BioVector? Phage R4C 玫瑰桿菌噬菌體

1. Overview and Taxonomy

Name: R4C

Type: Bacteriophage (a virus that infects bacteria).

Host Organism: Dinoroseobacter shibae DFL12T, a member of the ecologically important marine Roseobacter clade.

Isolation Source: Isolated from coastal seawater in Xiamen, China.

Morphology: Member of the family Siphoviridae (order Caudovirales). It has an icosahedral head (capsid) and a long, non-contractile tail.

2. Morphological Characteristics

Capsid: Icosahedral with a diameter of approximately 66 nm. It exhibits a T=7 quasi-icosahedral lattice and is composed of 415 copies of the major capsid protein. One of the five-fold vertices is replaced by a portal complex for genome delivery.

Tail: A long, rigid, and non-contractile tail, approximately 110 nm in length. This is atypical, as siphophages usually have flexible tails.

Structural Proteins: The virion comprises 12 distinct structural protein components.

3. Genomic Features

Genome Type: Double-stranded DNA (dsDNA).

Genome Size: 36,291 base pairs (bp) .

GC Content: High, at 66.75% .

Gene Content: Contains 49 predicted genes, with low homology to those in other known phages, indicating its novelty.

Packaging Mechanism: Utilizes a 'cohesive ends' (cos) DNA-packaging mechanism, similar to that of bacteriophage lambda (λ).

4. Biological and Life Cycle Features

Lifestyle: While isolated as a virulent phage, its genome encodes an integrase. This implies a potential for a lysogenic (temperate) life cycle, which is unusual for phages in its taxonomic group.

Host Range: Appears to be narrow, infecting its host D. shibae DFL12T. (The primary source tested 19 strains, mainly from the Roseobacter genus, and found no other infections).

Lipid Presence: Tested negative for lipids, as it was not sensitive to chloroform.

Latent Period: Approximately 3.5 hours.

Burst Size: Approximately 22 plaque-forming units (PFU) per infected cell.

5. Structural and Mechanistic Insights (from Cryo-EM)
Recent cryo-electron microscopy (cryo-EM) studies have provided detailed structural insights:

Capsid Structure: The major capsid protein has the canonical "HK97-fold" but with unique structural adaptations that allow for the formation of hexons and pentons.

DNA Delivery Mechanism: A ratchet-like mechanism is proposed to assist in DNA transmission through the tail tube. This is supported by a specific negative charge distribution along the inner surface of the tail tube.

Adsorption Device: The baseplate or adsorption apparatus structurally resembles that of the gene transfer agent (RcGTA) of Rhodobacter capsulatus, suggesting an evolutionary link.

6. Culturing and Isolation Methods

Isolation Method: Isolated from seawater using the double-layer agar method with its host, Dinoroseobacter shibae DFL12T.

Propagation: Can be propagated on D. shibae DFL12T in liquid culture or on solid media using standard bacteriological techniques for marine bacteria.

Plaque Assay: Forms plaques on a lawn of D. shibae DFL12T using the double-layer agar technique. Specific media formulations for the host are not detailed in the primary source but would involve marine broth/agar.

Visualization: Morphology can be visualized using transmission electron microscopy (TEM) .

7. Ecological Distribution

Habitat: Genes homologous to R4C are more prevalent in coastal marine areas than in the open ocean, suggesting it is adapted to coastal environments.

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