Identification of ribozyme mutations that increase chloramphenicol resistance.

<p><i>A</i>, Growth rates of <i>E.coli</i> cells containing different ribozyme variants, in medium containing 20 µg/mL chloramphenicol. The parent ribozyme (0 Mut), the most efficient, evolved ribozyme (12 Mut; black), and ribozyme M4 (4 Mut; green) are labeled with the number of mutations distant from the parent ribozyme. The label P8 (orange column) describes the ribozyme where the four mutations were transplanted from the P6b loop to the P8 loop of the parent ribozyme. All other variants (grey columns) are based on the 12-mutation ribozyme and show the effect of individual, labeled reversion mutations to the parent sequence. The absence of a column means that the doubling time was larger than 100 minutes in all experiments. Error bars are standard deviations from at least three experiments. <i>B</i>, Secondary structure of the ribozyme with the positions of the 12 mutations in (A) indicated by gray circles. The four beneficial mutations in the P6b stem-loop are shown in green.