Construction of a transducing virus from double-stranded RNA bacteriophage φ6: Establishment of carrier states in host cells

Bacteriophage φ6 contains three double-stranded RNA (dsRNA) genomic segments. We have constructed a plasmid that contains a cDNA copy of the middle (M) segment, with a gene for kanamycin resistance (kan) inserted into the PstI site. A transcript of this cDNA was incorporated in vitro into procapsids along with natural transcripts of the S and L segments. The procapsids were coated with nucleocapsid surface protein P8 and transfected into Pseudomonas syringae pv. phaseolicola. The resulting infectious virus, φ6 K1, was found to contain an M segment that was 1.2 kbp larger than the normal 4.1 kbp. K1 formed small, turbid plaques, and its genome was unstable. Preparations of K1 contained from about 0.1 to 10% large, clear-plaque forms of the virus which were usually missing the kan gene, and in some cases, the resulting segment M was smaller than its normal size. Cells picked from lawns of host cells infected with K1 yielded colonies that were resistant to kanamycin (Kan). These colonies could be passaged on kanamycin-containing medium. The cells were found to contain large amounts of dsRNA corresponding to the viral genomic segments. Some strains continued to produce viable phage, while others lost this ability. One strain completely lost the small genomic segment S. Approximately 1 in 10,000 infected cells acquired the carrier state with the original phage isolate K1. However, we isolated a viral mutant that was able to induce the carrier state in 10 to 20% of the infected cells. The ability to use drug resistance as a test for the carrier state makes this system very useful for the study of the mechanisms of induction of persistent infections.