Requirement of a limited segment of the sog gene for plasmid R64 conjugation

The sog gene of the IncI1 plasmid R64 was sequenced and characterized. The sog gene was shown to express two acidic proteins, SogL and SogS, with 1255 and 844 amino acid residues, respectively. The SogS protein was expressed by translational reinitiation within the SogL reading frame. Analysis of dnaG-suppression activity using the Escherichia coli dnaG strain indicated that the domain for this activity was located within the N-terminal one-third segment of the SogL protein. A Deltasog mutation was constructed by replacing most of the sog coding sequence with a DNA fragment encoding a tetracycline resistance gene. Introduction of the Deltasog mutation into an R64 derivative resulted in approximately a 50-fold reduction in transfer frequency. It was observed that only a limited portion of the SogL or SogS protein corresponding to an internal 0.94-kb EcoRV-SnaBI segment of the sog gene was required for the conjugal transfer of R64.