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Griffiths AJF, miller JH, Suzuki DT, et al. An introduction to genetic Analysis. 7th edition. New York: W. H. Freeman; 2000.


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Some phages space able come “mobilize” bacterial genes and also carry them native one bacterial cabinet to another through the procedure of transduction. Thus, transduction joins the battery of modes of genetic transfer in bacteria—along through conjugation, transmittable transfer the episomes, and also transformation.


Discovery that transduction

In 1951, Joshua Lederberg and also Norton Zinder were experimentation for recombination in the bacterium Salmonella typhimurium by using the techniques that had actually been successful with E. Coli. The researchers offered two various strains: one was phe− trp− tyr−, and also the other was met− his−. (We won’t worry about the nature of this markers other than to note that the mutant alleles confer nutritional requirements.) when either strain was plated on a minimal medium, no wild-type cells were observed. However, after ~ the 2 strains to be mixed, wild-type cells appeared at a frequency of around 1 in 105. Thus far, the case seems similar to the for recombination in E. Coli.

However, in this case, the researchers additionally recovered recombinants from a U-tube experiment, in i m sorry cell call (conjugation) to be prevented by a filter separating the two arms. By differing the dimension of the pores in the filter, they discovered that the agent responsible for recombination was around the size of the virus P22, a recognized temperate phage that Salmonella. More studies sustained the pointer that the vector the recombination is without doubt P22. The filterable agent and P22 are identical in properties of size, sensitivity come antiserum, and immunity to hydrolytic enzymes. Thus, Lederberg and also Zinder, instead of confirming shortcut in Salmonella, had found a new type of gene transfer mediated by a virus. They dubbed this process transduction. In the lytic cycle, some virus corpuscle somehow pick up bacterial gene that space then moved to an additional host, where the virus inserts that contents. Transduction has actually subsequently been presented to be quite common among both temperate and also virulent phages.

There are two kinds of transduction: generalized and also specialized. Generalised transducing phages can lug any part of the chromosome, whereas committed transducing phages carry only minimal parts of the bacterial chromosome.


Transducing phages and generalized transduction

How are transducing phages produced? In 1965, K. Ikeda and also J. Tomizawa threw light on this concern in part experiments ~ above the pleasant E. Coli phage P1. They uncovered that, once a donor cell is lysed by P1, the bacterial chromosome is broken up into small pieces. Occasionally, the forming phage particles mistakenly incorporate a piece of the bacter DNA right into a phage head in ar of phage DNA. This occasion is the origin of the transducing phage.

Because the phage cloak proteins recognize a phage’s ability to strike a cell, transducing phages have the right to bind come a bacterial cell and inject their contents, i beg your pardon now take place to be donor bacterial genes. When a transducing phage injects its materials into a recipient cell, a merodiploid situation is developed in i m sorry the transduced bacterial genes have the right to be included by recombination (Figure 7-26). Because any kind of of the host markers deserve to be transduced, this type of transduction is termed generalized transduction.


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Figure 7-26

The device of generalized transduction. In reality, only a very little minority the phage progeny (1 in 10,000) carries donor genes.


Phages P1 and also P22 both belong to a phage team that shows generalized transduction (that is, they move virtually any kind of gene of the organize chromosome). During their cycles, P22 most likely inserts into the organize chromosome, whereas P1 stays free, favor a big plasmid. However both transduce by faulty head stuffing in lysis.


Linkage data indigenous transduction

Generalized transduction permits us come derive link information about bacterial genes as soon as markers space close sufficient that the phage can pick castle up and also transduce castle in a single piece the DNA. For example, mean that we want to find the linkage between met and also arg in E. Coli. We might collection up a cross of a met+ arg+ strain through a met− arg− strain. We can grow phage P1 ~ above the donor met+ arg+ strain, permit P1 to infect the met− arg− strain, and also select because that met+ colonies. Then, we can note the percentage of met+ swarms that came to be arg+. Strains transduced to both met+ and arg+ are referred to as cotransductants.

Linkage values space usually expressed as cotransduction frequencies (Figure 7-27). The better the cotransduction frequency, the closer two genetic markers are.


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Figure 7-27

Genetic map of the purB- to-cysB region of E. Coli established by P1 cotransduction. The numbers given are the averages in percent because that cotransduction frequencies acquired in several experiments. Where transduction the cross were performed in both directions, (more...)


Using an expansion of this approach, we have the right to estimate the size of the piece of organize chromosome that a phage have the right to pick up. The following type of experiment provides P1 phage:

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We can select for one or much more donor mite in the recipient and then (in true merozygous genes style) look for the existence of the other unselected markers, together outlined in Table 7-5. Experiment 1 in Table 7-5 tells united state that leu is reasonably close to azi and also distant from thr, leaving us through two possibilities:

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By choosing for thr+ and also leu+ in the transducing phages in experiment 3, we view that the transduced piece of genetic material never has the azi locus.

If enough markers were studied to produce a an ext complete linkage map, we could estimate the size of a transduced segment. Together experiments suggest that P1 cotransduction wake up within approximately 1.5 minute of the E. Coli chromosome map (1 minute equals the length of chromosome transferred by an Hfr in 1 minute’s time in ~ 37°C).


Lysogeny

In the 1920s, long prior to E. Coli ended up being the favorite organism of microbial geneticists, some amazing results were obtained in the research of phage infections of E. Coli. Some bacterial strains were discovered to be resistant to epidemic by certain phages, but these resistant bacteria brought about lysis the nonresistant bacteria as soon as the two bacterial strains were combined together. The resistant bacteria that induced lysis in other cells were said to it is in lysogenic bacteria or lyso-gens. When non-lysogenic bacteria to be infected with phages acquired from a lysogenic strain, a small portion of the infected cell did no lyse yet instead came to be lysogenic themselves.

Apparently, the lysogenic bacteria could somehow “carry” the phages while continuing to be immune to your lysing action. Initially, little attention to be paid to this phenomenon after part studies appeared to show that the lysogenic bacteria were merely contaminated with external phages that can be eliminated by cautious purification. However, in the mid-1940s, André Lwoff examined lysogenic strains of Bacillus megaterium and followed the habits of a lysogenic strain through numerous cell divisions. Closely observing his culture, that separated each pair the daughter cells instantly after division. One cell to be put right into a culture; the various other was observed till it divided. In this way, Lwoff derived 19 cultures representing 19 generations (19 consecutive cabinet divisions). Every 19 cultures were lysogenic, however tests of the tool showed no totally free phages at any type of time during these divisions, in order to confirming that lysogenic behavior is a character that persists through reproduction in the absence of free phages.

On rarely occasions, Lwoff it was observed spontaneous lysis in his cultures. Once the medium was spread out on a lawn of nonlysogenic cells after one of these spontaneous lyses, plaques appeared, reflecting that totally free phages had actually been exit in the lysis. Lwoff had the ability to propose a theory to define all his observations: every bacterium of the lysogenic strain has a noninfective element that is pass from bacterial generation to generation, however this element occasionally provides rise come the manufacturing of infective phages (without the existence of complimentary phages in the medium). Lwoff dubbed this element the prophage due to the fact that it somehow appeared to have the ability to induce the formation of a “litter” the infective phages. Later on studies proved that a selection of agents, such as ultraviolet light or specific chemicals, might activate the prophage, inducing lysis and also infective phage relax in a large portion of a populace of lysogenic bacteria.

We currently know precisely how Lwoff’s observations occur. A lysogenic bacterium has a prophage, which in which method protects the cell against extr infection, or superinfection, from totally free phages and also which is duplicated and also passed on come daughter cell in division. In a small fraction of the lysogenic cells, the prophage is induced, or activated, developing infective phages. This process robs the cabinet of the protection against the phage; that lyses and releases infective phages right into the medium, hence infecting any type of nonlysogenic cells existing in the culture.

Phages deserve to be categorized right into two types. Virulent phages have actually an infectious cycle the is always lytic—for this phages, there room no lysogenic bacteria. (Resistant bacter mutants might exist for virulent phages, yet their resistance is not due to lysogeny.) Temperate phages follow a riot cycle under part circumstances, but they commonly initiate a lysogenic cycle, in which the phage exists as a prophage within the bacterial cell. In this case, the lysogenic bacterium becomes resistant come superinfection, an “immunity” conferred through the existence of the prophage, i m sorry is transmitted gene through many bacterial generations. Temperate phages likewise cause lysis when the prophage is induced, or activated. Figure 7-28 diagrams the lytic and lysogenic transmittable cycles the a typical temperate phage.


Figure 7-28

Alternative cabinet cycles that a pleasant phage and also its host. (After A. Lwoff, Bacteriological evaluate 17, 1953, 269.)


MESSAGE

Virulent phages cannot end up being prophages; castle are constantly lytic. Temperate phages can exist within the bacterial cell together prophages, permitting their master to survive as lysogenic bacteria; castle are additionally capable of direct bacterial lysis.


Genetic communication of lysogeny

What is the nature the the prophage? top top induction, the prophage is capable of directing the manufacturing of a complete mature phage, so all of the phage genome should be current in the prophage. Yet is the prophage a little particle cost-free in the bacter cytoplasm—a plasmid—or is it somehow connected with the bacter genome? Fortuitously, the original strain that E. Coli used by Lederberg and Tatum (page 209) confirmed to be lysogenic for a warm phage referred to as lambda (λ). Phage λ has become the many intensively studied and best-characterized phage. Crosses in between F+ and F− cells have actually yielded interesting results. It transforms out the F+ × F−(λ) crosses yield recombinant lysogenic recipients, conversely, the reciprocal cross F+(λ) × F− virtually never provides lysogenic recombinants.

These results became much more understandable as soon as Hfr strains to be discovered. In the cross Hfr × F−(λ), lysogenic F− exconjugants through Hfr genes are readily recovered. However, in the mutual cross Hfr(λ) × F−, the early genes native the Hfr chromosome room recovered among the exconjugants, yet recombinants for late markers (those intended to carry after a certain time in mating) space not recovered. Furthermore, lysogenic exconjugants are practically never recovered native this reciprocal cross. What is the explanation? The monitorings make sense if the λ prophage is behaving like a bacter gene locus (that is, like part of the bacter chromosome). In interrupted-mating experiments, the λ prophage always enters the F− cell at a certain time, very closely linked to the gal locus. Thus, we deserve to assign the λ prophage to a specific locus alongside the festival region.

In the overcome of a lysogenic Hfr v a nonlysogenic (nonimmune) F− recipient, the entry of the λ prophage into the nonimmune cell immediately triggers the prophage into a revolt cycle; this procedure is dubbed zygotic induction. But, in the overcome Hfr(λ) × F−(λ), any kind of recombinants are readily recovered; the is, no induction the the prophage, and consequently lysis, occurs (Figure 7-29). It would certainly seem that the cytoplasm that the F− cell must exist in two various states (depending on even if it is the cell includes a λ prophage), so contact in between an beginning prophage and also the cytoplasm that a nonimmune cell immediately induces the revolt cycle. Us now understand that a cytoplasmic element specified by the prophage represses the multiplication of the virus. Entry of the prophage into a nonlysogenic environment immediately dilutes this repressing factor, and also therefore the virus reproduces. But, if the virus mentions the repressing factor, climate why doesn’t the virus close up door itself turn off again? plainly it does, due to the fact that a portion of infected cells do become lysogenic. Over there is a race in between the λ gene signals for reproduction and also those specifying a shutdown. The model of a phagedirected cytoplasmic repressor nicely defines the immune of the lysogenic bacteria, because any kind of superinfecting phage would automatically encounter a repressor and also be inactivated. We existing this design in much more detail in chapter 11.


Prophage attachment

How is the prophage attached come the bacter genome? Allan Campbell suggest in 1962 the λ attaches come the bacter chromosome through a reciprocal crossover between the one λ chromosome and the one E. Coli chromosome, as presented in number 7-30. The crossover suggest would be in between a certain site in λ, the λ attachment site, and a site in the bacter chromosome located between the genes gal and also bio, since λ integrates in ~ that position in the E. Coli chromosome.


Figure 7-30

Campbell’s model for the integration of phage λ right into the E. Coli chromosome. Reciprocal recombination take away place in between a specific attachment site on the one λ DNA and a specific region on the bacterial chromosome between (more...)


One attraction the Campbell’s proposal is the it enables predictions the geneticists deserve to test by utilizing phage λ:

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Integration of the prophage right into the E. Coli chromosome should boost the hereditary distance between flanking bacter markers, as can be watched in figure 7-30 because that gal and bio. In fact, studies present that time-of-entry or recombination distances between the bacterial genes are boosted by lysogeny.

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Deleting bacter segments nearby to the prophage site need to delete phage genes at least some of the time. Speculative studies also confirm this prediction.


Specialized transduction

We deserve to now understand the procedure of specialized transduction, in which only specific host markers can be transduced.

Lambda is a an excellent example that a devoted transducing phage. Together a prophage, λ always inserts between the gal region and the bio region of the organize chromosome (see number 7-30). In transduction experiments, λ deserve to transduce only the gal and bio genes. Let’s visualize the system of λ transduction.

The recombination between regions the λ and the bacterial chromosome is catalytic analysis by a details enzyme system. This system normally ensures that λ integrates at the same suggest in the chromosome and, when the lytic cycle is induced (for instance, by ultraviolet light), the ensures that the λ prophage excises at specifically the correct suggest to create a common circular λ chromosome. An extremely rarely, cut is abnormal and also can result in phage particles the now bring a nearby gene and leave behind some phage gene (Figure 7-31a). In λ, the nearby genes are gal on one side and also bio top top the other. The result particles space defective as result of the genes left behind and also are referred to as λdgal (λ-defective gal), or λdbio. this defective particles carrying adjacent genes have the right to be packaged into phage heads and also can infect various other bacteria. In the visibility of a second, typical phage particle in a dual infection, the λdgal can incorporate into the chromosome at the λ-attachment website (Figure 7-31b). In this manner, the gal genes in this case are transduced into the 2nd host. Because this transduction device is restricted to genes really near the original combined prophage, the is called dedicated transduction.

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Figure 7-31

Specialized transduction mechanism in phage λ. (a) A lysogenic bacterial society can develop normal λ or, rarely, an abnormal particle, λdgal, i m sorry is the transducing particle. (b) Transducing by the mixed lysate can create (more...)


MESSAGE

Transduction occurs as soon as newly developing phages acquire host genes and transfer castle to other bacterial cells. Generalized transduction can transfer any host gene. It occurs once phage packaging by chance incorporates bacterial DNA instead of phage DNA. Committed transduction is as result of faulty separation the the prophage from the bacter chromosome, therefore the brand-new phage has both phage and also bacterial genes. The transducing phage can transfer only specific host genes.