Lecture 9 - Bacteriophage MS2

What family does bacteriophage MS2 belong to?

Leviviridae (Latin: levis, meaning light).

What host does bacteriophage MS2 infect?

Escherichia coli (E. coli).

Is MS2 an enveloped virus?

No, it is a non-enveloped (naked) virus.

What is the shape and size of the MS2 capsid?

Icosahedral; 26 nm in diameter.

What is the type of genome found in MS2?

(+) ssRNA, monopartite, linear, ~3.5 kb.

Is the MS2 genome ribosome-ready?

Yes, it acts directly as mRNA.

What are the structural proteins encoded by MS2?

Maturation and coat proteins.

What are the non-structural proteins encoded by MS2?

Lysis and replicase proteins.

How many copies of the coat protein are in the MS2 virion?

180 copies

How many copies of the maturation protein are present in MS2?

Only one.

What is the function of the replicase protein?

It is an RNA-dependent RNA polymerase (RdRp) for genome replication.

How does MS2 enter E. coli cells?

Via attachment to the F-pili using the maturation protein.

What happens to the maturation protein upon entry?

It is cleaved to release the (+)ssRNA genome into the cell.

How does MS2 do replication?

By making an intermediate (-)ssRNA to make more positive strands.

Is replication symmetric or asymmetric in MS2?

Asymmetric—many more (+) strands than (-) strands.

What protein is translated first in the MS2 life cycle?

Coat protein.

Why is the coat protein translated first?

Its start codon is in an unstructured region of the RNA, making it accessible to ribosomes.

What is the role of RNA structure in regulating MS2 protein translation?

RNA secondary structures hide or expose ribosome binding sites for different proteins.

What regulates the switch between translation and replication in MS2?

Competition between ribosome and replicase for access to the coat start codon.

How does ribosome backtrack to the lysis start codon for limited translation?

The Ribosomes ends at terminating coat protein seqeunce for translation and may go backwards to the lysis start codon or forward to the replicase start codon.

When is the maturation protein translated?

Only during synthesis of nascent (+) RNA before folding completes.

What is the role of EF-Tu in MS2 replication?

Carries GTP and initiates RNA synthesis.

What is the function of S1 protein in MS2?

Helps replicase bind to (+)ssRNA.

What unwinds the 3’ hairpin of MS2 RNA?

A protein referred to as "host factor".

How is the MS2 virion assembled?

Coat protein dimers bind the operator hairpin, maturation protein binds the MS2 stem-loop, leading to RNA encapsidation.