Phage T4, the archetype of lytic bacterial viruses, needs only 62 genes to propagate under standard laboratory conditions. Interestingly, the T4 genome contains more than 100 putative genes of unknown function, with few detectable homologues in cellular genomes. To characterize this uncharted territory of genetic information, we have identified several T4 genes that prevent bacterial growth when expressed from plasmids under inducible conditions. Here, we report on the various phenotypes and molecular characterization of 55.1, one of the genes of unknown function. High-level expression from the arabinose-inducible P(BAD) promoter is toxic to the bacteria and delays the intracellular accumulation of phage without affecting the final burst size. Low-level expression from T4 promoter(s) renders bacteria highly sensitive to UV irradiation and hypersensitive to trimethoprim, an inhibitor of dihydrofolate reductase. The delay in intracellular phage accumulation requires UvsW, a T4 helicase that is also a suppressor of 55.1-induced toxicity and UV sensitivity. Genetic and biochemical experiments demonstrate that gp55.1 binds to FolD, a key enzyme of the folate metabolism and suppressor of 55.1. Finally, we show that gp55.1 prevents the repair of UV-induced DNA photoproducts by the nucleotide excision repair (NER) pathway through interaction with the UvrA and UvrB proteins.