Regulatory changes in the fucose system associated with the evolution of a catabolic pathway for propanediol in Escherichia coli
Regulatory changes in the fucose system associated with the evolution of a catabolic pathway for propanediol in Escherichia coli
复制标题
与大肠杆菌中丙二醇分解代谢途径进化相关的岩藻糖系统的调节变化
作者：
A. Hacking;E. Lin
大肠杆菌野生型菌株无法将L - 1,2 - 丙二醇用作碳源和能源。菌株3是一种因能够在该化合物上以逐渐加快的速度生长而被筛选出的突变体，它组成型地合成一种烟酰胺腺嘌呤二核苷酸连接的丙二醇氧化还原酶。这种酶在L - 果糖上厌氧生长时通常作为乳醛还原酶被合成。丙二醇作为该发酵过程的最终产物，不可挽回地散失到培养基中。利用丙二醇的突变体在有或没有分子氧的情况下都不再能在果糖上生长。在本研究中，对九条独立的丙二醇阳性突变体系进行了特征分析。一个突变体，菌株418，在丙二醇上达到了接近菌株3的生长速率，且没有丧失在果糖上生长的能力。然而，在所有检测的案例中，在丙二醇上长期筛选确实导致了果糖阴性突变体的出现。所有这些突变体都具有与菌株3相似的酶模式；即，果糖通透酶、果糖异构酶和果糖激酶不可诱导，而果糖 - 1 - 磷酸醛缩酶是组成型的。在菌株418以及其他突变体的果糖阳性前体中，该途径中的前四种酶仍然是可诱导的，就像在野生型菌株中一样。在丙二醇上生长速率的提高似乎主要反映了在进化早期阶段氧化还原酶活性水平的提高。根据转导分析，影响在丙二醇上生长能力的突变以及那些影响果糖途径中第一种酶表达的突变是紧密连锁的。在果糖上生长能力的丧失被认为是有利于利用新碳源的调控系统重塑所附带的一种机制性结果。
Wild-type strains of Escherichia coli are unable to use L-1,2-propanediol as a carbon and energy source. Strain 3, a mutant selected for the ability to grow on this compound at progressively more rapid rates, synthesizes constitutively a nicotinamide adenine dinucleotide-linked propanediol oxidoreductase. This enzyme is normally synthesized during anaerobic growth on L-fucose when it functions as a lactaldehyde reductase. Propanediol, the end product of this fermentation process, escapes irretrievably into the medium. The propanediol-utilizing mutant can no longer grow on fucose in either the presence or absence of molecular oxygen. In the present study nine independent lines of propanediol-positive mutants were characterized. One mutant, strain 418, attained a propanediol growth rate close to that of strain 3 without loss of the ability to grow on fucose. In all cases examined, however, prolonged selection on propanediol did result in the emergence of fucose-negative mutants. All of these mutants had enzyme patterns similar to that of strain 3; namely, fucose permease, fucose isomerase, and fuculose kinase were noninducible, whereas fuculose 1-phosphate aldolase was constitutive. In strain 418 and in the fucose-positive predecessors of the other mutants, the first four enzymes in the pathway remained inducible, as in the wild-type strain. Improvements in the growth rate on propanediol appeared to reflect principally the increased activity level of the oxidoreductase during the early stages of evolution. According to transductional analysis, the mutations affecting the ability to grow on propanediol and those that affect the expression of the first enzymes in the fucose pathway were very closely linked. The loss of the ability to grow on fucose is thought to be a mechanistic consequence incidental to the remodeling of the regulatory system in favor of the utilization of the novel carbon source.