生物谷報(bào)道：日前，美國科學(xué)家發(fā)現(xiàn)一種巨大無比的細(xì)菌（Epulopiscium sp）,，其體積有一粒鹽大,，是大腸桿菌體積的1000倍，肉眼清晰可見,。這樣龐大的身軀可能與它自身基因組有數(shù)以萬計(jì)的拷貝有關(guān),。
這種細(xì)菌共生在澳大利亞大堡礁一種熱帶魚的腸子當(dāng)中。研究人員表示,，通常細(xì)菌細(xì)胞內(nèi)的基因組只有幾十個(gè)到幾百個(gè)拷貝,，但是這種巨型細(xì)菌卻含有上萬套拷貝。大部分細(xì)菌體型小,，結(jié)構(gòu)簡單不能像真核生物一樣獲取營養(yǎng),，擁有各種細(xì)胞器，形成較大體型,，因此細(xì)菌只能保持較小的體型利于自身代謝進(jìn)行,。但是這種巨型細(xì)菌利用其大量的基因組拷貝，能夠及時(shí)在細(xì)胞各處合成大量RNA和蛋白質(zhì)使生命活動(dòng)順利完成,。
相關(guān)研究結(jié)果發(fā)表在5月6日的《美國國家科學(xué)院院刊》上,。（生物谷www.bioon.com）
生物谷推薦原始出處：
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.
