Mendell Jennifer E, Clements Kendall D, Choat J Howard, Angert Esther R
Department of Microbiology, Cornell University, Ithaca, NY 14853, USA.
Proc Natl Acad Sci U S A. 2008 May 6;105(18):6730-4. doi: 10.1073/pnas.0707522105. Epub 2008 Apr 29.
Cells rely on diffusion to move metabolites and biomolecules. Diffusion is highly efficient but only over short distances. Although eukaryotic cells have broken free of diffusion-dictated constraints on cell size, most bacteria and archaea are forced to remain small. Exceptions to this rule are found among the bacterial symbionts of surgeonfish; Epulopiscium spp. are cigar-shaped cells that reach lengths in excess of 600 mum. A large Epulopiscium contains thousands of times more DNA than a bacterium such as Escherichia coli, but the composition of this DNA is not well understood. Here, we present evidence that Epulopiscium contains tens of thousands of copies of its genome. Using quantitative, single-cell PCR assays targeting single-copy genes, we have determined that copy number is positively correlated with Epulopiscium cell size. Although other bacteria are known to possess multiple genomes, polyploidy of the magnitude observed in Epulopiscium is unprecedented. The arrangement of genomes around the cell periphery may permit regional responses to local stimuli, thus allowing Epulopiscium to maintain its unusually large size. Surveys of the sequences of single-copy genes (dnaA, recA, and ftsZ) revealed genetic homogeneity within a cell consistent with only a small amount ( approximately 1%) of the parental DNA being transferred to the next generation. The results also suggest that the abundance of genome copies in Epulopiscium may allow for an unstable genetic feature, a long mononucleotide tract, in an essential gene. With the evolution of extreme polyploidy and large cell size, Epulopiscium has acquired some of the advantages of eukaryotic cells.
细胞依靠扩散作用来移动代谢物和生物分子。扩散作用效率很高，但仅在短距离内有效。尽管真核细胞已摆脱了扩散对细胞大小的限制，但大多数细菌和古细菌仍被迫保持较小的体积。栉齿刺尾鱼的细菌共生体是这一规则的例外；费氏刺骨鱼菌属的细胞呈雪茄状，长度超过600微米。一个大型的费氏刺骨鱼菌所含的DNA比大肠杆菌等细菌多出数千倍，但其DNA的组成尚不清楚。在此，我们提供证据表明费氏刺骨鱼菌含有其基因组的数万份拷贝。通过针对单拷贝基因的定量单细胞PCR分析，我们确定拷贝数与费氏刺骨鱼菌的细胞大小呈正相关。尽管已知其他细菌拥有多个基因组，但在费氏刺骨鱼菌中观察到的如此大规模的多倍体现象却是前所未有的。基因组在细胞周边的排列方式可能允许对局部刺激做出区域反应，从而使费氏刺骨鱼菌能够维持其异常大的体积。对单拷贝基因（dnaA、recA和ftsZ）序列的调查显示，细胞内的基因具有同质性，这与只有少量（约1%）的亲代DNA传递给下一代是一致的。研究结果还表明，费氏刺骨鱼菌中丰富的基因组拷贝数可能使得一个必需基因中存在不稳定的遗传特征，即长的单核苷酸序列。随着极端多倍体和大细胞大小的进化，费氏刺骨鱼菌获得了一些真核细胞的优势。
