The interaction of a phage-encoded topiosomerase I with specific sites to allow DNA recombination is being studied by: 1) genetic analysis of the inhibition of this reaction by various topoisomerase I mutants, and 2) biochemical analyses using attachment sites cloned onto a plasmid vector. Functional domains of the protein and recombination intermediates are being identified. A new role for integration host factor (IHF) has been defined. This accessory factor of Lambda site-specific cleavage is also involved in the packaging of Lambda DNA. Control regions of the genome of HSV-1 have been identified in the dDNA that occurs as reiterated sequences in virion particles. An origin of replication and packaging sites are evident. When Lambda HSV hybrid clones are transfected into eukaryotic cells with HSV helper, substantial editing of the Lambda sequences is seen.