- Bacteriophage ϕX174 is a well-studied virus that infects Escherichia coli and related bacteria. It belongs to the family Microviridae and is one of the smallest known DNA viruses, both in genome size and physical dimensions.
- Discovered in the early 1950s, ϕX174 gained historical importance as the first DNA-based genome to be fully sequenced, a landmark achieved by Frederick Sanger and colleagues in 1977. Its relatively simple genetic makeup and rapid replication cycle make it a model organism in molecular biology, genetics, and virology.
- The ϕX174 genome is a circular, single-stranded DNA (ssDNA) molecule comprising approximately 5,386 nucleotides. Despite its compact size, it encodes 11 proteins, many of which have overlapping reading frames, demonstrating the virus’s efficient use of genetic material. These proteins include those involved in structural functions, such as capsid formation (e.g., F, G, H, and J proteins), and non-structural proteins essential for replication and host cell lysis, such as A and E proteins. Protein A plays a crucial role in initiating DNA replication, while protein E causes host cell lysis by inhibiting cell wall synthesis, thereby releasing progeny phages.
- ϕX174 infects E. coli by attaching to lipopolysaccharides on the outer membrane and injecting its ssDNA genome into the cytoplasm. Once inside, the host’s replication machinery synthesizes a complementary strand, forming a double-stranded replicative form (RF DNA). This RF DNA serves as a template for transcription and replication, leading to the production of viral proteins and progeny genomes. The assembly of new virions occurs in the cytoplasm, followed by the lysis of the host cell to release the new phage particles.
- Due to its simplicity and experimental tractability, ϕX174 has served as a prototype for studying fundamental aspects of molecular biology, including DNA replication, transcriptional regulation, gene expression, and genome evolution. It has also been used in synthetic biology, where its genome has been artificially synthesized to explore the limits of genome design and viral engineering. Overall, ϕX174 remains an iconic model system, bridging historical significance with modern scientific inquiry.
