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Bacteriophage phi-X174: A Virus with Both DNA and RNA

Bacteriophage phi-X174 is a small, non-enveloped virus that infects the bacterium Escherichia coli. It has a circular, single-stranded DNA genome of about 5.4 kilobases (kb) that encodes 11 proteins involved in the virus replication, transcription, and assembly. However, unlike most other DNA viruses, phi-X174 also has a single-stranded RNA molecule that is packaged inside the virion and serves as a primer for the DNA replication.

DNA replication in phi-X174:

1. Attachment and penetration: The virus binds to a specific receptor on the bacterial cell surface and injects its DNA into the host cytoplasm by puncturing the cell wall and membrane.

2. Priming and elongation: The RNA primer anneals to a complementary sequence on the DNA and serves as a template for the synthesis of the complementary DNA strand by the viral DNA polymerase. This process is called rolling-circle replication because the circular DNA strand is continuously copied and displaced as a long linear concatemer.

3. Cleavage and packaging: The concatemer is cut by a viral endonuclease into unit-length genomes that are selectively packaged into pre-formed capsids that also contain the RNA primer and other viral proteins.

RNA function in phi-X174:

1. Primer for DNA replication: The RNA molecule, called A protein, has a sequence that matches a specific region at one end of the DNA strand and can base-pair with it. This allows the DNA polymerase to start copying the DNA using the RNA as a primer.

2. Inhibition of host translation: The A protein also binds to the ribosome, the cellular machinery that translates messenger RNA into proteins, and blocks its function. This ensures that the host cell produces fewer proteins that could interfere with the virus replication.

Implications for virology:

Bacteriophage phi-X174 is an interesting example of a virus that has evolved a hybrid genome consisting of both DNA and RNA. This kind of molecular diversity may have advantages and disadvantages in different environments and hosts. For example, having an RNA primer can increase the speed and efficiency of DNA replication, especially in cells with limited resources. On the other hand, having both DNA and RNA may require more complex regulation of gene expression and replication, and may restrict the range of hosts that the virus can infect. Understanding the biology and evolution of phi-X174 and other viruses with mixed genomes can shed light on the origins and diversity of viruses and their interactions with hosts.

Retro virus