Directed mutation: paradigm postponed

Claims have been made in recent years that spontaneous mutational events are "directed" by (i) the presence in a selective medium of a single carbon source which the cells are unable to utilize (e.g. lactose), or (ii) the absence from a selective medium of an amino acid which the cells are unable to make for themselves (e.g. tryptophan). In resurrecting the previously rejected hypothesis of a "directed" origin for spontaneous mutants, it was noted that selecting for valine-resistant mutants did not allow comparable increases in colony numbers to be observed when overlays containing valine and glucose were added to plates which had accumulated (or were still accumulating) large numbers of "directed" Lac+ or Trp+ mutants. The present paper shows that Valr mutants do occur in much greater numbers if a carbon source other than glucose is added to the plates along with the valine-containing overlays; the evidence is that even small amounts (< 0.02%) of glucose in the overlays prevent the recovery of Valr mutants. From these results, it is argued that the apparent "directed" nature of the spontaneous mutation process is actually a manifestation of the long-known phenomenon of glucose repression.