The role of single-mutant intermediates in the generation of trpAB double revertants during prolonged selection.

Selection-induced mutations are nonrandom mutations that occur as specific, direct responses to environmental challenges and that occur more often when they are selectively advantageous than when they are selectively neutral. One of the most puzzling examples of selection-induced mutations involved the simultaneous reversions of two mutations, one in trpA and the other in trpB, at rates that were several orders of magnitude greater than would have been predicted if the two mutations had occurred as independent events (B. G. Hall, Proc. Natl. Acad. Sci. USA 88:5882-5886, 1991). Here I examine the possibility that the double mutations might be accounted for by sequential mutations with intervening growth.