Transduction was discovered by Zinder and Lederberg in 1952 when they were looking for an E. coli type conjugation system in Salmonella typhimurium. A mixture of two cultures of auxotrophic mutants of this bacterium differing in contrasting characters was found to produce small number of prototrophic recombinants. For example, two populations of auxotroph’s having phenotypes, A+B– and A–B+ were mixed in growth medium, and, on incubation, they produced some bacteria having A+B+ phenotype.
That it was not due to transformation was also proved by treatment with DNase which destroys free naked DNA. It was finally proved that the gene exchange was mediated by the bacteriophage P22 which infects S. typhimurium and some other species of Salmonella. This new mode of gene exchange leading to genetic recombination was designated as transduction.
In generalized transduction, the transducing phage infects a bacterial host in the usual way by attachment and injection of its DNA. Phage DNA and proteins are synthesized, and the bacterial chromosome is broken down to small fragments. During maturation of progeny phage particles, the host DNA fragments having approximately the same size as that of phage DNA are packaged inadvertently in some of phage heads.
After release by lysis of the infected host cell, such aberrant or defective phage particles may infect new host cells and inject the DNA into the cell. But because the DNA of these phages is of bacterial origin and does not contain phage genes necessary for replication, its life-cycle is not completed. Instead, the DNA can be integrated into the bacterial chromosome by homologous recombination.
In this way, one or more genes can be transferred from one bacterium to another. For example, if a transducing phage carrying a bacterial gene controlling motility infects a non-motile mutant bacterium, the latter may acquire the property of motility.
A characteristic feature of the transducing phages mediating generalized transduction is that any part of the bacterial chromosome can be transferred without any restriction regarding the site. In general, the fragment in a particular phage head is about one-hundredth part of the bacterial chromosome. Also, the frequency of an aberrant phage carrying bacterial DNA in its head in the total phage population is very low, not more than 1 in 105 to 107.
A well-studied example of generalized transduction is by phage PI of E. coli K12. PI is a temperate phage which has also a lytic cycle, like other temperate phages. PI DNA has a molecular weight of 5.9 x 107 Daltons. It encodes a DNase which can cleave bacterial chromosome into fragments having molecular weight of 1 x 107 to 1 x 108 Daltons. As the phage particles are assembled following active replication, one of the host DNA fragments may be taken up by mistake and packaged into the head. Such phage particles become defective and they are the transducing phages.
However, as the DNA of the transducing PI phages is of bacterial origin, it can be integrated into the chromosome of E. coli. For example, when phage PI is allowed to infect an ampicillin resistant strain of E. coli (ampr), some of the transducing phages will package the fragment containing the ampr gene. Such transducing phages on infecting an ampicillin sensitive strain of E. coli (ampS) will transfer the amp’ gene, making the sensitive strain resistant.
A schematic representation of generalized transduction is shown in Fig. 9.100:
Like co-transformation, generalized transduction can also be used as a tool for gene mapping. In case a single fragment of bacterial chromosome contains two closely linked marker genes, both can be transduced into a host cell together resulting in co-transduction.
In the E. coli-Pl phage system, a number of pairs of genes have been found to be co-transduced quite often indicating their close linkage. For example, the genes controlling threonine (thr) and leucine (leu) synthesis, or leucine synthesis and azide resistance (aziy), or streptomycin resistance (sirr) and malate utilization (mal) are co-transduced quite frequently. On the other hand, markers which are located so far apart from each other that they cannot be included in a fragment of the size that can be packaged into a single phage head are never co-transduced together.
Thus, in E. coli-PI phage system, thr and leu pair or leu-aziy pair are separately co-transduced, but thr-aziy pair is never co-transduced. This means that thr-aziy pair of genes are too far apart on E. coli chromosome to be included in a fragment that fits in the phage head. It is also evident that the order of genes is thr-leu-aziy in the bacterial chromosome.
From the frequency of cotransductants, the relative distances of the marker genes can be determined. For example, if leu-aziy pair is more frequent among cotransductants than thr-leu, it would mean that leu is nearer to aziy than to thr. Thus, by analysis of a large number contransductions, it is possible to prepare a gene-map of the organism concerned (Fig. 9.101).
Besides being used as an instrument for gene mapping, generalized transduction has been utilized widely for transferring genes in genetical research. The development of modern recombinant DNA technology has largely replaced the need of transduction being used for gene transfer. But in natural environmental conditions, transduction probably plays a significant mechanism for horizontal gene transfer.
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