Coliphage MS2 containing 5-fluorouracil: I. Preparation and physical properties

5-Fluorouracil inhibits the formation of MS2 measured as infective units or physical phage particles. Addition of the base analog during the first 13 minutes after infection has a greater effect than addition at a later time. Whereas 30 μg/ml. 5-fluorouracil added at five minutes after infection results in a marked decrease in progeny phage particles, later addition of FU† allows up to 50% normal yield of physical phage, some of which contains FU. FU phage has a greater buoyant density in CsCl than normal MS2, the density being directly related to the degree of replacement of uracil by FU. This density shift is due to an increased buoyant density of FU-RNA, measured in cesium sulphate. FU phage and FU-RNA have normal sedimentation coefficients, and the T_m of FU-RNA, in which 80% of the uracil is replaced by FU, is the same as that of normal MS2 RNA. The specific infectivity of this highly replaced FU phage is about 20% of normal.