The central objective of this project is the analysis of error-prone repair in bacteriophage T4, with related explorations into other mechanisms of mutagenesis and into a novel mechanism for avoiding ultraviolet-induced killing. Most chemical and all radiation mutagenesis in T4 occur via error-prone repair and depend on the functions of the genes uvsW, uvsX and uvsY, most or all of the genes required for DNA replication, and perhaps several other genes as well. We have recently characterized temperature-sensitive alleles of the X and Y genes, alleles that differentially affect mutagenesis and inactivation. Some of the mutant X proteins are being isolated in order to determine which of their kinetic parameters correlate with mutagenesis and which with survival. A number of tests have been or will be conducted to characterize the role of the WXY system in photodynamically induced mutagenesis and killing, to determine if other T4 genes are involved in WXY action (especially genes 46, 47, 49, 58, 59 and uvsZ), to more deeply explore the role of the uvsW gene, and to characterize a newly discovered role in survival after UV irradiation for the T4 gene 32 protein (the major viral DNA-binding protein).