A general mechanism for viral resistance to suicide gene expression

Bacteriophage T7 was challenged with either of two toxic genes expressed fr om plasmids. Each plasmid contained a different gene downstream of a T7 pro moter; cells harboring each plasmid caused an infection by wild-type T7 to abort. T7 evolved resistance to both inhibitors by avoidance of the plasmid expression system rather than by blocking or bypassing the effects of the specific toxic gene product. Resistance was due to a combination of mutatio ns in the T7 RNA polymerase and other genes expressed at the same time as t he polymerase. Mutations mapped to sites that are unlikely to alter polymer ase specificity for its cognate promoter but the basis for discrimination b etween phage and plasmid promoters in vivo was not resolved. A reporter ass ay indicated that, relative to wild-type phage, gene expression from the pl asmid was diminished several-fold in cells infected by the evolved phages. A recombinant phage, derived from the original mutant but lacking a mutatio n in the gene for RNA polymerase. exhibited intermediate activity in the re porter assay and intermediate resistance to the toxic gene cassettes, Alter ations in both RNA polymerase and a second gene are thus responsible for re sistance, These findings have broad evolutionary parallels to other systems in which viral inhibition is activated by viral regulatory signals such as defective-interfering particles, and they may have mechanistic parallels t o the general phenomena of position effects and gene silencing.