RESUMO
TtcA catalyzes the post-transcriptional thiolation of cytosine 32 in some tRNAs. The enzyme from Escherichia coli was homologously overexpressed in E. coli. The purified enzyme is a dimer containing an iron-sulfur cluster and displays activity in in vitro assays. The type and properties of the cluster were investigated using a combination of UV-visible absorption, EPR and Mössbauer spectroscopy, as well as by site-directed mutagenesis. These studies demonstrated that the TtcA enzyme contains a redox-active and oxygen-sensitive [4Fe-4S] cluster, chelated by only three cysteine residues and absolutely essential for activity. TtcA is unique tRNA-thiolating enzyme using an iron-sulfur cluster for catalyzing a non-redox reaction.
Assuntos
Proteínas de Escherichia coli/química , Escherichia coli/enzimologia , Glutarredoxinas/química , Proteínas Ferro-Enxofre/química , Sulfurtransferases/química , Clonagem Molecular , Cisteína/química , Proteínas de Escherichia coli/genética , Proteínas de Escherichia coli/isolamento & purificação , Proteínas de Escherichia coli/metabolismo , Expressão Gênica , Glutarredoxinas/genética , Glutarredoxinas/isolamento & purificação , Glutarredoxinas/metabolismo , Proteínas Ferro-Enxofre/genética , Proteínas Ferro-Enxofre/isolamento & purificação , Proteínas Ferro-Enxofre/metabolismo , Sulfurtransferases/genética , Sulfurtransferases/isolamento & purificação , Sulfurtransferases/metabolismoRESUMO
Coenzyme Q (ubiquinone or Q) is a redox-active lipid found in organisms ranging from bacteria to mammals in which it plays a crucial role in energy-generating processes. Q biosynthesis is a complex pathway that involves multiple proteins. In this work, we show that the uncharacterized conserved visC gene is involved in Q biosynthesis in Escherichia coli, and we have renamed it ubiI. Based on genetic and biochemical experiments, we establish that the UbiI protein functions in the C5-hydroxylation reaction. A strain deficient in ubiI has a low level of Q and accumulates a compound derived from the Q biosynthetic pathway, which we purified and characterized. We also demonstrate that UbiI is only implicated in aerobic Q biosynthesis and that an alternative enzyme catalyzes the C5-hydroxylation reaction in the absence of oxygen. We have solved the crystal structure of a truncated form of UbiI. This structure shares many features with the canonical FAD-dependent para-hydroxybenzoate hydroxylase and represents the first structural characterization of a monooxygenase involved in Q biosynthesis. Site-directed mutagenesis confirms that residues of the flavin binding pocket of UbiI are important for activity. With our identification of UbiI, the three monooxygenases necessary for aerobic Q biosynthesis in E. coli are known.
