Coliphage MS2 containing 5-fluorouracil. II. RNA-deficient particles formed in the presence of 5-fluorouracil

The addition of 5-fluorouracil to Escherichia coli infected with MS2 leads to the formation of two classes of fluorouracil-containing phage: one with buoyant density greater than that of normal MS2 and another with buoyant density lower than normal. The latter are non-infectious and do not adsorb to E. coli receptor sites as measured by inhibition of infective center formation. Serologic tests revealed slight differences between normal and defective phage, but no difference in the fingerprints of the coat proteins was detectable. The RNA content of defective phage is about 65% of normal. The isolated RNA is heterogeneous, in keeping with the density heterogeneity of the particles, and has an average S₂₀ value of 20. Although the RNA is not infectious, it is more active than normal MS2 RNA in directing the synthesis of phage-specific proteins in cell extracts. As with normal MS2 RNA, the major product is phage coat protein, which yields a normal fingerprint. The results suggest that the RNA fragment lacks a specific part of the phage RNA molecule.