Genetic analysis of the filamentous bacteriophage packaging signal and of the proteins that interact with it.

The single-stranded DNA of filamentous phages (f1, fd, M13, Ike) contains a region that can fold into a hairpin structure that serves to earmark the DNA for encapsidation. Second-site suppressor mutants of f1 that can compensate for deletion of this packaging signal have been isolated and characterized. The mutations lie in three genes, two that encode virion proteins located at the end of the particle that is first to emerge from the cell, the end at which the packaging signal is located, and the third in a gene whose product is required for assembly but which is not itself a part of the virion. Analysis of base substitution and deletion mutations in the packaging signal suggests that both structural and sequence elements are important to its proper function.