Fig. 4.

Growth curves and mutant appearance frequencies in soft agar plates containing 0.5 % tryptone (a) or 2.5 % tryptone (b). For (a), overnight cultures of CAT_Km^R (wild-type, Km^R, 2×10⁹ c.f.u. ml⁻¹; squares) and a CAT_ IS5 mutant (Cm^R, 2×10⁶ c.f.u. ml⁻¹; circles) were mixed and inoculated into soft agar plates (0.5 % tryptone, 0.5 % NaCl and 0.3 % agar) at 30 °C. For (b), the same strains were inoculated into soft agar with 2.5 % tryptone plus NaCl and 0.3 % agar. All soft agar plates were incubated at 30 °C. Every 3 h, the cells were collected and plated on LB and LB+Cm plates to measure the two inoculated populations. The emerging insertion mutant populations (Cm^R Km^R, triangles) were determined on LB+Cm+Km plates. All measurements were conducted in triplicate, and the error, expressed in sd, was less than 10 % of the measured values. Error bars are often obscured by the symbols representing the averages of the three determinations.