Construction of a microphage variant of filamentous bacteriophage.

The intergenic region in the genome of the Ff class of filamentous phage (comprising strains fl, fd and M13) genome constitutes 8% of the viral genome, and has essential functions in DNA replication and phage morphogenesis. The functional domains of this region may be inserted into separate sites of a plasmid to function independently. Here, we demonstrate the construction of a plasmid containing, sequentially, the origin of (+)-strand synthesis, the packaging signal and a terminator of (+)-strand synthesis. When host cells harboring this plasmid (pLS7) are infected with helper phage they produce a microphage particle containing all the structural elements of the mature, native phage. The microphage is 65 A in diameter and about 500 A long. It contains a 221-base single-stranded circle of DNA coated by about 95 copies of the major coat protein (gene 8 protein).