決定氨基酸的密碼子通常不止一個,，三聯(lián)體密碼子中第三個核苷酸的置換往往并不影響蛋白質(zhì)在該處的氨基酸組成,，因此研究人員認為最終得到的蛋白質(zhì)結(jié)構(gòu)和功能也不會產(chǎn)生變化，這樣的突變被稱為“沉默突變”,。
但是現(xiàn)在,，這一細胞生物學理論卻有可能被顛覆。美國食物和藥品管理局的Chava Kimchi-Sarfaty和他的同事日前在《科學》雜志在線報告說,，這種沉默突變并不“沉默”,，在某種情況下可以決定蛋白質(zhì)的最終表現(xiàn),。
沉默突變不影響氨基酸順序可能是因為蛋白質(zhì)是由三個核苷酸編碼的，每個核苷酸都負責在蛋白質(zhì)鏈上加一個特定的氨基酸,。突變的核苷酸也可能依然添加上同樣的氨基酸,。因此氨基酸組成和蛋白質(zhì)結(jié)構(gòu)也被認為不會變化。
然而,，每隔一段時間,，就會有一些數(shù)據(jù)不符合這個假設，比如一種叫做多藥物排斥-1(MDR-1)的基因,。該基因已經(jīng)被發(fā)現(xiàn)在人類癌細胞中頻繁地發(fā)生特殊的沉默突變,。MDR-1編碼的蛋白P-gp可以把化學療法的藥物排出癌細胞，從而使藥物失效,。對為什么叫做C3435T的沉默突變比對癌細胞存活沒有作用的突變發(fā)生得更頻繁,，研究人員感到好奇。
Kimchi-Sarfaty的小組得到了以下幾個版本的細胞系：正常MDR-1基因細胞系,，攜帶C3435T 突變的MDR-1基因細胞系,，攜帶常伴隨C3435T出現(xiàn)的兩個突變(一個是沉默突變，另外一個是非沉默突變,，但對蛋白的功能沒有影響)的MDR-1基因細胞系,，和一些同時攜帶兩種或三種突變的MDR-1細胞系。
研究結(jié)果顯示,，各種突變單獨看來似乎都不受影響：每個版本的基因變量編碼的P-gp蛋白都能精確地把藥物排出細胞,。但是攜帶了C3435T突變和另外一兩種突變的MDR-1基因的細胞可以更好地使癌細胞擺脫藥物，從而使細胞活得更久,。然而如果突變的P-gp擁有和正常的P-gp相同的氨基酸順序的話,，怎么會發(fā)生這種情況呢？
令研究人員吃驚的是,，一項生化測試顯示,，突變的P-gp的三維結(jié)構(gòu)略有不同。Kimchi-Sarfaty表示,，沉默突變也許發(fā)生在細胞不常使用的三聯(lián)體核苷酸上,，這些三聯(lián)體核苷酸可以減緩細胞的蛋白質(zhì)制造機制。類似Silly String牌噴彩摩絲以不同速度噴射出去的設計,，氨基酸鏈三維結(jié)構(gòu)的折疊也是由速度決定的,，較慢的折疊可以導致蛋白質(zhì)最終形式的改變。細胞可能可以彌補一次沉默突變,，但對那些多重的極少使用的三聯(lián)體密碼子則不行,。
波特蘭俄勒岡健康和科學大學的細胞生物學家William Skach表示，這是一個極具爭議的結(jié)論，沉默突變可能有這樣的效果是“一個全新的概念”,。他預言,，更多的研究人員將會開始研究沉默突變。
英文原文：
Noise.
The normal gene for P-gp (left) and a triple mutant (right) make the same amount of protein in the cell. But the protein's structure and function are altered.
Credit: C. Kimchi-Sarfaty et al.
The Sound of a Silent Mutation
Another dogma in cell biology seems about to be toppled: If a mutation in a gene doesn't change the basic sequence of building blocks, then it has no effect. Chava Kimchi-Sarfaty of the U.S. Food and Drug Administration in Bethesda, Maryland, and colleagues report online this week in Science that such "silent mutations" can, under certain circumstances, determine how well a final protein performs--an "extremely provocative" result, says cell biologist William Skach of Oregon Health & Science University in Portland.
Silent mutations occur when the change of a single DNA nucleotide within a protein-coding portion of a gene does not affect the sequence of amino acids that make up the gene's protein. That's possible because proteins are encoded by "triplets" of nucleotides, each responsible for adding a particular amino acid to the protein chain. A change in one nucleotide, however, doesn't always change the triplet's meaning; the mutated triplet may still add the same amino acid. And when the amino acids of a protein stay the same, researchers believed, so do its structure and function.
But every once in a while, data crop up that don't make sense; for example, a gene called multidrug resistance-1 (MDR-1) has been found to frequently have a particular silent mutation in human cancer cells. MDR-1 produces P-gp, a protein that pumps chemotherapy drugs out of cancer cells, thus making the drugs useless. Researchers wondered why the silent mutation, called C3435T, showed up much more frequently than expected for a change that doesn't have an effect on the cancer cells' survival.
Kimchi-Sarfaty's team made cell lines that had either the normal MDR-1 gene, a version with the C3435T mutation, versions with either of two other mutations known to occur sometimes along with C3435T (one of them silent as well, the other nonsilent but without an effect on protein function), and versions with various combinations of two or three of the mutations.
They found that the mutations individually appeared to have no effect: The P-gp proteins encoded by each gene variant were just as proficient at pumping drugs out of cells. But cells with the MDR-1 gene containing the C3435T mutation plus one or two of the other two mutations did a much better job of ridding cancer cells of the drugs, allowing the cells to live another day.
How is this possible if the variant P-gp has the same string of amino acids as the normal one? To the researchers' surprise, a biochemical test suggested that the mutant P-gp has a slightly different three-dimensional shape. Perhaps, Kimchi-Sarfaty says, the silent mutations reside in nucleotide triplets that cells don't use very often, which could slow down the cell's proteinmaking machinery. Like designs made with Silly String spraying out at different velocities, the folding of an amino acid chain into a 3D structure is somewhat speed-dependent, and slower production could cause the protein to take an altered final form. The cell might be able to compensate for one silent mutation but not for multiple rarely used triplets.
The idea that silent mutations might have such effects is "an entirely new concept," Skach says. His prediction: More researchers will start listening to what silent mutations have to say.
