Visible light-sensitive mutants of Escherichia coli

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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.