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Nature: 固氮出现在时间很早

标签:进化 生物地理
摘要 : “固氮酶”(生物通过它能固定大气中的氮)的酶的出现显然是生命史中的一大事件,但不太确定的是这个事件是什么时候出现的。

名为“固氮酶”(生物通过它能固定大气中的氮)的酶的出现显然是生命史中的一大事件,但不太确定的是这个事件是什么时候出现的。Eva Stüeken等人确定了距今32亿年和27.5亿年间的海洋和河流沉积岩中的氮同位素比例。这些比例最容易通过生物固氮得到解释,而在固氮过程中钼可能是一个辅因子。这表明固氮至少是距今32亿年前出现的,并且否定了以前所提出的认为海洋中的钼在“大氧化事件”之前极少的观点。


Isotopic evidence for biological nitrogen fixation by molybdenum-nitrogenase from 3.2 Gyr


Innate immunity serves as the first line of defence against invading pathogens such as bacteria and viruses1. Toll-like receptors (TLRs) are examples of innate immune receptors, which sense specific molecular patterns from pathogens and activate immune responses2. TLR9 recognizes bacterial and viral DNA containing the cytosine–phosphate–guanine (CpG) dideoxynucleotide motif3, 4. The molecular basis by which CpG-containing DNA (CpG-DNA) elicits immunostimulatory activity via TLR9 remains to be elucidated. Here we show the crystal structures of three forms of TLR9: unliganded, bound to agonistic CpG-DNA, and bound to inhibitory DNA (iDNA). Agonistic-CpG-DNA-bound TLR9 formed a symmetric TLR9–CpG-DNA complex with 2:2 stoichiometry, wheras iDNA-bound TLR9 was a monomer. CpG-DNA was recognized by both protomers in the dimer, in particular by the amino-terminal fragment (LRRNT–LRR10) from one protomer and the carboxy-terminal fragment (LRR20–LRR22) from the other. The iDNA, which formed a stem-loop structure suitable for binding by intramolecular base pairing, bound to the concave surface from LRR2–LRR10. This structure serves as an important basis for improving our understanding of the functional mechanisms of TLR9.

来源: Nature 浏览次数:0


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