The consequences of growth of a mutator strain of Escherichia coli as measured by loss of function among multiple gene targets and loss of fitness.

Abstract

We have examined the composition of members of mutator populations of Escherichia coli by employing an extensive set of phenotypic screens that allow us to monitor the function of >700 genes, constituting approximately 15% of the genome. We looked at mismatch repair deficient cells after repeated cycles of single colony isolation on rich medium to generate lineages that are forced through severe bottlenecks, and compared the results to those for wild-type strains. The mutator lineages continued to accumulate mutations rapidly with each increasing cycle of colony isolation. By the end of the 40th cycle, after approximately 1000 generations, most of the lineages had reduced colony size, 4% had died out, 55% had auxotrophic requirements (increasing to 80% after 60 cycles), and 70% had defects in at least one sugar or catabolic pathway. In addition, 33% had a defect in cell motility, and 26% were either temperature-sensitive or cold-sensitive lethals. On the other hand, only 3% of the wild-type lineages had detectable mutations of any type after 40 cycles. By the 60th cycle, the typical mutator cell carried 4-5 inactive genes among the 15% of the genome being monitored, indicating that the average cell carried at least 24-30 inactivated genes distributed throughout the genome. Remarkably, 30% of the lineages had lost the ability to utilize xylose as a carbon source. DNA sequencing revealed that most of the Xyl(-) mutants had a frameshift in a run of eight G's (GGGGGGGG) in the xylB gene, either adding or deleting one -G-. Further analysis indicated that rendering E. coli deficient in mismatch repair unmasks hypermutable sites in certain genes or intergenic regions. Growth curves and competition tests on lineages that passed through 90 cycles of single colony isolation showed that all lineages suffered reduced fitness. We discuss these results in terms of the value of mutators in cellular evolution.