The role of transient hypermutators in adaptive mutation in Escherichia coli

Microbial populations under nonlethal selection can give rise to mutations that relieve the selective pressure, a phenomenon that has come to be calle d "adaptive mutation." One explanation for adaptive mutation is that a smal l proportion of the cells experience a period of transient hypermutation, a nd that these hypermutators account for the mutations that appear. The expe riments reported here investigated the contribution that hypermutators make to the mutations occurring in a Lac(-) strain of Escherichia coli during s election for lactose utilization. A broad mutational screen, loss of motili ty, was used to compare the frequency of nonselected mutations in starved L ac(-) cells, in Lac(+) revertants, and in Lac(+) revertants carrying yet an other nonselected mutation. These frequencies allowed us to calculate that the hypermutating subpopulation makes up approximate to 0.06% of the popula tion and that its mutation rate is elevated approximate to 200-fold. From t hese numbers we conclude that the hypermutators are responsible for nearly all multiple mutations but produce only approximate to 10% of the adaptive Lac(+) mutations.