8. During development and differentiation, there is a dynamic programme of differential expression of sets of genes. In bacteria, phage infections are among the simplest examples of developmental process. Typically, only a subset of the phage genome, offer referred to as immediate early genes, are expressed in the host immediately after phage infection. As time passes, early genes start to be expressed, and the immediate early genes and bacterial genes are turned off. In the final stage of phage infection, the early genes give way to late genes. One of the simplest way it is achieved is through
(A) expression of cascade of σ factors
(B) expression of new RNA polymerases
(C) expression of different holoenzymes
(D) expression of different transcription factors
The correct reasons are
(1) A, D (2) A, C, D
(3) A, B, D (4) A, B, C
During bacteriophage infection, a tightly regulated program of gene expression unfolds in a sequential manner. This dynamic process ensures that the phage genome is expressed in stages—starting with immediate early genes, followed by early genes, and finally late genes. Such temporal regulation is crucial for efficient viral replication and assembly within the host bacterium.
The Challenge of Temporal Gene Regulation in Phages
Phages infect bacterial cells and must coordinate the expression of hundreds of genes in a defined order. Initially, only a subset of genes called immediate early genes are expressed right after infection. These genes often encode regulatory proteins that control the expression of subsequent gene sets.
As infection progresses:
-
Immediate early genes are turned off.
-
Early genes are activated, which often encode enzymes and proteins needed for DNA replication.
-
Finally, late genes are expressed, encoding structural proteins and factors necessary for phage assembly and host cell lysis.
The question is: How do phages achieve this precise temporal regulation of gene expression?
Mechanisms of Temporal Regulation in Phage Gene Expression
Several molecular strategies have evolved in phages to regulate gene expression in a cascade fashion. Among the most important are:
(A) Expression of a Cascade of Sigma (σ) Factors
Sigma factors are subunits of bacterial RNA polymerase that direct the enzyme to specific promoter sequences. Phages exploit this by encoding or inducing the expression of different sigma factors at various stages of infection.
-
The initial host sigma factor recognizes immediate early promoters.
-
Phage-encoded sigma factors or modified host sigma factors then redirect RNA polymerase to early and late promoters.
-
This cascade ensures that different gene sets are transcribed at the right time.
(C) Expression of Different Holoenzymes
A holoenzyme is the complete form of RNA polymerase, including its core enzyme plus sigma factor. By expressing or modifying different sigma factors, phages effectively change the composition of the holoenzyme.
-
This altered holoenzyme recognizes distinct promoters.
-
It allows the switch from immediate early to early and late gene transcription.
(D) Expression of Different Transcription Factors
Phages also produce transcription factors that regulate RNA polymerase activity and promoter recognition.
-
These factors can activate or repress specific promoters.
-
They fine-tune the timing and level of gene expression.
-
Examples include antiterminators and activators that modify transcription elongation or initiation.
Why Not (B) Expression of New RNA Polymerases?
While some phages like T7 encode their own RNA polymerase, many rely on the host RNA polymerase. The question focuses on the simplest and most typical mechanisms. Expressing new RNA polymerases is less common and more complex compared to modifying the host enzyme via sigma factors and transcription factors.
Correct Combination of Mechanisms
Given these points, the correct combination of mechanisms used by phages to regulate temporal gene expression is:
-
(A) Expression of cascade of sigma factors
-
(C) Expression of different holoenzymes (via different sigma factors)
-
(D) Expression of different transcription factors
Thus, the correct answer is (2) A, C, D.
Supporting Examples from Phage Biology
-
Phage T4 modifies host RNA polymerase with phage-encoded factors to recognize middle and late promoters.
-
Phage lambda uses transcription factors like CI and Cro to regulate the switch between lysogenic and lytic cycles.
-
Phage T7 encodes its own RNA polymerase but also uses host factors for early gene expression.
Summary Table of Mechanisms
| Mechanism | Role in Phage Gene Expression Regulation | Typical Usage in Phages |
|---|---|---|
| Expression of cascade of σ factors | Redirects RNA polymerase to different promoters | Common in many phages (e.g., T4, lambda) |
| Expression of new RNA polymerases | Phage-encoded RNA polymerase transcribes specific genes | Used by some phages (e.g., T7) |
| Expression of different holoenzymes | Different σ factors form distinct holoenzymes | Integral to temporal regulation |
| Expression of different transcription factors | Activators/repressors modulate transcription | Widely used for fine control |
Conclusion
Phages achieve temporal regulation of their gene expression primarily through a combination of expressing a cascade of sigma factors, forming different holoenzymes, and producing various transcription factors. These mechanisms allow the phage to sequentially express immediate early, early, and late genes efficiently during infection. While some phages encode their own RNA polymerases, the simplest and most common strategy involves modifying the host RNA polymerase specificity and activity.
Understanding these molecular strategies provides deep insights into phage biology and gene regulation, with broader implications for molecular genetics and biotechnology.
