兩個基因共轉(zhuǎn)導的頻率

1、兩個基因共轉(zhuǎn)導的頻率Both bacteria and bacteriophages(Bacterial viruses) demonstrate mechanisms by which genetic recombination occurs, processes that can serve as the basis for genetic mappingAs a result, both groups have been the subject of extensive analysis. KEY CONCEPTSfertility factor (F) permits bacteria

2、l cells to transfer DNA to other bacteria cells through conjugationF can be integrated or cytoplasmicwhen integrated, F can transfer host chromosome markers through conjugationbacteriophages can transfer DNA from one bacterial cell to another in two ways . generalized transduction is the transfer of

3、 randomly incorporated bacterial chromosome fragmentsspecialized transduction is the transfer of specific genes near phage integration sitesthese methods of gene transfer facilitate construction of detailed maps of bacterial genomesBacteria also contain extrachromosomal DNA in the form of plasmids,w

4、hich can house a fertility factor that plays a key role in genetic recombination.Some E. coli cells carry a circular plasmid called the sex factor, F. Factor F can exist in a free state in the cytoplasm or it can be integrated into the circular bacterial chromosome. In the nonintegrated state, F can

5、 pass into F-free cells during cell conjugation.When F is integrated, the bacterial chromosome is transferred linearly to an F-free cell during conjugation. Bacteriophages can transfer (transduce) DNA from one bacterial cell to another.During generalized transduction, random chromo-some fragments ar

6、e incorporated into the heads of phages and transferred to other cells by infection. During specialized transduction, specific genes near the phage-integration sites on the bacterial chromosome are mistakenly incorporated into the phage genome and transferred to other cells by infection. DNA from th

7、e medium can enter a bacterial cell and integrate into the chromosome, thereby transforming the genotype. The different methods of gene transfer in bacteria generate partial diploids that permit the study of recombination and gene interaction. 9.1 Bacterial Mutation and Growth Genetic 9.2 Recombinat

8、ion in Bacteria: Conjugation9.3 Rec Proteins and Bacterial Recombination F Factors and Plasmids Bacterial Transformation The Genetic Study of BacteriophagesPhage T4: Structure and Life CycleThe Plaque Assay, Lysogeny Transduction: Virus-Mediated Bacterial . DNA TransferThe Lederberg-Zinder Experimen

9、t The Nature of Transduction Transduction and Mapping Intergenic Recombination and Mapping in Bacteriophages9.1 Bacterial Mutation and GrowthTo do genetics, we need phenotypic variation.Prior to1943 The adaptation hypothesis, spontaneous mutationsMorphology/resistance/prototroph(autotroph)/auxotroph

10、 and so on fluctuation test, Bacterial Phenotypesstrainsprototrophs = wild typegrow on minimal mediumauxotrophs = mutantsdo not grow on minimal mediumnutritioncarbon sourceresistant mutantsprototroph auxotroph medium minimal,compelet, and supplemented selectionBacterial Phenotypes To do genetics, we

11、 need phenotypic variationDeduce the genotypes of the following four E. coli strains:Bacterial PhenotypesFIGURE 9-1 Typical bacterial population growth curve showing the initial lag phase, the subsequent log phase where exponential growth occurs, and the stationary phase that occurs when nutrients a

12、re exhaustedCells grown in liquid medium can be quantified by transferring them to semisolid medium in a Petri dishWe need only select the dish in which the number of colonies can be counted accurately. Because each colony arose from a single bacterium, the number of colonies multiplied by the dilut

13、ion factor represents the number of bacteria in each milliliter of the initial inoculum used to start the serial dilutions. In Figure 9-2, the rightmost dish has 15 colonies. The dilution factor for a 10-5 dilution is 105. Therefore, the initial number of bacteria is 15 105 per milliliter10-3 10 -4

14、10 -59.2 Genetic Recombinationin Bacteria: ConjugationLederberg and Tatum 1946 coli K12. Strain A required methionine and biotin in order to grow, while strain Strain B required threonine, leucine, and thiamine (Figure 9-3). Neither strain would grow on minimal medium. The two strains were first gro

15、wn separately in supplemented media, and cells from both were mixed and grown together for several more generations and then plated on minimal medium. Any bacterial cells that grew on minimal medium were prototrophs. highly improbable spontaneous mutation at two or threeConjugation Lederberg & Tatum

16、, 1946 auxotrophs in E. coli auxotrophs mixed 107 cells plated on minimal medium reversion vs. ? shown cell contact requiredIn this experiment, prototrophs were recovered at a rate of 1/107 (10-7) cells plated. The controls for this experiment involved separate plating of cells from strains A and B

17、on minimal medium. No prototrophs were recovered. Based on these observations, Lederberg and Tatum proposed that genetic exchange had occurred.FIGURE 94 When strain A and B auxotrophs are grown in a common medium but separated by a filter, no recombination occurs and no prototrophs are produced.The

18、apparatus shown is a Davis U-tube.ParasexualstrainA:met-thr+leu+thi + 需甲硫氨酸 B:met+thr-leu-thi-需蘇氨酸，亮氨酸和硫胺 出現(xiàn)原養(yǎng)型菌落 不出現(xiàn)原養(yǎng)型菌落 說明菌株A和B在雜交中的作用不同，有受體和供體之分. A為遺傳物質(zhì)的供體(donor)，相當于雄性；而B為受體(recipient)，相當于雌性。含有鏈霉素的根本培養(yǎng)基Str處理但并不殺死只阻礙分裂繁殖 A StrB AB StrF+ and F- Bacteriainitial crosses of Lederberg and Tatum (Figu

19、re 9-3) can be designated as follows:STRAIN A F+DONORSTRAIN BFRECIPIENTIsolation of the F factor confirmed these conclusions.Like the bacterial chromosome, though distinct from it, the F factor has been shown to consist of a circular, double-stranded DNA molecule, equivalent to about 2 percent of th

20、e bacterial chromosome (about 945,000 nucleotide pairs). Contained in the F factor, among others, are 19 genes, the products of which are involved in the transfer of genetic information. These include those essential to the formation of the sex pilus.As we soon shall see, the F factor is in reality

21、an autonomous genetic unit referred to as a plasmid. However, in our historical coverage of its discovery, we will continue to refer to it as a factor.Conjugation Pilus needed for cell contact DNA synthesis needed for transferrolling circle replicationbegins at origin (ori)one strand nickedfertility

22、 geneConjugationThe F factor is replicated during transferMGA2e Fig. 4-10Hayes, et. al., 1950stransfer one-way, not reciprocaldonor and recipient strains (+ and strains) donor: F+ recipient: F-mating of F+ x F- 2 F+ F is F plasmidbacterial gene transfer rareTo summarize, an E. coli cell may or may n

23、ot contain the F factor. When this factor is present, the cell is able to form a sex pilus and potentially serves as a donor of genetic information. During conjugation, a copy of the F factor is almost always transferred from the F+ cell to the F recipient, converting it to the F+ state. The questio

24、n remained as to exactly why such a low proportion of cells involved in these matings (107) also results in genetic recombination. The answer awaited further experimentation.Hfr Bacteria and Chromosome MappingIn 1950, Cavalli-Sforza treated an F+ strain of E. coli K12 with nitrogen mustard, a chemic

25、al known to induce mutations. From these treated cells, he recovered a genetically altered strain of donor bacteria that underwent recombination at a rate of 1/104 (or 104), 1000 times more frequently than the original F+ strains. In 1953, Hayes isolated another strain that demonstrated an elevated

26、frequency. Both strains were designated Hfr, for high-frequency recombination. Because Hfr cells behave as donors, they are a special class of F+ cells.F+ F- F+ (low rate of recombination)Hfr F F (higher rate of recombination)In the mid-1950s, experimentation by Ellie Wollman and Francois Jacob expl

27、ained the difference between Hfr and F+Interrupted mating techniqueand mapping chromosome 8 no10azi ,no other15 70% azi 30% ton2025,It appeared that the chromosome of the Hfr bacterium was transferred linearly and that the gene order and distance between genes, as measured in minutes, could be predi

28、cted from such experiments (Figure 9-8). This information served as the basis for the first genetic map of the E. coli chromosome. Minutes in bacterial mapping are equivalent to map units in eukaryotes.mapping in E. coli by interrupted-matingdistance measured in time (min)Wollman and Jacob then repe

29、ated the same type of experiment with other Hfr strains, obtaining similar results with one important difference. While genes were always transferred linearly with time, as in their original experiment, which genes entered first and which followed later seemed to vary from Hfr strain to Hfr strain F

30、igure 9-9(a). The major difference between each strain was simply the point of origin (O) and the direction in which entry proceeded from that point Figure 9-9(b).Mapping gene by recombaniaton紫紅色菌落：lac+ade+ 780白色或粉紅色菌落：lac- ade+ 220Hfr: lac+ade+strs X F-: lac-ade-strr混合作用60min在含鏈霉素的基本培養(yǎng)基上涂板只有重組子能夠存活

31、：F-: ade+strr 1000影印到EMB培養(yǎng)基上兩者在根本培養(yǎng)基上都不能生長Lac- ade+ strrLac+ ade+ strrLac+ ade+ strs Lac- ade- strrLac- ade- strsLac+ ade+ strs Lac- ade- strr recombinantLac+ ade- strs重組頻率的計算RFlac-ade = *100% lac-ade+ lac+ade+lac-ade+ = *100% lac-ade+ade+ 用重組頻率RF所測得的基因距離與用中斷雜交技術(shù)以時間為單位的基因距離根本上是成正比的，大致是1分鐘相當于20%重組值，即

32、：1min=20cM = *100%=22%=22cM 220 1000 大腸桿菌的遺傳圖譜 根據(jù)中斷雜交實驗和基因重組實驗，以及其他基因定位技術(shù)的結(jié)果，已經(jīng)繪制出大腸桿菌的環(huán)形遺傳學圖，即基因連鎖圖，圖據(jù)單位為分鐘，以thr座位為起點0分鐘，總長度為100分鐘。圖中標出了常用的52個基因座位。Recombination in F+ x F Matings:A ReexaminationThe F State and MerozygotesIn 1959, during experiments with Hfr strains of E. coli, Edward Adelberg disco

33、vered that the F factor could lose its integrated status, causing the cell to revert to the F+ state (Figure 9-11, step 1). When this occurs, the F factor frequently carries several adjacent bacterial genes along with it (step 2). Adelberg labeled this condition F to distinguish it from F+ and Hfr.

34、F, like Hfr, is thus another special case of F+. This conversion is described as one from Hfr to F.Ffactor and sxe-duction1959年，Adel berg發(fā)現(xiàn)了一種新的F因子，并稱之為F因子(F-prime factor),這是帶有一小段細菌染色體基因的F因子。它能使F-變成F菌株，也能轉(zhuǎn)移細菌基因，形成局部二倍體，但是頻率較Hfr為低。如圖2. 利用F因子形成的局部二倍體，將供體細胞基因?qū)胧荏w的過程，叫做性導(sex-duction or F-duction)。一、F因子

35、F+與Hfr兩種菌株可以相互轉(zhuǎn)變，也就是說因子既可以插入到染色體中去，形成Hfr菌株，又可通過規(guī)那么的交換和剪切，從染色體上完整地游離下來形成F+菌株圖6-12，但是偶爾也會出現(xiàn)不規(guī)那么的環(huán)出，形成F因子攜帶一段相鄰的細菌染色體圖6-13。這種帶有插入細菌基因的環(huán)狀F因子是一個復(fù)制子，這種新的F因子稱為F因子F-factor。F因子在細菌染色體上插入位置不同而構(gòu)成不同的Hfr品系，由此可形成不同的F因子。它們各自攜帶細菌的不同基因，有的F因子攜帶著干個基因的一大段細菌的染色體。F與dgal或dbio顆粒不同，F(xiàn)攜帶細菌的基因，但并不減少本身的基因，如果本身的基因喪失，轉(zhuǎn)移就可能停頓了。此外F因

36、子也不存在蛋白質(zhì)外殼包裝的問題，所以它的長度不為包裝所限制，可以攜帶不同長度的細菌DNA片段。F因子和F+一樣是能感染的，并把F因子轉(zhuǎn)移給F-細胞，同時也能轉(zhuǎn)移細菌基因，其結(jié)果使F-變成F菌株，并形成局部二倍體。二、性導F因子轉(zhuǎn)移細菌基因不同于Hfr菌株。如比較以下兩個雜交結(jié)果：Hfr：thr+leu+strsF-：thr-leu-strr結(jié)果：篩選出F-：thr+leu+strr重組子F：thr+leu+strsF-：thr-leu-strr結(jié)果：篩選出F：thr+leu+strr重組子這兩個雜交一個是Hfr菌株，另一個是來源于它的F菌株。雜交中把混合培養(yǎng)物涂布在含鏈霉素的根本培養(yǎng)基上，第一

37、個雜交選出的菌株都是F-，因此不能將thr+leu+轉(zhuǎn)入F-菌株，而第二個雜交選出的菌株都帶有活性的F因子，能與其他F-菌株雜交，并能將thr+leu+轉(zhuǎn)入F-菌株，而且這些菌株也具有F因子，所以都是F+，仍具有感染能力。產(chǎn)生F因子的Hfr菌株仍保持單倍體狀態(tài)，當F因子轉(zhuǎn)入到受體細胞之后，由于引入了供體細胞的局部基因，從而構(gòu)成了局部二倍體。如圖6-13中Flac+可轉(zhuǎn)移到F-lac-后構(gòu)成Flac+/F-lac-局部二倍體。這種利用F因子將供體細胞的基因?qū)耸荏w形成局部二倍體的過程叫性導sexduction或F-duction。性導在大腸桿菌的遺傳學分析中十分有用。這種局部二倍體如果不發(fā)生重組

38、，那么F因子自主復(fù)制，可在細菌細胞中延續(xù)下去；性導所形成的局部二倍體可用作不同突變型之間的互補測驗，以確定這兩個突變型是屬于同一個基因或者是兩個不同的基因；觀察由性導形成的雜合的局部二倍體中某一性狀的表現(xiàn)，可以確定這一性狀的等位基因中的顯隱性關(guān)系；不同的F因子帶有不同的細菌DNA片段，因此利用不同的F因子性導可以測定不同基因在一起性導的頻率來作圖。局部二部體中也會出現(xiàn)重組，即F因子所帶的供體細菌染色體同受體細菌染色體之間的同源重組，如果發(fā)生單交換，就導致F整合形成Hfr品系，同時F因子上所攜帶的基因發(fā)生重組；如果雙交換，那么形成F品系，只是F因子的細菌基因和受體染色體上的等位基因之間發(fā)生互換。

39、三種致育因子F, F，Hfr的關(guān)系是：(1). 有F因子的細菌為F+，沒有F因子的為F-，具有致育因子F, F或Hfr的菌株就是雄性菌株(male strains) 。(2). F因子可以整合到細菌染色體上，形成Hfr染色體。不同的Hfr菌株F因子的整合位點不同。(3). F因子又可以從Hfr染色體上剪切下來，產(chǎn)生F因子。如果剪切不準確而帶有一段細菌染色體，那么稱為F因子。(4). F因子很容易轉(zhuǎn)移到F-細胞中， F+ F-F+，但是供體染色體的轉(zhuǎn)移頻率那么很低, 重組頻率很低。(5). Hfr能以高頻率把細菌染色體基因轉(zhuǎn)移到F-細菌中，卻極少使F-變?yōu)镕+因為F因子位于Hfr染色體的最末端；

40、(6). F因子的性質(zhì)介于F+和Hfr之間，即可轉(zhuǎn)移自身，又可以轉(zhuǎn)移細菌基因。但頻率較低。Conjugation F+ x F- Hfr x F- Conjugation Plasmid exit from genome: Hfr F+ Hfr F Conjugation Plasmid exit from genome: Hfr F+ Hfr F recipient is a partial diploidRecombination bringing new gene combinations togetherEukaryotes - crossing over during meiosis

41、reciprocal exchangeProkaryotes - transfer of genes from one cell to anotherone-way transfer of genesDNA transferred: exogenoterecipient DNA: endogenotepartial diploid may be formed Transformation Conjugation Transduction Limited transfer: one gene to a few genes closely related cells9.3 Rec Proteins

42、 and BacterialRecombinationOnce researchers established that a unidirectional transfer of DNA occurs between bacteria, they were interested in determining how the actual recombination event occurs in the recipient cell. Just how does the donor DNA replace the comparable region in the recipient chrom

43、osome? As with many systems, the biochemical mechanism by which recombination occurs was deciphered through genetic studies. Major insights were gained as a result of isolating a group of mutations representing rec genes.The first relevant observation in this case involved a series of mutant genes l

44、abeled recA, recB, recC, and recD. The first mutant gene, recA, RecA protein diminished genetic recombination in bacteria 1000-fold, nearly eliminating it altogether. The other rec mutations RecBCD protein reduced recombination by about 100 times. Clearly, the normal wild-type products of these gene

45、s play some essential role in the process of recombination.These characteristics place the F factor in the more general category of genetic structures called plasmids. These structures contain one or more genesoften, quite a few. Their replication depends on the same enzymes that replicate the chrom

46、osome of the host cell, and they are distributed to daughter cells along with the host chromosome during cell division.Plasmids are generally classified according to the genetic information specified by their DNA. The F factor confers fertility and contains genes essential for sex pilus formation, u

47、pon which genetic recombination depends. Other examples of plasmids include the R and the Col plasmids.9.4 F Factors and Plasmids染色體DNA作為細胞中的主要遺傳因子，攜帶有在所有生長條件下所必需的基因，這些基因有時稱之為“持家基因(housekeeping gene)，而質(zhì)粒所含的基因?qū)λ拗骷毎话闶欠潜匦璧?，只是在某些特殊條件下，質(zhì)粒能賦予宿主細胞以特殊的機能，從而使宿主得到生長優(yōu)勢。例如抗藥性質(zhì)粒和降解性質(zhì)粒能使宿主細胞在具有相應(yīng)藥物或化學毒物的環(huán)境中生存，而且

48、在細胞分裂時恒定的傳遞給子代細胞。 根據(jù)質(zhì)粒所編碼的功能和賦予宿主的表型效應(yīng)，可將其分為各種不同的類型：1.致育因子(Fertility factor，F(xiàn)因子) 又稱F質(zhì)粒，其大小約100kb，這是最早發(fā)現(xiàn)的一種與大腸桿菌的有性生殖現(xiàn)象(接合作用)有關(guān)的質(zhì)粒。攜帶F質(zhì)粒的菌株稱為F+菌株(相當 于雄性)，無F質(zhì)粒的菌株稱為F-菌株(相當于雌性)。F質(zhì)粒整合到宿主細胞染色體上的菌株稱之為高頻重組菌株(high frequence recombination, 簡稱Hfr)。由于F因子能以游離狀態(tài)(F+)和以與染色體相結(jié)合的狀態(tài)(Hfr)存在于細胞中，所以又稱之為附加體(episome)。 F質(zhì)粒

49、在大腸桿菌的接合作用(conjugation)中起主要作用。當Hfr菌株上的F因子通過重組回復(fù)成自主狀態(tài)時，有時可將其相鄰的染色體基因一起切割下來，而成為攜帶某一染色體基因的F因子，例如F-lac、F-gal、F-pro等。因此將這些攜帶不同基因的F因子統(tǒng)稱為F，帶有這些F因子的菌株也常用F表示。 2.抗性因子(Resistance factor，R因子) 這是另一類普遍而重要的質(zhì)粒，主要包括抗藥性和抗重金屬二大類，簡稱R質(zhì)粒。帶有抗藥性因子的細菌有時對于幾種抗生素或其他藥物呈現(xiàn)抗性。例如R1質(zhì)粒(94kb)可使宿主對以下五種藥物具有抗性：氯霉素(Chlorampenicol, Cm)、鏈霉素

50、(Streptomycin, Sm)、磺胺(Sulfonamide, Su)、氨芐青霉素(Ampicillin, Ap)和卡那霉素(Kanamycin, Km)，并且負責這些抗性的基因是成簇地存在于R1抗性質(zhì)粒上。 許多R質(zhì)粒能使宿主細胞對許多金屬離子呈現(xiàn)抗性，包括碲(Te6+)、砷(As3+) 、汞(Hg2+)、鎳(Ni2+)、鈷(Co2+)、銀(Ag+)、鎘(Cd2+)等。 在腸道細菌中發(fā)現(xiàn)的R質(zhì)粒，約有25%是抗汞離子的，而銅綠假單胞菌中約占75%。 因這類質(zhì)粒首先發(fā)現(xiàn)于大腸桿菌中而得名，該質(zhì)粒含有編碼大腸菌素的基因，大腸菌素是一種細菌蛋白，只殺死近緣且不含Col質(zhì)粒的菌株，而宿主不受其

51、產(chǎn)生的細菌素的影響。由G+細菌產(chǎn)生的細菌素通常也是由質(zhì)?；蚓幋a，有些甚至有商業(yè)價值，例如一種乳酸細菌產(chǎn)生的細菌素NisinA能強烈抑制某些G+細菌的生長，而被用于食品工業(yè)的保藏。4.毒性質(zhì)粒(virulence plasmid 現(xiàn)在越來越多的證據(jù)說明，許多致病菌的致病性是由其所攜帶的質(zhì)粒引起的，這些質(zhì)粒具有編碼毒素的基因，例如產(chǎn)毒素大腸桿菌是引起人類和動物腹瀉的主要病原菌之一，其中許多菌株含有為一種或多種腸毒素編碼的質(zhì)粒。有些使昆蟲 致病乃至死亡的細菌毒素也是由質(zhì)粒編碼的，蘇云金桿菌產(chǎn)生的毒素是這種類型的典型例子 。研究說明，蘇云金桿菌含有編碼內(nèi)毒素(伴孢晶體中)的質(zhì)粒，伴孢晶體的構(gòu)造基因及

52、調(diào)節(jié)基因位于質(zhì)粒上。 此外，目前廣泛應(yīng)用于轉(zhuǎn)基因植物載體的是一種經(jīng)過人工改造后的Ti質(zhì)粒(tumor-inducing-plasmid)(見第十章)，Ti質(zhì)粒是引起雙子葉植物冠癭瘤的致病因子，其宿主是一種根癌土壤桿菌(Agrobacterium tumefaciens)。是引起植物冠癭瘤的致病因子，其機制是Ti質(zhì)粒上的一段特殊DNA片段轉(zhuǎn)移至植物細胞內(nèi)并整合其染色體上，導致細胞無控制的瘤狀增生，合成正常植物所沒有的冠癭堿(opines)化合物，該DNA片段稱為T-DNA其上含有三個致癌基因。 發(fā)根土壤桿菌(Agrobacterium rhizogenes)引起許多雙子葉植物患毛根瘤，而致病因子

53、是該菌所含的Ri質(zhì)粒。Ri質(zhì)粒在功能上與Ti質(zhì)粒有廣泛的同源性，也有一段特殊的DNA片段(T-DNA)，在侵染過程中，能轉(zhuǎn)移進植物基因組，也可用于轉(zhuǎn)基因植物載體。 5.代謝質(zhì)粒(Metabolic plasmid) 這類質(zhì)粒上攜帶有能降解某些基質(zhì)的酶的基因，含有這類質(zhì)粒的細菌，特別是假單胞菌，能將復(fù)雜的有機化合物降解成能被其作為碳源和能源利用的簡單形式。尤其是對一些有毒化合物，如芳香簇化合物(苯)、農(nóng)藥(2,4-dichlorophenox yacetic acid)、辛烷和樟腦等的降解，在環(huán)境保護方面具有重要的意義(見第十一章)。因此這類質(zhì)粒也常被稱為降解質(zhì)粒，每一種具體的質(zhì)粒常以其降解的底

54、物而命名。如樟腦質(zhì)粒 (camphor, CAM)、辛烷質(zhì)粒(octadecane, OCT)、二甲苯質(zhì)粒(xylene, XYL)等。 此外，代謝質(zhì)粒中還包括一些能編碼固氮功能的質(zhì)粒。例如根瘤菌中與結(jié)瘤(nod)和固氮(fix)有關(guān)的所有基因均位于共生質(zhì)粒中。放線菌中也已發(fā)現(xiàn)許多大的線型質(zhì)粒(500kb以上) 含有抗生素合成的基因。6.隱秘質(zhì)粒(cryptic plasmid) 以上所討論的質(zhì)粒類型均具有某種可檢測的遺傳表型，但隱秘質(zhì)粒不顯示任何表型效應(yīng)，它們的存在只有通過物理的方法，例如用凝膠電泳檢測細胞抽提液等方法才能發(fā)現(xiàn)。他們存在的生物學意義，目前幾乎不了解。酵母的2m質(zhì)粒不授予宿主任

55、何表型效應(yīng)，也屬于隱秘型質(zhì)粒。 除了根據(jù)質(zhì)粒賦予宿主的遺傳表型將質(zhì)粒分成不同類型外，還可根據(jù)質(zhì)粒的拷貝數(shù)、宿主范圍等將質(zhì)粒分成不同類型。例如：有些質(zhì)粒在每個宿主細胞中可以有10-100個拷貝，稱為高拷貝數(shù)(high copy number)質(zhì)粒，另一些質(zhì)粒在每個細胞中只有1-4個拷貝，為低拷貝數(shù)(Low copy number)質(zhì)粒。前者又稱松弛型質(zhì)粒(relaxed plasmid)，后者又稱嚴謹型質(zhì)粒(stringent plasmid)。此外，還有一些質(zhì)粒的復(fù)制起始點(origin of replication)較特異，只能在一種特定的宿主細胞中復(fù)制，稱為窄宿主范圍質(zhì)粒(narrow h

56、ost range plasmid)；復(fù)制起始點不太特異，可以在許多種細菌中復(fù)制，稱為廣宿主范圍質(zhì)粒(broad host range plasmid)。能整合進染色體而隨染色體的復(fù)制而進展復(fù)制的質(zhì)粒又稱附加體(episome)。 Most R plasmids consist of two components: the RTF (resistance transfer factor) and one or more r-determinants Figure 9-12(b). The RTF encodes genetic information essential to transfer

57、ring the plasmid between bacteria, and the r-determinants are genes that confer resistance to antibiotics. Bacteria bearing such plasmids are of great medical significance, not only because of their multiple resistance, but because of the ease with which the plasmids can be transferred to other bact

58、eria.While RTFs are similar to a variety of plasmids from different bacterial species, r-determinants, each of which is specific for resistance to one class of antibiotic, vary widely. Sometimes, a bacterial cell contains r-determinant plasmids but no RTF. Such a cell is resistant but cannot transfe

59、r the genetic information for resistance to recipient cells. The most commonly studied plasmids, however, contain the RTF as well as one or more r-determinants. Resistance to tetracycline, streptomycin, ampicillin, sulfonamide, kanamycin, and chloramphenicol are most frequently encountered. Sometime

60、s these occur in a single plasmid, conferring multiple resistance to several antibiotics Figure 9-12(b). The Col plasmid, ColEl, derived from E. coli, is clearly distinct from R plasmids. It encodes one or more proteins that are highly toxic to bacterial strains that do not harbor the same plasmid.