoded hydrolase rendered the host strain sensitive to all three beta-glucosides as potent inhibitors. This sensitivity arises from the accumulation of the phosphorylated beta-glucosides. Because even the fully active genes conferred some degree of beta-glucoside sensitivity, the effects of cellobiose on a series of five Cel+ mutants of independent origin were investigated. Although each of those strains utilizes cellobiose as a sole carbon and energy source, cellobiose also acts as a potent inhibitor that reduces the growth rate on glycerol 2.5-16.5-fold. On the other hand, wild-type strains that cannot utilize cellobiose are not inhibited. The observation that the same compound can serve either as a nutrient or as an inhibitor suggests that, under most conditions in which cellobiose will be present together with other resources, there is a strong selective advantage to having the cryptic (Cel0) allele. In those environments in which cellobiose is the sole, or the best, resource, mutants that express the genes (Cel+) will have a strong selective advantage. It is suggested that temporal alternation between these two conditions is a major factor in the maintenance of these genes in E. coli populations. This alternation of environments and fitnesses was predicted by the model for cryptic-gene maintenance that was previously published.
在大肠杆菌中，利用纤维二糖的基因通常是隐性的。以纤维二糖系统作为模型来理解沉默基因在微生物群体中得以维持的过程。此前报道了：（1）分离出一株表达纤维二糖利用（Cel）基因的突变菌株；（2）这些基因的表达使得该菌株能够利用三种β-葡萄糖苷糖：纤维二糖、熊果苷和水杨苷。现在已从该突变菌株中克隆出Cel基因簇。在将Cel基因定位到克隆的DNA片段的过程中，发现Cel编码的水解酶失活会使宿主菌株对所有这三种β-葡萄糖苷糖敏感，因为它们是强效抑制剂。这种敏感性源于磷酸化β-葡萄糖苷的积累。由于即使是完全活性的基因也会赋予一定程度的β-葡萄糖苷敏感性，因此研究了纤维二糖对一系列五个独立来源的Cel +突变体的影响。尽管这些菌株中的每一株都能将纤维二糖用作唯一的碳源和能源，但纤维二糖也作为一种强效抑制剂，使甘油上的生长速率降低2.5至16.5倍。另一方面，不能利用纤维二糖的野生型菌株不受抑制。同一化合物既可以作为营养物质又可以作为抑制剂这一观察结果表明，在大多数纤维二糖与其他资源共存的条件下，拥有隐性（Cel0）等位基因具有很强的选择优势。在纤维二糖是唯一或最佳资源的环境中，表达这些基因的突变体（Cel +）将具有很强的选择优势。有人提出，这两种条件之间随时间的交替是这些基因在大肠杆菌群体中得以维持的一个主要因素。环境和适应性的这种交替是先前发表的隐性基因维持模型所预测的。
