Visible light-sensitive mutants of Escherichia coli

我们分离出对可见光敏感的大肠杆菌K-12突变体。这种现象是由编码铁螯合酶的visA （hemH）基因的缺陷引起的。该基因的缺陷导致原卟啉IX的积累，这是一种光敏剂，并产生一种对细胞有害的活性氧。我们从一株签证缺失菌株中分离出抗光敏回变菌株，发现第二突变位于铁螯合酶催化反应前参与血红素生物合成途径的其他基因中。利用这些突变体，得到以下结果：1)突变株vsr751在hemG基因产物原卟啉原氧化酶（PPO）中存在缺陷。利用该菌株进行互补分析，分离并鉴定了该PPO基因。2)我们发现了hemA下游一个新基因的突变。我们分析了这个被命名为血红素K的新基因。3)我们之前的研究表明，大肠杆菌hemH突变体对蓝光的敏感度是其野生型亲本的100倍以上。在这里，我们发现hemG突变体AW823对蓝光的反应至少与hemH突变体一样敏感。4)利用lambda载体构建拟南芥cDNA文库，筛选大肠杆菌hemG突变体，获得原卟啉原氧化酶cDNA。5)我们发现对血红素的饥饿引起了严格的反应，这种饥饿促进了卟啉的合成，但对hemA-lacZ融合基因的表达没有影响。我们提出了卟啉合成的调节模型，其中卟啉的合成与蛋白质的合成协调。

We isolated the mutants of E.coli K-12 that were sensitive to visible light. This phenomenon was brought about by a defect in the visA (hemH) gene that encodes ferrochelatase. A defect in this gene causes the accumulation of protoporphyrin IX,which is a photosensitizer and produces an active species of oxygen that is harmful to the cell. We isolated photoresistant revertants from a visA-deleted strain and found that the second mutations were located in other genes involved in the heme biosynthetic pathway at steps prior to the reaction catalyzed by ferrochelatase. Utilizing those mutants, the following results were obtained :1) The mutant strain VSR751must be defective in protoporphyrinogen oxidase (PPO), the product of the hemG gene. By complementation analysis using this atrain we isolated and identified this PPO gene of E.coli.2) We found a mutation in a new gene downstream of hemA.This new gene, designated heme K,was analyzed.3) We showed previously that an E.coli hemH mutant is somewhat better than 100-fold more sensitive than its wild-type parent in tumbling to blue light. Here we found that the hemG mutant AW823 tumbles to blue light at least as sensitively as the hemH mutant.4) A cDNA encoding protoporphyrinogen oxidase, was obtained from a library of Arabidopsis thaliana cDNA constructed in a lambda vector by screening for complementation of a hemG mutant of E.coli.5) We found that starvation for heme caused the stringent response, and such starvation promote the synthesis of porphyrins without having any effect on the expression of the hemA-lacZ fusion gene. We suggested a model for the regulation of porphyrin synthesis wherein the synthesis of porphyrins is coordinated with that of proteins.