Nostoc sp. PCC 7120 are filamentous cyanobacteria capable of both oxygenic photosynthesis and nitrogen fixation, with the latter taking place in specialized cells known as heterocysts that terminally differentiate from vegetative cells under conditions of nitrogen starvation. Cyanobacteria have existed on earth for more than 2 billion years and are thought to be responsible for oxygenation of the earth's atmosphere. Filamentous cyanobacteria such as Nostoc sp. PCC 7120 may also represent the oldest multicellular organisms on earth that undergo cell differentiation. Pentapeptide repeat proteins (PRPs), which occur most abundantly in cyanobacteria, adopt a right‐handed quadrilateral β‐helical structure, also referred to as a repeat five residue (Rfr) fold, with four‐consecutive pentapeptide repeats constituting a single coil in the β‐helical structure. PRPs are predicted to exist in all compartments within cyanobacteria including the thylakoid and cell‐wall membranes as well as the cytoplasm and thylakoid periplasmic space. Despite their intriguing structure and importance to understanding ancient cyanobacteria, the biochemical function of PRPs in cyanobacteria remains largely unknown. Here we report the crystal structure of Alr1298, a PRP from Nostoc sp. PCC 7120 predicted to reside in the cytoplasm. The structure displays the typical right‐handed quadrilateral β‐helical structure and includes a four‐α‐helix cluster capping the N‐terminus and a single α‐helix capping the C‐terminus. A gene cluster analysis indicated that Alr1298 may belong to an operon linked to cell proliferation and/or thylakoid biogenesis. Elevated alr1298 gene expression following nitrogen starvation indicates that Alr1298 may play a role in response to nitrogen starvation and/or heterocyst differentiation.

中文翻译：

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Alr1298的晶体结构，一种来自蓝细菌Nostoc sp。的五肽重复蛋白。PCC 7120，以2.1Å分辨率确定。

Nostoc sp。PCC 7120是丝状蓝细菌，能够同时进行氧气的光合作用和固氮，后者发生在称为异种囊的特殊细胞中，该异种囊在缺氮条件下与营养细胞最终分化。蓝细菌已经存在于地球上超过20亿年，并且被认为是造成地球大气氧化的原因。丝状蓝细菌如Nostocsp。PCC 7120也可能代表地球上经历细胞分化的最古老的多细胞生物。在蓝藻中最常见的五肽重复蛋白（PRP）具有右旋四边形β螺旋结构，也称为重复五残基（Rfr）折叠，其中四个连续的五肽重复序列构成了单个螺旋。 β螺旋结构。预计PRPs存在于蓝细菌的所有隔室中，包括类囊体和细胞壁膜以及细胞质和类囊体周质空间。尽管它们的结构和对理解古代蓝细菌的重要性，但是PRPs在蓝细菌中的生化功能仍然未知。在这里，我们报告了Nostoc的PRP Alr1298的晶体结构sp。预计PCC 7120驻留在细胞质中。该结构显示了典型的右手四边形β螺旋结构，并包括一个覆盖N端的四个α螺旋簇和一个覆盖C端的单个α螺旋。基因簇分析表明，Alr1298可能属于与细胞增殖和/或类囊体生物发生有关的操纵子。缺氮后alr1298基因表达升高表明Alr1298可能在响应缺氮和/或异种囊分化中起作用。