The Widely Conserved ebo Cluster Is Involved in Precursor Transport to the Periplasm during Scytonemin Synthesis in Nostoc punctiforme
The Widely Conserved ebo Cluster Is Involved in Precursor Transport to the Periplasm during Scytonemin Synthesis in Nostoc punctiforme
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DOI：
10.1128/mbio.02266-18
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发表时间：
2018-11-01
期刊：
影响因子：
6.4
通讯作者：
Garcia-Pichel, Ferran
中科院分区：
文献类型：
--
作者：
Klicki, Kevin;Ferreira, Daniela;Garcia-Pichel, Ferran
Scytonemin是一种二聚吲哚酚防晒剂，由一些蓝细菌在暴露于UVA辐射的条件下合成。虽然它的生物合成途径已被阐明只有部分，比较基因组学揭示，scytonemin操纵子往往包含一个集群的五个高度保守的基因（ebo集群）的未知功能，是广泛和保守的几个细菌和藻类门。我们试图通过构建和分析框内缺失突变体（每个ebo基因一个，整个簇一个）来阐明氰基细菌Nostoc punctiforme中ebo簇的功能。在UVA诱导的条件下，所有的ebo突变体scytoneminless，和所有积累了一个单一的化合物，scytonemin单体，明确暗示所有ebo基因scytonemin生产。我们发现，scytonemin单体也积累在一个诱导的缺失突变体的scyE，一个非ebo scytonemin基因，其产品被证明是针对周质。共聚焦自发荧光显微镜显示，积累仅限于细胞质中的所有ebo突变体，但这不是在scyE缺失的情况下，与一个完整的ebo集群，其中scytonemin单体也被分泌到周质。结果表明，ebo簇在出口的scytonemin单体的周质的最终氧化二聚体的ScyE。推而广之，ebo基因簇可能在许多细菌门的代谢物转运中发挥类似的作用。我们讨论了这种作用的基础上的结构和系统发育的考虑的ebo proteins.IMPORTANCE阐明scytonemin的生化和遗传基础构成了一个有趣的挑战，因为它的独特的结构和不寻常的事实，它是部分合成的周质空间的潜在机制。我们的工作点的ebo基因簇，与蓝细菌的scytonemin操纵子，作为负责排泄scytonemin中间体从细胞质到周质在生物合成。很少有保守的系统已被描述，促进小分子的膜易位。由于ebo簇在大量的细菌和藻类中是很保守的，但缺乏对其潜在功能的了解，我们的研究结果表明，小分子穿过质膜的易位可能是其在微生物中的一般作用。
Scytonemin is a dimeric indole-phenol sunscreen synthesized by some cyanobacteria under conditions of exposure to UVA radiation. While its biosynthetic pathway has been elucidated only partially, comparative genomics reveals that the scytonemin operon often contains a cluster of five highly conserved genes (ebo cluster) of unknown function that is widespread and conserved among several bacterial and algal phyla. We sought to elucidate the function of the ebo cluster in the cyano-bacterium Nostoc punctiforme by constructing and analyzing in-frame deletion mutants (one for each ebo gene and one for the entire cluster). Under conditions of UVA induction, all ebo mutants were scytoneminless, and all accumulated a single compound, the scytonemin monomer, clearly implicating all ebo genes in scytonemin production. We showed that the scytonemin monomer also accumulated in an induced deletion mutant of scyE, a non-ebo scytonemin gene whose product is demonstrably targeted to the periplasm. Confocal autofluorescence microscopy revealed that the accumulation was confined to the cytoplasm in all ebo mutants but that that was not the case in the scyE deletion, with an intact ebo cluster, where the scytonemin monomer was also excreted to the periplasm. The results implicate the ebo cluster in the export of the scytonemin monomer to the periplasm for final oxidative dimerization by ScyE. By extension, the ebo gene cluster may play similar roles in metabolite translocation across many bacterial phyla. We discuss potential mechanisms for such a role on the basis of structural and phylogenetic considerations of the ebo proteins.IMPORTANCE Elucidating the biochemical and genetic basis of scytonemin constitutes an interesting challenge because of its unique structure and the unusual fact that it is partially synthesized in the periplasmic space. Our work points to the ebo gene cluster, associated with the scytonemin operon of cyanobacteria, as being responsible for the excretion of scytonemin intermediates from the cytoplasm into the periplasm during biosynthesis. Few conserved systems have been described that facilitate the membrane translocation of small molecules. Because the ebo cluster is well conserved among a large diversity of bacteria and algae and yet insights into its potential function are lacking, our findings suggest that translocation of small molecules across the plasma membrane may be its generic role across microbes.