In vivo synthesis and properties of uracil-containing DNA.

T4 bacteriophage DNA containing as much as 30% of its thymine replaced by uracil can be synthesised in Escherichia coli deficient in both dUTPase and uracil--DNA glycosidase. This uracil-containing DNA is competent for RNA transcription, and can be packaged into phage which are viable, if the host cells are deficient in uracil--DNA glycosidase activity. If the host cells are not deficient in this glycosidase activity the infecting phage DNA is rapidly attacked, resulting in more than 50% acid-solubilisation of the DNA. The infected cells are inefficiently killed, presumably because of very limited, if any, expression of the phage DNA. These results indicate that this replacement of thymine by uracil in DNA does not seriously impair the biological functionality of T4 DNA, provided the DNA is not subjected to the breakdown (repair) pathway initiated by uracil--DNA glycosidase.