This research is designed to elucidate mechanisms for the regulation of expression of viral genes in host cells. The system being investigated is bacteriophage T5 and BF23 after infection of Escherichia coli. Phage mutants that display altered regulatory patterns of gene expression have been isolated and some of the phage-specific proteins that participate in these regulatory processes have been identified. These regulatory proteins are being isolated so that their mechanism of action can be determined in cell-free systems. A protein that shuts off the expression of both host genes and the first group of phage genes to be expressed after infection (pre-early genes) has been highly purified, and is currently being studied in a cell-free transcription system. Another protein that turns on the expression of late proteins is being studied in a similar way. Both T5 and BF23 modify the membranes of host cells that they infect. Colicinogenic factor Ib, an extra-chromosomal DNA element, also modifies the membranes of cells carrying it, and infection of these Ib-factor containing cells with T5 or BF23 leads to membrane disfunction due probably to over-modification of the cellular membranes. These membrane modifications are being studied from the point of view of protein:protein interactions among host, phage and Ib-factor specified membrane proteins.