Phi X 174

The phi X 174 (or ΦX174) bacteriophage is a single-stranded DNA (ssDNA) virus that infects Escherichia coli. This virus was isolated in 1935 by Nicolas Bulgakov in Félix d'Hérelle's laboratory at the Pasteur Institute, from samples collected in Paris sewers. Its characterization and the study of its replication mechanism were carried out from the 1950s onwards. It was the first DNA-based genome to be sequenced. This work was completed by Fred Sanger and his team in 1977. In 1962, Walter Fiers and Robert L. Sinsheimer had already demonstrated the physical, covalently closed circularity of ΦX174 DNA. Nobel prize winner Arthur Kornberg used ΦX174 as a model to first prove that DNA synthesized in a test tube by purified enzymes could produce all the features of a natural virus, ushering in the age of synthetic biology. In 1972–1974, Jerard Hurwitz, Sue Wickner, and Reed Wickner with collaborators identified the genes required to produce the enzymes to catalyze conversion of the single stranded form of the virus to the double stranded replicative form. In 2003, it was reported by Craig Venter's group that the genome of ΦX174 was the first to be completely assembled in vitro from synthesized oligonucleotides. The ΦX174 virus particle has also been successfully assembled in vitro. In 2012, it was shown how its highly overlapping genome can be fully decompressed and still remain functional.

Naming The naming of the virus follows a system that was developed by Vladimir Sertić and Nikolai Boulgakov in 1935. The Greek letter 'Φ' was used to indicate that it worked against multiple bacteria. The Roman numeral 'X' (ten) referred to an antigen-based classification and 174 was related to isolates against which it was tested.

Genome

This bacteriophage has a [+] sense circular single-stranded DNA genome of 5,386 nucleotides. The genome GC-content is 44% and 95% of nucleotides belong to coding genes. Because of the balance base pattern of the genome, it is used as the control DNA for Illumina sequencers.

Genes ΦX174 encodes 11 genes, named as consecutive letters of the alphabet in the order they were discovered, with the exception of A* which is an alternative start codon within the large A genes. Only genes A* and K are thought to be non-essential, although there is some doubt about A* because its start codon could be changed to ATT but not any other sequence. It is now known that the ATT is still likely capable of producing protein within E. coli and therefore this gene may in fact be essential. The first half of the ΦX174 genome features high levels of gene overlap with eight out of 11 genes overlapping by at least one nucleotide. These overlaps have been shown to be non-essential although the refactored phage with all gene overlaps removed had decreased fitness from wild-type. Phage ΦX174 has been used to try to establish the absence of undiscovered genetic information through a "proof by synthesis" approach.

Transcriptome In 2020, the transcriptome of ΦX174 was generated. Notable features of the ΦX174 transcriptome is a series of up to four relatively weak promoters in series with up to four Rho-independent (intrinsic) terminators and one Rho-dependent terminator.

Proteins ΦX174 encodes 11 proteins.

Proteome Identification of all ΦX174 proteins using mass spectrometry has recently been reported.

Infection cycle Infection begins when G protein binds to lipopolysaccharides on the bacterial host cell surface. H protein (or the DNA Pilot Protein) pilots the viral genome through the bacterial membrane of E. coli bacteria most likely via a predicted N-terminal transmembrane domain helix. However, it has become apparent that H protein is a multifunctional protein. This is the only viral capsid protein of ΦX174 to lack a crystal structure for a couple of reasons. It has low aromatic content and high glycine content, making the protein structure very flexible and in addition, individual hydrogen atoms (the R group for glycines) are difficult to detect in protein crystallography. Additionally, H protein induces lysis of the bacterial host at high concentrations as the predicted N-terminal transmembrane helix easily pokes holes through the bacterial wall. By bioinformatics, this protein contains four predicted coiled-coil domains which has a significant homology to known transcription factors. Additionally, it was determined that de novo H protein was required for optimal synthesis of other viral proteins. Mutations in H protein that prevent viral incorporation, can be overcome when excess amounts of protein B, the internal scaffolding protein, are supplied. The DNA is ejected through a hydrophilic channel at the 5-fold vertex. It is understood that H protein resides in this area but experimental evidence has not verified its exact location. Once inside the host bacterium, replication of the [+] ssDNA genome proceeds via negative sense DNA intermediate. This is done as the phage genome supercoils and the secondary structure formed by such supercoiling attracts a primosome protein complex. This translocates once around the genome and synthesizes a [−]ssDNA from the positive original genome. [+]ssDNA genomes to package into viruses are created from this by a rolling circle mechanism. This is the mechanism by which the double stranded supercoiled genome is nicked on the positive strand by a virus-encoded A protein, also attracting a bacterial DNA polymerase (DNAP) to the site of cleavage. DNAP uses the negative strand as a template to make positive sense DNA. As it translocates around the genome it displaces the outer strand of already-synthesised DNA, which is immediately coated by SSBP proteins. The A protein cleaves the complete genome every time it recognises the origin sequence. As D protein is the most abundant gene transcript, it is the most abundant protein in the viral procapsid. Similarly, gene transcripts for F, J, and G are more abundant than for H as the stoichiometry for these structural proteins is 5:5:5:1. The primosomes are protein complexes which attach/bind the enzyme helicase on the template. Primosomes gives RNA primers for DNA synthesis to strands.

Mutation rate The mutation rate of phiX174 is estimated to be 1.0 × 10−6 substitutions per base per round of copying, a value that is consistent with Drake's rule (0.003 mutations per genome per round of copying in DNA-based microorganisms).

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