A SEARCH FOR A GENERAL PHENOMENON OF ADAPTIVE MUTABILITY

The most prominent systems for the study of adaptive mutability depend on the specialized activities of genetic elements like bacteriophage Mu and the F plasmid. Searching for general adaptive mutability, we ha ve investigated the behavior of Salmonella typhimurium strains with ch romosomal lacZ mutations. We have studied 30 revertible nonsense, miss ense, frameshift, and insertion alleles. One-third of the mutants prod uced greater than or equal to 10 late revertant colonies (appearing th ree to seven days after plating on selective medium). For the prolific mutants, the number of late revertants showed rank correlation with t he residual beta-galactosidase activity; for the same mutants, reverta nt number showed no correlation with the nonselective reversion rate ( from fluctuation tests). Leaky mutants, which grew slowly on selective medium, produced late revertants whereas tight nongrowing mutants gen erally did not produce late revertants. However, the number of late re vertants was not proportional to residual growth. Using total residual growth and the nonselective reversion rate, the expected number of la te revertants was calculated. For several leaky mutants, the observed revertant number exceeded the expected number. We suggest that excess late revertants from these mutants arise from general adaptive mutabil ity available to any chromosomal gene.