Mechanisms causing rapid and parallel losses of ribose catabolism in evolving populations of Escherichia coli B

Twelve populations of Escherichia coli B all lost D-ribose catabolic functi on during 2,000 generations of evolution in glucose minimal medium, We soug ht to identify the population genetic professes and molecular genetic event s that caused these rapid and parallel losses. Seven independent Rbs(-) mut ants were isolated, and their competitive fitnesses were measured relative to that of their Rbs(+) progenitor. These Rbs(-) mutants mere all about 1 t o 2% more fit than the progenitor. A fluctuation test revealed an unusually high rate, about 5 x 10(-5) per cell generation, of mutation from Rbs' to Rbs-, which contributed to rapid fixation. At the molecular level, the loss of ribose catabolic function involved the deletion of part or all of the r ibose operon (rbs genes). The physical extent of the deletion varied betwee n mutants, but each deletion was associated with an IS150 element located i mmediately upstream of the rbs operon. The deletions apparently involved tr ansposition into various locations within the rbs operon; recombination bet ween the new IS150 copy and the one upstream of the rbs operon then led to the deletion of the intervening sequence. To confirm that the beneficial fi tness effect was caused by deletion of the rbs operon (and not some undetec ted mutation elsewhere), we used P1 transduction to restore the functional rbs operon to two Rbs(-) mutants, and we constructed another Rbs(-) strain by gene replacement with a deletion not involving IS150. All three of these new constructs confirmed that Rbs(-) mutants have a competitive advantage relative to their Rbs(+) counterparts in glucose minimal medium. The rapid and parallel evolutionary losses of ribose catabolic function thus involved both (i) an unusually high mutation rater such that Rbs(-) mutants appeare d repeatedly in all populations, and (ii) a selective advantage in glucose minimal medium that drove these mutants to fixation.