The pi-histidine factor of Salmonella typhimurium: a demonstration that pi-histidine factor integrates into the chromosome.

The Salmonella typhimurium pi-histidine episome was identified by Ames et al. (2) in an unstable partial revertant of a deletion mutation-containing strain, hisG203. HisG203 lacks the histidine operator, promoter, and part of the first structural gene. In this paper, we study some properties of pi factor and demonstrate a low frequency of pi integration into the chromosome at or near the histidine region.