Genetic inversion in the formation of an Hfr strain from a temperature-sensitive F' gal strain.

An Hfr strain (PB15) that carries a duplicated copy of the galactose operon genes flanking the integrated sex factor is unusually stable since it does not show excision of the repeated deoxyribonucleic acid segment. The right-hand galactose operon is in the normal orientation. Deletion mutations that eliminate the right-hand galactose genes, the sex factor, and some of the left-hand operon have been isolated. Mutants believed to have their left-hand galactose operon inverted were able to be induced for galactose epimerase synthesis by D-fucose but did not show escape synthesis on induction of bacteriophage lambda. Ribonucleic acid specific for the galactose operon was isolated after induction of lysogenic strains presumed to carry the galactose operon in the normal and inverted orientation. Hybridization to the isolated left and right strands of lambdapgal showed that the noninformational strand of the left-hand galactose operon of the deletion mutant of PB15 was transcribed on escape induction. These results show that inversion has occurred.