Filamentous phage assembly: Morphogenetically defective mutants that do not kill the host

Abstract

The filamentous phage virion is assembled without killing the host, by extrusion of the DNA through the envelope and concomitant acquisition of coat proteins from the inner membrane. When assembly is blocked, however, intracellular phage DNA and gene products accumulate and the host is killed. This “cell killing” is largely absent in phage fdtet, which carries a tetracycline-resistance determinant within the origin of minus-strand synthesis; as a result of the replication defect, phage DNA does not accumulate to high levels intracellularly when virion assembly is blocked. This allows morphogenetically defective mutants except those ablating gene V to be freely propagated in tetracycline-containing medium and studied in the absence of the confounding factor of cell morbidity. Because cultures can be initiated by transfection in the complete absence of input virions, extremely low levels of phage production can be assayed. Using this system, I show that genes III, VI, I, and IV are not required to form the complex between viral DNA and gene-V protein that is the intracellular precursor to mature virions; that genes I and/or IV are absolutely (or nearly absolutely) required for assembly; and that mos, a cis-acting sequence previously shown to enhance phage yield in some circumstances, is without such effect in others.