Cytochrome P450 monooxygenases from extremophiles

[摘要] English: Information indicate that CYP450s are prevalent in members of the bacterial phylum Deinococcus-Thermus as well as the archaeal family Halobacteriaceae that belong to the phyulm Euryarchaeota. A property shared by these phylogenetically distant extremophiles is the production of carotenoid pigments. It became the purpose of this study to use genome sequence information to clone and study new CYP450s from the genera Thermus and Halobacterium and to explore the role of these CYP450s in pigment production. The non-pigmented thermophilic bacterium Thermus scotoductus SA-01 was screened by PCR for the presence of a cytochrome P450 monooxygenase (CYP450). No CYP450 could be found and subsequent genome sequencing confirmed this finding. However, a CYP450 gene (CYP175A) was isolated from the closely related yellow pigmented strain Thermus sp. NMX2.A1 using oligonucleotides based on the DNA sequence of the β-carotene gene cluster from three Thermus strains. The genome sequence of T. scotoductus SA-01, revealed a ferredoxin (Fdx) and ferredoxin reductase (FNR) that were almost identical to those of Thermus thermophilus HB27. In T. thermophilus HB27 the Fdx and FNR are the native redox partners for CYP175A1, a β- carotene hydroxylase. After heterologous expression in Escherichia coli, we attempted to hydroxylate β-carotene with the CYP450 from Thermus sp. NMX2.A1 and the redox partners of T. scotoductus SA-01 using cell free extracts, but no products were detected. Thirty two CYP450s have been identified in the sequenced genomes of thirteen extremely halophilic archaea. Initial attempts to clone and heterologously express a CYP174A2- homologue from a Haloarcula LK-1 strain in E. coli and Pseudomonas fluorescens were unsuccessful. In order to study the physiological role of CYP450s in halophilic archaea and to create a strain that can be used for heterologous expression of CYP450s from halophiles CYP174A1 was deleted from H. salinarum R1. CYP174A1 is the only CYP450 in H. salinarum R1 and H. salinarum R1 is a genetically tractable strain. Upon culturing the wildtype and deletion strains, a difference in red pigmentation of stationary phase cultures was observed; implying that CYP174A1 might play a role in carotenoid synthesis. Microarray analyses revealed that the bop gene, which codes for bacterioopsin (BO) was severely repressed in stationary phase cultures of the deletion strain and sucrose gradient experiments showed a consequent loss of purple membrane (PM) in the deletion strain. The classical causes of bop repression e.g. insertion elements in the bop open reading frame as well as in the brz gene was ruled out by PCR screening. In addition to bop repression, the neighboring vng1459 and vng1468 genes (both part of the bopregulon) were also down regulated, but the genes normally involved in regulation of the bop gene were not affected. Currently the functions of vng1459 and vng1468 are unknown. Retinal, together with BO, is a key component of bacteriorhodopsin (BR) and essential for PM synthesis. Retinal is formed by the central cleavage of β-carotene which can be achieved by monooxygenases or dioxygenases.The Blh and Brp proteins in H. bacterium salinarum are very closely related to a confirmed bacterial 15,15�?β-carotene dioxygenase and studies have shown that deletion of both brp and blh results in complete abolishment of retinal and BR. It is therefore unlikely that CYP174A1 plays a role in retinal biosynthesis. Another possible function for CYP174A1 might be the hydroxylation of β-carotene, since it is known that H. salinarum strains produce hydroxylated carotenoids such as transastaxanthin, but no genes encoding typical β-carotene hydroxylases or ketolases have been identified in the genomes of H. salinarum strains. This will imply that hydroxylated carotenoids play a role in the regulation of bop.

[发布机构] University of the Free State