The Earth is a unique planet, which has been highly evolved, diversified and complicated through geologic time, and underwent many key events, including giant impact, magma ocean, core formation, large-scale mantle differentiation and late heavy bombardment, especially in its dawn. But, our knowledge of early Earth is limited due to the lack of the Hadean supracrustal rocks. The supracrustal rocks with the Eoarchean ages provide key evidence for the Earth's early evolution, but few supracrustal rocks have been comprehensively investigated. Therefore, we mapped in seven areas of the Saglek Block, northern Labrador, where ancient supracrustal sequences are interleaved with a diverse assemblage of orthogneisses. Early studies suggested that some of them have the Mesoarchean ages because of the lack of the Mesoarchean Saglek dyke, but we found the Saglek dykes in the areas to recognize the Eoarchean Nulliak supracrustal rocks and Uivak Gneiss in all the areas. Recent reassessment of U-Pb dating and cathodoluminescence observation of zircons from the oldest suites of the Uivak Gneiss showed that the Uivak Gneiss has the Eoarchean age, N 3.95 Ga, and forms the Iqaluk-Uivak Gneiss series. Because our geological survey clearly showed that the Iqaluk-Uivak Gneisses were intruded into the Nulliak supracrustal belts, the Nulliak supracrustal rocks are the oldest supracrustal rock in the world. The supracrustal belts consist of piles of fault-bounded blocks, which are composed of the ultramafic rocks, mafic rocks and sedimentary rocks in ascending order, similar to modern ocean plate stratigraphy (OPS). In addition, small-scale duplex structures are found over the areas. The presence of duplex structure and OPS indicates that the N 3.95 Ga Nulliak supracrustal belts originate from an accretionary complex. The presence of the accretionary complex, ophiolite and granitic continental crust provides the oldest evidence for the plate tectonics on the early Earth.
sp. strain SYK-6 converts four stereoisomers of arylglycerol-β-guaiacyl ether into achiral β-hydroxypropiovanillone (HPV) via three stereospecific reaction steps. Here, we determined the HPV catabolic pathway and characterized the HPV catabolic genes involved in the first two steps of the pathway. In SYK-6 cells, HPV was oxidized to vanilloyl acetic acid (VAA) via vanilloyl acetaldehyde (VAL). The resulting VAA was further converted into vanillate through the activation of VAA by coenzyme A. A syringyl-type HPV analog, β-hydroxypropiosyringone (HPS), was also catabolized via the same pathway. SLG_12830 (), which belongs to the glucose-methanol-choline oxidoreductase family, was isolated as the HPV-converting enzyme gene. An mutant completely lost the ability to convert HPV and HPS, indicating that is essential for the conversion of both the substrates. HpvZ produced in oxidized both HPV and HPS and other 3-phenyl-1-propanol derivatives. HpvZ localized to both the cytoplasm and membrane of SYK-6 and used ubiquinone derivatives as electron acceptors. Thirteen gene products of the 23 aldehyde dehydrogenase (ALDH) genes in SYK-6 were able to oxidize VAL into VAA. Mutant analyses suggested that multiple ALDH genes, including SLG_20400, contribute to the conversion of VAL. We examined whether the genes encoding feruloyl-CoA synthetase () and feruloyl-CoA hydratase/lyase ( and ) are involved in the conversion of VAA. Only FerA exhibited activity toward VAA; however, disruption of did not affect VAA conversion. These results indicate that another enzyme system is involved in VAA conversion. Cleavage of the β-aryl ether linkage is the most essential process in lignin biodegradation. Although the bacterial β-aryl ether cleavage pathway and catabolic genes have been well documented, there have been no reports regarding the catabolism of HPV or HPS, the products of cleavage of β-aryl ether compounds. HPV and HPS have also been found to be obtained from lignin by chemoselective catalytic oxidation by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone/-butyl nitrite/O, followed by cleavage of the β-aryl ether with zinc. Therefore, value-added chemicals are expected to be produced from these compounds. In this study, we determined the SYK-6 catabolic pathways for HPV and HPS and identified the catabolic genes involved in the first two steps of the pathways. Since SYK-6 catabolizes HPV through 2-pyrone-4,6-dicarboxylate, which is a building block for functional polymers, characterization of HPV catabolism is important not only for understanding the bacterial lignin catabolic system but also for lignin utilization.
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