Double, independent mutational events in the rpsL gene of Escherichia coli: an example of hypermutability?

Summary A proportion (up to 20%) of newly arising streptomycin‐dependent (Sm D ) colonies of Escherichia coli WP2 contain bacteria where, in addition to a known Sm D ‐determining (primary) mutation in the rpsL gene, there is a further ancillary mutation in the same gene. These ancillary mutations occur at 10 sites between 11 and 201 bp away from the primary mutation. Ancillary mutations have been found in mutant colonies arising both spontaneously and after treatment with ultraviolet light and some have been found repeatedly. Ancillary mutations were frequently found to occur in mixed clones with an excess of bacteria carrying only the primary Sm D mutation. No ancillary mutations were found in an adjacent non‐coding region and there were no coding sequence changes that did not alter the amino acid specified. Although a selective advantage for bacteria containing ancillary mutations could not be demonstrated directly in every case, some small advantage must be presumed to have occurred to explain the absence of mutations at the other sites and particularly at third (wobble) codon positions. Ancillary mutations appear to occur fairly early in the life of a newly arisen Sm D mutant clone in some sort of hypermutable process. Whether they are noticed appears to depend on their conferment of some selective advantage on the bacteria carrying them. While the ancillary mutations within rpsL lie close to, and may be consequent upon the formation of a primary Sm D mutation, their mechanism of formation appears to be at least to some extent independent and does not involve the recA or umuC genes. Ancillary mutations were also detected in bacteria lacking reverse transcriptase or transcription repair coupling factor.