fD

Fd, often written fd to reflect its naming in literature, is a filamentous bacteriophage that infects Escherichia coli. It is one of the Ff phages, a group that also includes f1 and M13, and is known for its slender, flexible rod-like virions. Particles are about 6 nanometers in diameter and can reach lengths of roughly 0.9 to 1.5 micrometers depending on coat protein assembly. The virion carries a circular single-stranded DNA genome encased by thousands of copies of the major coat protein, pVIII, with specialized proteins at the ends that mediate infection and assembly.

The fd genome is a circular single-stranded DNA molecule of about 6,400 nucleotides, encoding roughly 10 to 11 proteins. Replication proceeds via a replicative form inside the host cell and uses a rolling-circle mechanism. New virions are assembled at the inner membrane and extruded through the outer membrane, causing a chronic infection without host lysis.

Infection requires the F pilus of F+ Escherichia coli; the phage binds via its end proteins to the pilus, retracts it, and transfers its genome into the cell. The infection enables continuous production and secretion of progeny virions while the host remains viable.

Fd is widely used as a cloning and display vector in molecular biology. As with other Ff phages, it supports phage-display technologies, allowing peptides or proteins to be displayed on the tip protein (pIII) or on the major coat protein (pVIII). This enables ligand discovery and the construction of nanoscale filaments for research in biotechnology and structural biology.

Taxonomically, fd is classified in the family Inoviridae, genus Inovirus, Escherichia virus fd. It is a prototypical filamentous phage closely related to f1 and M13. The capitalization fd versus Fd varies across sources.