What is the Difference Between Generalized and Specialized Transduction

The main difference between generalized and specialized transduction is that bacteriophages can take any portion of the host genome in generalized transduction, whereas bacteriophages pick up only a specific part of the host’s genome in specialized transduction.  

Generalized and specialized transduction are two methods of transduction in which foreign DNA is introduced into the host cell by a virus.  

Key Areas Covered

1. What is Generalized Transduction
     – Definition, Features, Importance
2. What is Specialized Transduction
     – Definition, Features, Importance
3. Similarities Generalized and Specialized Transduction
     – Outline of Common Features
4. Difference Between Generalized and Specialized Transduction
     – Comparison of Key Differences

Key Terms

Generalized Transduction, Specialized Transduction

What is Generalized Transduction

Generalized transduction is a process in which phages package any bacterial DNA, including chromosomal and plasmid DNA, and transfer it to another bacterium. When bacterial DNA is packed into the head of the phage, the bacteriophage is called the transducing particle. Earlier, it was thought that host DNA was packaged into the head of the transducing particles in the lytic cycle instead of viral DNA. But the current understanding is that the unfragmented chromosomal DNA serves as the substrate for the packaging of headful DNA. This occurs in pac-type phages

Figure 1: Transduction Mechanism

Furthermore, the terminases in the phage mistakenly recognize pseudo-pac sites that are homologous to the pac-sites in both chromosomal and plasmid DNA and initiate packaging. Transduction frequency varies from different parts of the genome due to the degree of homology to the pac-sites and their distribution in the genome. In generalized transduction, any part of the genome can pack and transfer in this manner. Only one pseudo-pac site is necessary for headful packaging. However, cos-type phages do not involve in generalized transduction.  

What is Specialized Transduction

Specialized transduction is the second process of transduction which is limited to a set of genes. Therefore, it transduces a limited set of genes during transduction. Encapsidation of the bacterial host DNA as a hybrid is the mechanism of specialized transduction. According to the classical λ phage model, viral DNA joins with the bacterial host DNA in aberrant prophage excision events. The hybrid molecule replicates as normal and if a cos-site is present, the concatemeric hybrid DNA is packaged by cos-type packaging machinery. However, specialized transduction makes only a small contribution to the phage-mediated gene transfer due to the rare aberrant excision and restricted bacterial DNA transfer.

Figure 2: Generalized and Specialized Transduction

Moreover, both transductions have missteps in transduction that result in the packaging of host DNA into the phage particles. However, host gene transfer occurs in less frequency due to the less pseudo-pac-site recognition and the errors in the prophage excision.

Similarities Between Generalized and Specialized Transduction

• Generalized and specialized transduction are two methods of transduction, introducing foreign DNA into the host cell by viruses. For example, viruses transfer DNA from one bacterial cell to another by transduction.

Difference Between Generalized and Specialized Transduction

Definition

Generalized transduction refers to the process by which phages can package any bacterial DNA (chromosomal or plasmid) and transfer it to another bacterium, while specialized transduction refers to the process by which a restricted set of bacterial genes are transferred to another bacterium.

Type

Generalized transduction occurs due to pac-sites while specialized transduction occurs due to cos-sites.

Occurrence

Moreover, generalized transduction occurs in high frequency while specialized transduction occurs in low frequency.

Vector

Generalized transduction occurs in virulent phage or temperate phage in the lytic cycle, while specialized transduction occurs in temperate phage.   

Target Gene

Any donor gene can be transferred in generalized transduction, while certain donor genes can be transferred in specialized transduction.

Time

Furthermore, generalized transduction occurs in the bacteriolysis phase, but specialized transduction occurs in the lysogenic phase.

Cause

Generalized transduction occurs due to errors in assembly, while specialized transduction occurs in errors in excision.  

Conclusion

In brief, generalized and specialized transduction are two methods of transduction that occur during the gene transfer from one to another bacteria by a virus. Generalized transduction occurs in the virulent phage or temperate phage in the lytic cycle. Any donor gene can be transferred in generalized transduction. It occurs in bacteriolysis due to errors in assembly. On the other hand, specialized transduction occurs in temperate phage and certain donor genes only can be transferred into the bacteria in specialized transduction. It also occurs in the lysogenic phase due to errors in excision. Therefore, the main difference between generalized and specialized transduction is the type of genes transferred.  

References:                     

1. Chiang YN, Penadés JR, Chen J. Genetic transduction by phages and chromosomal islands: The new and noncanonical. PLoS Pathog. 2019 Aug 8;15(8):e1007878. doi: 10.1371/journal.ppat.1007878. PMID: 31393945; PMCID: PMC6687093.

Image Courtesy:

1. “Transduction image” By Momodou Bah – Own work (CC-BY SA 4.0) via Commons Wikimedia
2. “Transduction illustration” By Towns157 – Own Work (CC-BY SA 4.0) via Commons Wikimedia