Bacterial Transduction

Bacterial Transduction

Bacterial transduction may be defined as — ‘a phenomenon causing genetic recombination in bacteria wherein DNA is carried from one specific bacterium to another by a bacteriophage’.

It has been duly observed that a major quantum of bacteriophages, particularly the ‘virulent’ ones, predominantly undergo a rather quick lytic growth cycle in their respective host cells. During this phenomenon they invariably inject their nucleic acid, normally DNA, right into the bacterium, where it takes up the following two cardinal steps :

(a) undergoes ‘replication’ very fast, and

(b) directs the critical synthesis of new phage proteins.

Another school of thought may put forward another definition of bacterial transduction as - ‘the actual and legitimate transfer by a bacteriophage, serving as a vector, of a segment of DNA from one bacterium (a donor) to another (a recipient)’.

Zinder and Ledenberg (1952) first and foremost discovered the wonderful phenomenon during an intensive search for ‘sexual conjugations’ specifically amongst the Salmonella species.

Methodology :

The various steps that are involved in the bacterial transduction phenomenon are as stated under :

(1) Auxotrophic mutants were carefully mixed together ; and subsequently, isolated the prototrophic recombinant colonies from the ensuing selective nutritional media.

(2) U-Tube Experiment : The U-tube experiment was duly performed with a parental auxotrophic strain in each arm (viz., I and II), and adequately separated by a microporous fritted glass (MFG) filter, whereby the resulting ‘prototrophs’ distinctly appeard in one arm of the tube, as shown in Fig. 6.10.

As the MFG-filter particularly checked and prevented cell-to-cell contact, but at the same time duly permits the ‘free pas-sage’ of fluid between the said two cultures [i.e., strains I and II], it may be safely inferred that there must be certain ‘phenomenon’ other than the ‘conjugation’ was involved.

Besides, the process could not be radically prevented to DNAase (an enzyme) activity, thereby completely eliminating ‘transformation’ as the possible phenomenon involved for causing definite alterations in the recipient auxotrophs to prototrophs.

The bacteriophage was duly released in a substantial amount from a lysogenic (i.e., recipient) culture. Thus, the emerging phage critically passed via the MFG-filter, and adequately infected the other strain (i.e., donor) lyzing it exclusively.

Finally, during the ‘replication’ observed in the donor strain, the ensuing phage adventitiously comprised of the relevant portions of the critical bacterial chromosome along with it. Eventually, it gained entry via the MFG-filter once again ; thereby taking with it a certain viable segment of the respective donor’s ‘genetic information’ and ultimately imparting the same to the desired recipient strain.

Nevertheless, the ‘bacterial transduction’ may be further classified into two sub-heads, namely :

(a) Generalized transduction, and

(b) Specialized transduction.

(a) Generalized Transduction

In a situation when practically most of the fragments pertaining to the bacterial DNA* do get an obvious chance to gain entry right into a ‘transducting phage’, the phenomenon is usually termed as ‘generalized transduction’.

Modus Operandi : The very first step of the phage commences duly with the ‘lytic cycle’ whereby the prevailing ‘viral enzymes’ preferentially hydrolyze the specific bacterial chromosome essentially into several small fragments of DNA. In fact, one may most conveniently incorporate any portion of the ‘bacterial chromosome’ right into the ‘phage head’ in the course of the ensuing phage assembly ; and, therefore, it is not normally associated with any sort of ‘viral DNA’.

Example :

Transduction of Coliphage P1 : In fact, the coliphage P1 can effectively transduce a variety of genes in the bacterial chromosome**. After infection a small quantum of the phages carry exclusively the bacterial DNA as shown in Fig. 6.11.

Figure : 6.11 clearly illustrates the following salient features, namely :

Phage P1 chromosomes, after injection into the host cell, gives rise to distinct degradation of the specific host chromosome right into small fragments.

During maturation of different particles, a small quantum of ‘phage heads’ may, in fact, envelop certain fragments of the bacterial DNA instead of the phage DNA.

Resulting bacterial DNA on being introduced into a new host cell may get integrated into the bacterial chromosome, thereby causing the transference of several genes from one host cell to another.

It has been observed that the ‘frequency’ of such defective phage particles usually range between 10^{– 5} to 10^{– 7} with respect to corresponding ‘progeny phage’ generated. As this particular DNA more or less matches the DNA of the newer bacterium thus infected, the ‘recipient bacterium’ shall not be rendered lysogenic* for the respective P1 phage. Instead, the injected DNA shall be duly integrated right into the chromosome of the available recipient cell. In this manner, the so called ‘genetic markers’ duly present in the DNA would precisely detect the very presence of all defective P1 phages essentially bearing the E. coli DNA.

Advantages :

The various glaring advantages of the generalized transduction are as given below :

(1) Just like bacterial conjugation (see Section 2.10) and bacterial transformation (see Sec-tion 2.7) the generalized transduction also caters for the typical ways for ‘mapping* bac-terial genes’, by virtue of the fact that the fragments duly transferred by the bacteriophage are invariably big enough to safely accomodate hundreds of genes.

(2) To test actually the exact quantum of such ‘recombinants’ that have inherited from other ‘donor markers’ due to the growth occurring on other culture media.

(3) Strategic closeness of the ‘two markers’ on the bacterial chromosome ascertains the fact that they would be inherited together more likely by the aid of a single transducing phage.

(b) Specialized Transduction

Based on enough scientific evidences it has been duly proved and established that the ‘bacterial genes’ may also be adequately transduced by means of bacteriophage in another equally interesting and thought provoking phenomenon usually termed as ‘specialized transduction’. In fact, this phenomenon confirms duly that certain template phage strains may be capable of transferring merely a handful of ‘restricted genes’ belonging categorically to the ‘bacterial chromosomes’.

In other words, the ensuing phages particularly transduce exclusively such bacterial genes that are strategically positioned quite adjacent to the prophage in the bacterial chromosome. Therefore, this particular process is sometimes also referred to as ‘restricted transduction’. Interestingly, in an event when such a phage duly infects a cell, it invariably carries along with it the specified group of bacterial genes which ultimately turns out to be an integral part of it. Consequently, such genes may recombine meticulously with the homologous DNA of the prevailing infected cell.

Phage Lambda (λ) of E. coli. : In a broader perspective, the most elaboratedly researched spe-cialized transducting phage is duly represented by the phage lambda (λ) of E. coli. The exact location of the ensuing λ prophage present in the bacterial chromosome invariably lies between the bacterial genes gal and bio. It may be observed that whenever phages duly carrying either a gal or bio genes do infect an altogether ‘new host’, then the desired recombination either with the gal or bio genes of the respective may take place articulately. Fig. 6.12 depicts vividly the phenomenon of specialized transduction.

Salient Features :

The various salient features highlighting the process of specialized transduction in Figure 6.12 are stated as under :

(1) Practically ‘all phages’ which essentially carry certain bacterial genes solely on account of ‘‘incorrect’’ excision are obviously found to the ‘defective’ with respect to certain highly important functions.*

(2) Thorough passage via the entire ‘replication cycle’ cannot be accomplsihed ; whereas, the ensuing cell may suitably give rise to certain phages, provided it is also duly infected with a rather ‘complete phage’.

Explanations :

The various stages illustrated in Fig. 6.12 are as follows :

(1) When a cell gets duly infected by phage λ, its DNA is precisely inserted right into the bacterial genome next to the genes meant for galactose metabolism (i.e, gal genes).

(2) Invariably when such a cell is being induced, the λ DNA emerges out promptly, get repli-cated, and subsequently turned into a normal phage.

(3) Sometimes, the respective λ DNA is excised imperfectly thereby taking along with it the gal genes ; and hence leaving behind certain quantum of itself that may finally lead to λ dg (i.e., defective-galactose transducing phage.)