Selection and Endpoint Distribution of Bacterial Inversion Mutations

This paper describes the isolation and characterization of spontaneous inversion mutants of Salmonella typhimurium. The mutants are selected by demanding that an unexpressed hisD gene acquire a new promoter. Chromosome rearrangements that juxtapose the hisD gene and a foreign promoter are obtained by this selection. Although a number of inversions are found, the frequency was lower than expected. The breakpoint of these inversions are not distributed randomly either in the his operon or on the chromosome. The his breakpoint lies in the hisG-hisD intercistronic region, a sequence known to occur at several places on the bacterial chromosome. In most of the inversions, the 'non-his' breakpoint lies across the chromosome, so that the inverted region includes the origin or terminus of DNA replication. The significance of these results is discussed.