Regulation of gene expression by active oxygen species

检测了大肠杆菌硫氧还蛋白和硫氧还蛋白还原酶对过氧化氢(H_2O_2)和超氧化物生成剂的保护作用。在稳定期后期，硫氧还蛋白和硫氧还蛋白还原酶缺陷突变株trxA和trxB对这些氧化剂的敏感性均高于野生型菌株。TrxA突变体的指数生长细胞对H_2O_2的敏感性也高于野生型菌株。另一方面，处于指数生长期的trxB突变体对H_2O_2的抗性高于野生型菌株。过氧化氢酶-氢过氧化物酶I(HPI)由katG编码，是过氧化氢的主要防御酶，可被H_2O_2诱导。我们检测了经H_2O_2处理后，trxA和trxB突变体中HPI的表达水平。有趣的是，trxB突变促进了katG：：lacZ融合基因的表达，而trxA表达降低。这些结果表明，硫氧还蛋白至少通过两种不同的机制来保护大肠杆菌细胞免受H_2O_2的伤害，一种是作为还原形式的自由基清除剂，另一种是通过刺激氧化形式的HPI的诱导。

The ability of Escherichia coli thioredoxin and thioredoxin reductase was examined with respect to protect the cells against hydrogen peroxide (H_2O_2) and super-oxide-generating agents. In late stationary phase, thioredoxin-and thioredoxin reductase-deficient mutant, trxA and trxB,respectively, exhibited increased sensitivity to these oxidizing agents compared with wild-type strain. Exponentially growing cells of trxA mutant was also more sensitive to H_2O_2 than wild-type strain. On the other hand, trxB mutant in exponentially growing phase acquired resistance to H_2O_2 over the level of wild-type strain. Catalase-hydroperoxidase I (HPI), encoded by the katG,is known to be the major defense enzyme against peroxides and inducible by H_2O_2. We examined the level of HPI expression in trxA and trxB mutants after treatment with H_2O_2. Interestingly, the trxB mutation stimulated the expression of katG : : lacZ fusion gene, while the trxA reduced. These results suggested that thioredoxin protects E.coli cells against H_2O_2 by at least two different mechanisms, by serving as a radical scavenger in its reduced form and by stimulating the induction of HPI in its oxidized form.