Uyemura D, Eichler D C, Lehman I R
J Biol Chem. 1976 Jul 10;251(13):4085-9.
DNA polymerase I has been purified to homogeneity from an Escherichia coli K12 strain bearing the temperature-sensitive conditionally lethal mutation, polAex1. The purified enzyme shows no defect in its polymerase or 3' leads to 5'-exonuclease activities; however, its 5' leads to 3'-exonuclease activity is abnormally low at both 30 degrees and 43 degrees. Although the mutant enzyme is able to catalyze the coordinated 5' leads to 3' polymerization and 5' leads to 3' exonucleolytic hydrolysis of nucleotides at a nick in duplex DNA ("nick translation") at a measurable rate at 30 degrees, this reaction is undetectable at 43 degrees. This defect is very likely responsible for the retarded joining of nascent DNA fragments and the consequent loss of viability that occur in the mutant at this temperature.
已从携带温度敏感型条件致死突变 polAex1 的大肠杆菌 K12 菌株中纯化出均一的 DNA 聚合酶 I。纯化后的酶在其聚合酶或 3' 到 5' 外切核酸酶活性方面没有缺陷；然而，其 5' 到 3' 外切核酸酶活性在 30 摄氏度和 43 摄氏度时均异常低。尽管突变酶能够在 30 摄氏度以可测量的速率催化双链 DNA 缺口处核苷酸的 5' 到 3' 聚合和 5' 到 3' 外切核酸水解（“切口平移”），但在 43 摄氏度时该反应无法检测到。这种缺陷很可能是导致突变体在此温度下新生 DNA 片段连接延迟以及随之而来的活力丧失的原因。
