Question

The RNA genome of phage \(Q \beta\) is the nontemplate strand, or coding strand, and when introduced into the cell, it functions as an mRNA. Suppose the RNA replicase of phage \(\mathrm{Q} \beta\) synthesized primarily template-strand RNA and uniquely incorporated this, rather than nontemplate strands, into the viral particles. What would be the fate of the template strands when they entered a new cell? What enzyme would have to be included in the viral particles for successful invasion of a host cell?

Short answer

The template strands would require RNA replicase to produce mRNA, allowing translation in the host cell.

Step-by-step solution

Understanding the Template and Nontemplate Strands

The RNA genome of phage \(Q \beta\) typically acts as the nontemplate (coding) strand, functioning directly as mRNA. This means it can immediately be translated into proteins after entering a cell.

Role of Template Strand RNA

If the template-strand RNA were packaged into viral particles instead, it would be complementary to the mRNA form. Hence, upon entering a new cell, it cannot be translated directly into proteins as it does not have the correct sequence.

Required Enzyme for Transcription

For the template strand to be functional, an RNA-dependent RNA polymerase (also known as RNA replicase) would need to be present in the viral particles. This enzyme would synthesizes a complementary RNA strand, effectively converting the template-strand RNA back into the mRNA form.

The Process After Cell Entry

Upon entry into a new host cell, the RNA replicase would use the template-strand RNA to generate new nontemplate strands. These new strands would then be able to function as mRNA, allowing translation to occur and viral proteins to be produced.

Key concepts

Phage Q beta

Phage Q beta is a fascinating type of ribovirus, specifically a single-stranded RNA bacteriophage. It infects bacterial cells, particularly the well-studied Escherichia coli (E. coli). One remarkable aspect of phage Q beta is its RNA genome which acts as the nontemplate strand, or coding strand.
The RNA genome is capable of immediate function as messenger RNA (mRNA) once it enters the bacterial cell. This allows the quick translation of viral proteins necessary for the reproduction of the virus. Since it operates directly as mRNA, the genetic material of phage Q beta can be expressed without needing transcription into a different form.
Phage Q beta is an excellent model for studying RNA life cycles and provides insight into RNA replication strategies and RNA virus evolution.

RNA strand coding

The concept of RNA strand coding refers to the portion of an RNA molecule that can directly function as mRNA, meaning it can be immediately translated into proteins. In the case of phage Q beta, its genome serves as the coding strand. This RNA strand resembles the sense strand of DNA, which is used during transcription to generate RNA.
A coding RNA strand contains the correct sequence of nucleotides necessary for protein synthesis. Each triplet of nucleotides, known as a codon, corresponds to a specific amino acid in a polypeptide chain.
By functioning as the coding strand, the RNA genome of phage Q beta enables efficient protein synthesis and virus proliferation after entering the host cell. Its ability to bypass the transcription stage speeds up the infection process significantly.

RNA-dependent RNA polymerase

RNA-dependent RNA polymerase is a crucial enzyme for RNA virus replication. Instead of using DNA as a template for RNA synthesis (as in most cellular organisms), it uses an RNA template to create a new RNA strand.
This enzyme is often referred to as RNA replicase and is vital in the replication of RNA virus genomes.
For phage Q beta, if the viral particle were to package the template-strand RNA rather than the coding strand, RNA-dependent RNA polymerase would be essential. It would synthesize a complementary RNA strand from the template-strand RNA.
By doing so, the enzyme converts the template strand into a functional nontemplate strand, capable of acting as mRNA and allowing the continuation of the viral life cycle, such as protein production and virus assembly.

Viral genome translation

Viral genome translation is the process through which the mRNA of a virus is decoded to synthesize viral proteins by the host cell's ribosomes. Since phage Q beta RNA acts as the coding strand, it circumvents the need for initial transcription and directly undergoes translation upon entry into the host cell.
This process begins when ribosomes bind to the viral mRNA. The ribosomes read the sequence of codons, directing the incorporation of amino acids into polypeptides, which eventually fold into functional viral proteins.

• Viral proteins are crucial for the replication and packaging of new viral particles.
• Efficient translation allows phage Q beta to rapidly produce the necessary components to infect more cells.

The success of viral genome translation is integral to the propagation and survival of RNA viruses like phage Q beta, showcasing the importance of understanding RNA-mediated mechanisms in virology.