This proposed research is concerned with the molecular mechanisms involved in the control of virus gene expression. We proceed with the bacteriphage T-7 - Escherichia coli system as a simple virus - host model system, and aim to obtain thorough understanding of virus development in the host which is applicable for the studies of other viruses inclucing animal viruses and tumor viruses. T7 genes and corresponding mRNAs and proteins have been subject to extensive genetic and biochemical studies. It is now well established that the early genes of T7 are transcribed by the host E. coli RNA polymerase, and the late genes are transcribed by the T7-coded RNA polymerase which is one of the early gene products. In addition to this transcriptional switch using two RNA polymerases at different times after infection, T7 phage has a "host shut-off" function which results in an inactivation of the host RNA polymerase thus terminates the synthesis of host RNA and T7 early messenger RNA. An inhibitor protein of E. coli RNA polymerase has been isolated and purified from T7 infected cells. It was found that T7 gene 2 codes for this inhibitor, termed I protein. Further characterization of the "host shut-off" mechanism which involved both gene 2 product (I protein) and gene 0.7 product (protein kinase) is proposed. New W. coli mutants have been isolated. T7 DNA processing and packaging are interfered in these mutants, resulting in an accumulation of T7 progeny DNA which is cleaved shorter than the T7 genome concomitant with an accumulation of large amounts of prohead and newly found head precursor particle, termed X particle. T7 DNA processing and head assembly process will be investigated using these host mutants.