Reversion Instability of an Extreme Polar Mutant of the Galactose Operon

HE mutation gul-3 of the galactose operon of Escherichia coli (MORSE, LEDER-BERG and LEDERBERG 1956) has been shown to be a mutant of the extreme polar type (KALCKAR, KURAHASHI and JORDAN 1959; The extreme polar mutants (formerly called Oo) were thought to be changes of the operator region but are now believed to be nonsense mutations in the structural gene proximal to the ' Supparled by Public Health Service Grant AI-01345-09. Contribution No. 264, Department of Biophysics ' Cui-eer Development Awardee, U.S. Public Health Service. types were estimated from the median of the distribution (LEA and COWISON 1W) and the mean number of, bactenal divisions. The rates were: R l phenotype, 1.2 x lP/bacterium/division; other phenotyqes, 3.0 X 10-8/bacterium/division. Total revertibility, 3.7 x lO-a/bacterium/division. Instabillty appears to be restncted to the R1 phenotype, but not all RI-appearing colonies are unstable.