Abstract
A DNA protein complex that is a precursor of mature .PHI.X174 phage was isolated. The complex sedimented with an S value of 50 in a sucrose gradient and contained phage DNA consisting of a replicative form molecule with an extended tail of single-stranded viral DNA. The viral-strand DNA ranged from 1-2 genomes in length. Proteins coded on the .PHI.X174 genome and on the host genome were associated with the viral DNA in the 50S precursor complex. Viral DNA synthesis and cleavage of the growing viral-strand DNA apparently occurred in the 50S complex.