Life Without DNA Replication Origins

所有的细胞都含有生物体DNA的完整拷贝，被包装成染色体。在细胞分裂之前，它们的染色体必须复制。这个过程被称为DNA复制，从染色体上被称为复制起点的特定位置开始。细菌只有一个复制起点，但是像人类这样的具有大染色体的生物体需要多个复制起点。我们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 et al. 2013 Nature 503,544 -7）。我们的DNA复制研究是在古细菌的成员火山盐藻中进行的。生命之树分为三大类：真核生物、细菌和古细菌。古生菌是一种以生活在极端条件下而闻名的微生物，比如酸性水池和盐湖。火山盐藻来自死海，我们选择它是因为古细菌中进行DNA复制的酶与真核生物中使用的酶相似。火山盐黄貂鱼使用几个起源来复制它的染色体。但当所有这些来源都被移除时，细胞实际上生长得更快。在人类身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。那么火山盐藻是如何存活下来的呢？没有起源的细胞使用一种叫做重组的替代方法来开始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，而不是局限于有限数量的起源，这使得这个过程更快。但这带来了一个难题：如果使用重组的替代过程更有效，那么为什么会有复制起源呢？我们认为火山盐藻的起源是自私的基因。自私的起源并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复制与细胞分裂的时间。在像人类这样的复杂生物体中，起源已经与细胞过程结合在一起，不可能在不产生有害影响的情况下删除它们。我们在火山藻中发现的不寻常的DNA复制模式与癌症有相似之处。火山盐黄貂鱼的染色体有很多拷贝，这被称为多倍体，这有助于它在复制和细胞分裂不再协调时存活下来。许多癌细胞在控制DNA复制的基因中有突变，多倍体是癌症的一个共同特征。不协调复制的另一个后果是癌细胞比普通细胞生长得快。这种加速的生长让人想起原生的火山盐藻，它使用另一种复制模式来超越其他细胞。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存在的假设会产生令人惊讶的结果。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？- Haloferax volcanii一直使用它吗？如果不是，它是如何被“正常”复制所控制的？最重要的是，为什么没有起源的火山盐藻生长得更快，而其他细胞会死亡？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，来检查重组和多倍体对复制的影响。这项工作对所有生物体的DNA复制都有意义——它可能有助于我们理解癌细胞如何逃避对复制的检查，并深入了解DNA在“自私”起源进化之前是如何复制的。

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.