生物谷报道：日前，美国科学家发现一种巨大无比的细菌（Epulopiscium sp），其体积有一粒盐大，是大肠杆菌体积的1000倍，肉眼清晰可见。这样庞大的身躯可能与它自身基因组有数以万计的拷贝有关。
这种细菌共生在澳大利亚大堡礁一种热带鱼的肠子当中。研究人员表示，通常细菌细胞内的基因组只有几十个到几百个拷贝，但是这种巨型细菌却含有上万套拷贝。大部分细菌体型小，结构简单不能像真核生物一样获取营养，拥有各种细胞器，形成较大体型，因此细菌只能保持较小的体型利于自身代谢进行。但是这种巨型细菌利用其大量的基因组拷贝，能够及时在细胞各处合成大量RNA和蛋白质使生命活动顺利完成。
相关研究结果发表在5月6日的《美国国家科学院院刊》上。（生物谷www.bioon.com）
生物谷推荐原始出处：
PNAS ,May 6, 2008 ,vol. 105,no. 18 ，pnas.0707522105
Extreme polyploidy in a large bacterium
Jennifer E. Mendell*, Kendall D. Clements, J. Howard Choat, and Esther R. Angert*,
*Department of Microbiology, Cornell University, Ithaca, NY 14853; School of Biological Sciences, University of Auckland, Auckland 1142, New Zealand; and School of Marine and Tropical Biology, James Cook University, Townsville, Queensland 4811, Australia
Edited by James M. Tiedje, Michigan State University, East Lansing, MI, and approved March 11, 2008 (received for review August 10, 2007)
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 µm. 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 (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.
