The primary objective of this research is to understand the molecular mechanisms underlying F-mediated restriction of T7 and of mutants of the related organism, bacteriophage T3. Genetic and biochemical studies are being employed to investigate the pleiotropic physiological defects associated with the abortive infection. These include perturbations in gene expression, both transcriptional and translational, an inhibition of phage DNA replication and possible membrane dysfunctions. The work will include studies both on the phage genes and those chromosomal or F-plasmid encoded genes which determine F-mediated restriction. Pseudorevertants of mutant T3 strains that have regained the ability to overcome F-restriction will be analyzed by biochemical means to investigate the intracellular role of the major capsid protein in controlling the transcriptionally-mediated entry of DNA into the cell and in early steps in phage DNA metabolism. These studies will also be conducted in parallel with phage T7, using its abortive infection of Shigella sonnei as a primary biological assay system.