UV irradiation impairs in vivo encapsidation of bacteriophage T4 DNA.

T4 DNA structural requirements for encapsidation in vivo were investigated, using thin-section electron microscopy to quantitate the kinetics and yields of head intermediates after synchronous DNA packaging into accumulated processed proheads. UV irradiation (254 nm) of T4-infected bacteria just before initiation of encapsidation resulted in a reduction in the rate of DNA packaged measured by electron microscopy and in the yield of viable phage progeny. In UV-irradiated infections with excision-deficient mutants (denV-), the extent of packaging decline was proportional to the UV dose and phage yields were lower than expected based on the packaging levels observed by microscopy. Rescue analysis of progeny from such infections revealed elevated levels of nonviable virions. Pyrimidine dimers were encapsidated in denV- infections, but in excision-competent infections (denV+) dimers were not packaged. A UV-independent, 15 to 20% packaging arrest was also observed when denV endonuclease was inactive during encapsidation, indicating a denV requirement to achieve normal T4 packaging levels. Pyrimidine dimers apparently represent or induce transient blockage of DNA encapsidation or both, causing a decline in the rate. This is in contrast to other DNA structural blocks to packaging induced by mutations in T4 genes 30 and 49, which appear to arrest the process.