Escherichia coli strain MAS90 [E. coli K-12, recA1 D(pro-lac) thi ara F 0 : lacI q1 lacZ::Tn5 proAB + ]. Liquid cultures were grown in minimal MOPS media (32) using glycerol as carbon source. Cultures were incubated with shaking at 37 C for at least 10 generations in the log phase prior to being used in frameshift assays.