Life Without DNA Replication Origins

所有细胞都含有生物体DNA的完整副本，并将其打包成染色体。在细胞分裂之前，它们的染色体必须复制。这一过程称为DNA复制，始于染色体上称为复制起点的特定位置。细菌只有一个复制起点，但拥有大染色体的生物，如人类，需要多个复制起点。我们已经发现起源是不必要的，没有起源的细胞可以比正常细胞生长得更快(Hawkins等人。2013自然503,544-7)。我们对DNA复制的研究是在古细菌成员Haloferax Volcanii中进行的。生命之树分为三类：真核生物、细菌和古生菌。古生菌是以生活在酸池和盐湖等极端条件下而闻名的微生物。Haloferax Volcanii来自死海，我们之所以选择它，是因为在古生物中进行DNA复制的酶与真核生物中使用的酶相似。Haloferax Volcanii使用几个来源来复制它的染色体。但当所有这些起源都被移除时，细胞实际上生长得更快。在人体上做这些实验是不可能的。当真核生物或细菌的起源被消除时，它会阻止DNA复制并导致死亡。那么，盐生植物火山是如何存活的呢？没有起源的细胞使用一种称为重组的替代方法来启动DNA复制。重组是DNA修复的一种形式，它用于修复染色体上的断裂。我们发现，重组在染色体上的随机位置开始DNA复制，而不是限制在有限数量的起始点，这使得这个过程更快。但这提出了一个问题：如果使用重组的替代过程更有效，为什么会有复制来源？我们提出，盐生植物火山的起源是自私的基因。自私的起源不需要为宿主细胞提供任何优势，但它们增加了自己的频率，因为它们劫持了DNA复制机制。在进化过程中，宿主细胞已经找到了一种调节起源的方法，这使得它们能够协调DNA复制的时间和细胞分裂。在像人类这样的复杂生物中，起源已经与细胞过程结合在一起，删除它们而不产生有害影响是不可能的。我们在Haloferax Volcanii中发现的不同寻常的DNA复制模式与癌症相似。Haloferax Volcanii的染色体有许多副本，这被称为多倍体，当复制和细胞分裂不再协调时，它可以帮助它存活下来。许多癌细胞在控制DNA复制的基因上发生了突变，多倍体是癌症的常见特征。不协调复制的另一个后果是癌细胞比普通细胞生长得更快。这种加速的生长让人想起无起源的Haloferax火山II，它使用另一种复制模式来超过其他细胞。我们对死海微生物的研究表明，在不寻常的生物体中测试长期持有的假设可以得出多么令人惊讶的结果。但它给我们带来的问题和答案一样多：-这种DNA复制的替代机制是如何工作的？它对细胞有负面影响吗？-盐生植物火山一直在使用它吗？如果不是，它是如何通过正常的复制来控制的？-最重要的是，为什么在没有起源的情况下，当其他细胞会死亡的时候，盐生植物会生长得更快？我们认为这种生物的两个方面是关键：重组和多倍体。我们将使用我们开发的遗传和生化工具的组合，来研究重组和多倍体对复制的影响。这项工作对所有生物体中的DNA复制都有影响--它可能有助于我们理解癌细胞是如何逃避复制检查的，并让我们深入了解DNA是如何在“自私”起源进化之前复制的。

All cells contain a complete copy of the organism's DNA, packaged into chromosomes. Before cells can divide, their chromosomes must be duplicated. This process is called DNA replication and begins at specific locations on the chromosome called replication origins. Bacteria have a single replication origin but organisms with large chromosomes, such as humans, need many origins. We have found that origins are unnecessary, and that cells without them can grow faster than normal (Hawkins et al. 2013 Nature 503, 544-7).Our research on DNA replication was carried out in Haloferax volcanii, a member of the archaea. The tree of life is split into three groups: eukaryotes, bacteria and archaea. Archaea are microbes renowned for living in extreme conditions such as acid pools and salt lakes. Haloferax volcanii comes from the Dead Sea, we chose it because the enzymes that carry out DNA replication in archaea are similar to those used in eukaryotes.Haloferax volcanii uses several origins to replicate its chromosome. But when all of these origins are removed, the cells actually grow faster. Doing these experiments in humans would be impossible. When origins are eliminated from eukaryotes or bacteria, it prevents DNA replication and leads to death. So how is Haloferax volcanii able to survive?Cells without origins use an alternative method called recombination to start DNA replication. Recombination is a form of DNA repair, it is used to mend breaks in the chromosome. We found that recombination starts DNA replication at random locations on the chromosome, instead of being restricted to a limited number of origins, and this makes the process faster. But this poses a puzzle: if the alternative process using recombination is more efficient, why have replication origins at all?We propose that origins in Haloferax volcanii are selfish genes. Selfish origins need not offer any advantage to the host cell, but they increase their own frequency because they have hijacked the DNA replication machinery. Over the course of evolution, host cells have found a way to regulate origins and this has allowed them to coordinate the timing of DNA replication with cell division. In complex organisms such as humans, origins have become integrated with cellular processes and it is impossible to delete them without detrimental effects.The unusual mode of DNA replication we have discovered in Haloferax volcanii has parallels with cancer. Haloferax volcanii has many copies of its chromosome, this is called polyploidy and helps it to survive when replication and cell division are no longer coordinated. Many cancer cells have mutations in the genes that control DNA replication, and polyploidy is a common feature of cancer. Another consequence of uncoordinated replication is that cancer cells grow faster than ordinary cells. Such accelerated growth is reminiscent of origin-less Haloferax volcanii, which use an alternative mode of replication to outpace other cells.Our work on a microbe from the Dead Sea has shown how surprising results can come from testing long-held assumptions in unusual organisms. But it has given us as many questions as answers:- How does this alternative mechanism of DNA replication work? Does it have negative consequences for the cell?- Does Haloferax volcanii use it all the time? If not, how is it kept in check by 'normal' replication?- Above all, why does Haloferax volcanii grow faster without origins, when other cells would die? We believe that two aspects of this organism are key: recombination and polyploidy.We will use a combination of genetic and biochemical tools that we have developed, to examine the effects of recombination and polyploidy on replication. This work has implications for DNA replication in all organisms - it may contribute to our understanding of how cancer cells evade the checks on replication, and give an insight into how DNA was replicated before the evolution of 'selfish' origins.