Bacteriophage T7 is a relatively simple virus that has been well characterized physiologically and genetically. Mutations are now available in 25 of the 30 or so T7 genes, including many of the genes involved in replication and maturation of T7 DNA. Mutant strains of E. coli that do not permit normal replication of T7 DNA are also becoming available. Thus, the T7 - E-coli system is an excellent one in which to study the molecular details of DNA replication. Intermediates produced during replication and maturation of T7 DNA will be identified and isolated by sedimentation or gel electrophoresis of extracts of infected cells. Kinetics of movement of parental and progeny DNA through such forms will be followed for wild-type and mutant phages, and restriction endonucleases will be used to analyze their structure. Other aspects of DNA metabolism, such as protection of T7 DNA after infection, selective degradation of host DNA, initiation of replication, and mechanism of formation of deletions will also be studied. Attempts will be made to clone segments of the late region of T7 DNA in a bacterial plasmid. If the proposed experiments are successful, host containing such clones would be used for physical mapping of T7 mutants and as helper strains for isolation of deletions in the late region of T7 DNA. Such deletions would open the late region of T7 DNA to the types of genetic and physiological analysis that have been so successful with the early region.