The sog gene of the large plasmid ColIdrd-1 has previously been shown to encode a DNA primase and a smaller antigenically related polypeptide. Genesis of these two products has been examined using Sog+ recombinant plasmids. Effects of amber mutations, isolated after in vitro mutagenesis, and deletions into or within sog suggest that the smaller polypeptide is a separate translation product which is encoded by DNA specifying the C-terminal region of the larger protein. Under control of the lac promotor, synthesis of both polypeptides is reduced when transcription is repressed. These findings imply that transcription of sog yields a single transcript which is translated from two initiation sites.