Single-Stranded DNA Viruses Use a Double-Stranded Intermediate in Their Life Cycles

Most DNA viruses are double-stranded, but several important viruses with single-stranded (ss) DNA genomes have been described. The life cycles of ssDNA viruses are similar to those of dsDNA viruses with one major exception.

An additional step must occur in the synthesis stage because the ssDNA genome needs to be converted to a dsDNA molecule. A few ssDNA viruses are discussed next.

Bacteriophages  φ fi 174 and fd

The life cycle of fX174 (family Microviridae, species Enterobacteria phage phiX174) begins with attachment to the cell wall of its E. coli host. The circular ssDNA genome enters the cell with the help of a pilot protein called protein H.

Protein H is thought to form a tubelike structure through which the DNA passes; the protein capsid remains outside the cell. The fX174 ssDNA genome has the same base sequence as viral mRNA and is therefore plus-strand DNA.

For either transcription or genome replication to occur, the phage DNA must be converted to a double-stranded replicative form (RF) (figure 27.17). This is catalyzed by the host’s DNA polymerase. The RF directs the synthesis of more RF copies and plus-strand DNA, both by rolling-circle replication (see figure).

Assembly of fX174 virions begins with formation of a procapsid. This involves scaffolding proteins that interact with capsid proteins, causing them to undergo conformational changes and form a procapsid with the proper morphology. Once the procapsid is generated, the ssDNA genome is inserted, creating the mature virion.

The Multiplication Strategy of fX174
The Multiplication Strategy of fX174, a Plus-Strand DNA Phage.

After hat blocks peptidoglycan synthesis. Enzyme E inhibits the activity of the bacterial protein MraY, which catalyzes the transfer of peptidoglycan precursors to lipid carriers (see figure). Blocking cell wall synthesis weakens the host cell wall, causing the cell to lyse and release the progeny virions.

We include Escherichia coli phage fd (family Inoviridae) in our discussion because of one striking feature: its ability to be released from its host without lysis; it is also a good example of viral selfassembly. It, like other filamentous phages (e.g., Pseudomonas phage Pf1), is released by a secretory process. Extrusion begins when phage coat proteins are inserted into the host cell’s plasma membrane.

The coat then assembles around the viral DNA as it is secreted through the host plasma membrane (figure 27.18). Although the host cell is not lysed, it grows and divides at a slightly reduced rate. This mechanism is rare among bacterial viruses but is more common among archaeal viruses, as we note in blog.


Parvoviruses (family Parvoviridae) infect numerous animal hosts, including crustaceans, dogs, cats, mice, and humans. Since its discovery in 1974, human parvovirus B19 (genus Erythrovirus) has emerged as a significant human pathogen.

Parvovirus virions are uniform, icosahedral, nonenveloped particles approximately genome is so small that it directs the synthesis of only three proteins and some smaller polypeptides. None has enzymatic activity. Human parvovirus B19 infection

Release of Pf1 Phage.
Release of Pf1 Phage.
 The filamentous Pf1 phage is released from Pseudomonas aeruginosa without lysis. In this illustration, the blue cylinders are hydrophobic α-helices that span the plasma membrane, and the red cylinders are amphipathic helices that lie on the membrane surface before virion assembly.
 In each protomer, the two helices are connected by a short, flexible peptide loop (yellow). It is thought that the blue helix binds with circular, single-stranded viral DNA (green) as it is extruded through the membrane. The red helix simultaneously attaches to the growing viral coat that projects from the membrane surface. Eventually the blue helix leaves the membrane and also becomes part of the capsid 26 nm in diameter.

Their genomes are composed of one ssDNA molecule of about 5,000 bases. Most of the genomes are negative-strand DNA molecules. That is, their sequence of nucleotides is complementary to that of the viral mRNA (figure). Parvoviruses are among the simplest of the DNA viruses.

The genome is so small that it directs the synthesis of only three proteins and some smaller polypeptides. None has enzymatic activity. Human parvovirus B19 infection (section) Each parvovirus uses a particular host cell molecule for attachment.

For instance, the B19 virus uses a molecule found only on the surface of red blood cell progenitors. The virion then enters by receptor-mediated endocytosis. The virion escapes the endosome and is thought to be transported to the nucleus by the host cell’s microtubules.

The Rolling Hairpin Replication Used by Parvoviruses to
The Rolling-Hairpin Replication Used by Parvoviruses to Replicate Their DNA.

The nucleocapsid enters the nucleus, followed by release of viral DNA from the capsid. Since the parvovirus genome does not code for any enzymes, the virus must use host cell enzymes for all biosynthetic processes. Thus viral DNA can only be replicated in the nucleus during the S phase of the cell cycle, when the host cell replicates its own DNA.

Because the genome is negative-strand DNA, it serves as the template for mRNA synthesis. Some of the RNA products encode polypeptides required for the interesting way the virus’s genome is replicated. The ends of the parvovirus genome are palindromic sequences that can fold back on themselves. Formation of a hairpin at the 39 end of the genome provides the primer needed for replication (figure).

This is recognized by the host DNA polymerase, and DNA replication ensues by a process that is somewhat similar to rolling-circle replication. The parvovirus version of this replication method is often called rolling-hairpin replication because DNA polymerase seems to shuttle back and forth as it synthesizes genomes. The process involves a dsDNA intermediate, much like fX174.