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tion). The complete life cycle of a temperate phage is shown in gure 7.24.
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TRANSDUCTION
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Before lysis, when phage DNA is being packaged into phage heads, an occasional error occurs that causes bacterial DNA to be incorporated into the phage head instead. When this happens, bacterial genes can be transferred to another bacterium via the phage coat. This process, called transduction, has been of great use in mapping the bacterial chromosome.Transduction occurs in two patterns: specialized and generalized.
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Specialized Transduction
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The process of specialized or restricted transduction was rst discovered in phage by Lederberg and his students. Specialized transduction is analogous to sexduction it depends upon a mistake made during a loopingout process. In sexduction, the error is in the F factor. In specialized transduction, the error is in the prophage. Figure 7.25 shows the prophage looping out incorrectly to create a defective phage carrying the adjacent gal locus. Since only loci adjacent to the phage attachment site can be transduced in this process, specialized transduction has not proven very useful for mapping the host chromosome.
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II. Mendelism and the Chromosomal Theory
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7. Linkage and Mapping in Prokaryotes and Bacterial Viruses
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Linkage and Mapping in Prokaryotes and Bacterial Viruses
Bacterial chromosome prophage
Exogenote
Endogenote
A B
C+ C
Defective phage
Figure 7.25 Imprecise excision, or looping out, of the
Two crossovers Four crossovers
prophage, resulting in a defective phage carrying the gal locus.
Generalized Transduction
Generalized transduction, which Zinder and Lederberg discovered, was the rst mode of transduction discovered. The bacterium was Salmonella typhimurium and the phage was P22. Virtually any locus can be transduced by generalized transduction. The mechanism, therefore, does not depend on a faulty excision, but rather on the random inclusion of a piece of the host chromosome within the phage protein coat. A defective phage, one that carries bacterial DNA rather than phage DNA, is called a transducing particle. Transduction is complete when the genetic material from the transducing particle is injected into a new host and enters the new host s chromosome by recombination. For P22, the rate of transduction is about once for every 105 infecting phages. Since a transducing phage can carry only 2 to 2.5% of the host chromosome, only genes very close to each other can be transduced together (cotransduced). Cotransduction can thus help to ll in the details of gene order over short distances after interrupted mating or transformation is used to ascertain the general pattern. Transduction is similar to transformation in that cotransduction, like co-occurrence in transformation, is a relative indicator of map distance.
A+ B+
A B
A+ B+
A B
Mapping with Transduction
Transduction can be used to establish gene order and map distance. Gene order can be established by twofactor transduction. For example, if gene A is cotransduced with gene B and B with gene C, but A is never cotransduced with C, we have established the order A B C
Figure 7.26 The rarest transductant requires four crossovers.
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II. Mendelism and the Chromosomal Theory
7. Linkage and Mapping in Prokaryotes and Bacterial Viruses
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Transduction
Table 7.5 Gene Order Established by Two-Factor
Cotransduction*
Transductants A B A C B C A B C Number 30 0 25 0
* An A B C strain of bacteria is infected with phage. The lysate is used to infect an A B C strain. The transductants are scored for the wild-type alleles they contain. These data include only those bacteria transduced for two or more of the loci. Since AB cotransductants and BC cotransductants occur, but no AC types, we can infer the A B C order.
(table 7.5). This would also apply to quantitative differences in cotransduction. For example, if E is often cotransduced with F and F often with G, but E is very rarely cotransduced with G, then we have established the order E F G. However, even more valuable is three-factor transduction, in which we can simultaneously establish gene order and relative distance. Three-factor transduction is especially valuable when the three loci are so close that it is very dif cult to make ordering decisions on the basis of two-factor transduction or interrupted mating. For example, if genes A, B, and C are usually cotransduced, we can find the order and relative distances by taking advantage of the rarity of multiple crossovers. Let us use the prototroph (A B C ) to make transducing phages that then infect the A B C strain of bacteria. To detect cells that have been transduced for one, two, or all three of the loci, we need to eliminate the
nontransduced cells. In other words, after transduction, there will be A B C cells in which no transduction event has taken place. There will also be seven classes of bacteria that have been transduced for one, two, or all three loci (A B C , A B C , A B C , A B C , A B C , A B C , and A B C ). The simplest way to select for transduced bacteria is to select bacteria in which the wild-type has replaced at least one of the loci. For example, if, after transduction, we grow the bacteria in minimal media with the requirements of B and C added, all the bacteria that are A will grow. (Without the requirement of A bacteria, no A bacteria will grow.) Hence, although we lose the A B C , A B C , and A B C categories, we also lose the A B C , untransduced bacteria. In this example, the A locus is the selected locus; we must keep in mind that we have an incomplete, although informative, data set. Replica-plating allows us to determine genotypes at the B and C loci for the A bacteria. In this example, colonies that grow on complete medium without the requirement of the A mutant are replica-plated onto complete medium without the requirement of the B mutant and then onto complete medium without the requirement of the C mutant. In this way, each transductant can be scored for the other two loci (table 7.6). Now let us take all these selected transductants in which the A allele was incorporated. These can be of four categories: A B C , A B C , A B C , and A B C . We can now compare the relative numbers of each of these four categories. The rarest category will be caused by the event that brings in the outer two markers, but not the center one, because this event requires four crossovers ( g. 7.26). Thus, by looking at the number of transductants in the various categories, we can determine that the gene order is A B C (table 7.7), since the A B C category is the rarest.
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