Nakayashiki T, Nishimura K, Inokuchi H
Department of Biophysics, Faculty of Science, Kyoto University, Japan.
Gene. 1995 Feb 3;153(1):67-70. doi: 10.1016/0378-1119(94)00805-3.
We have isolated a number of porphyrin (Por)-synthesis mutants as light-resistant revertants of a light-sensitive strain delta visA (hemH) of Escherichia coli that accumulates protoPor IX in the cell. Among such mutants, we found a double mutant (H103) with mutations in hemA and in a new gene downstream of hemA. This new gene, designated hemK, was located at 27 min on the linkage map of the E. coli chromosome. By nucleotide (nt) sequencing, it was demonstrated that hemK forms part of the hemA-prfA-hemK operon and encodes 225 amino acids that show no significant homology to any protein in the standard databases. The mutant strain H103 formed small colonies and showed no catalase activity even in the presence of 5-aminolevulinic acid (ALA), indicating its inability to catalyze a step in the biosynthesis of heme from ALA. An extract of H103 cells has readily detectable ALA dehydratase and porphobilinogen deaminase activities. H103 cells carrying a plasmid that included only hemA as an insert accumulated protoPor and coproPor, but showed no sensitivity to light, a result that suggests that it may be deficient in protoporphyrinogen oxidase activity.
我们从大肠杆菌的光敏感菌株δvisA（hemH）的耐光回复突变体中分离出了一些卟啉（Por）合成突变体，该菌株在细胞中积累原卟啉IX。在这些突变体中，我们发现了一个双突变体（H103），其hemA和hemA下游的一个新基因发生了突变。这个新基因命名为hemK，位于大肠杆菌染色体连锁图上27分钟处。通过核苷酸（nt）测序表明，hemK是hemA-prfA-hemK操纵子的一部分，编码225个氨基酸，与标准数据库中的任何蛋白质均无明显同源性。突变菌株H103形成小菌落，即使在存在5-氨基乙酰丙酸（ALA）的情况下也没有过氧化氢酶活性，表明它无法催化从ALA生物合成血红素的一个步骤。H103细胞提取物具有易于检测到的ALA脱水酶和胆色素原脱氨酶活性。携带仅包含hemA作为插入片段的质粒的H103细胞积累了原卟啉和粪卟啉，但对光不敏感，这一结果表明它可能缺乏原卟啉原氧化酶活性。
