Complementation Between Filamentous F-specific and I-specific Bacteriophages

Two groups of filamentous bacterial viruses attacking Escherichia coli are known. One of these (Ff; F specific, filamentous, see review by Marvin & Hohn, 1969) adsorbs specifically to the tips of F-pili while the other (If) adsorbs only to the tips of I-pili. The F-pili are produced by cells which possess the F episome or certain derepressed R factors, while the I-pili are produced by cells harbouring Colicine factor I or certain other derepressed R factors (Lawn et al. 1967). Several species of Ff are known (f1, fd, M13, ZJ/2, Ec9) but only two isolates of If (If1 and If2). The virus particles of both groups are long flexible rods about 5.5 nm. wide but they differ in length. All the Ff phages are about 850 nm. long whereas the If phages are 1300 nm. long. There is a corresponding difference in the size of the genomes.