Growth rate effects of mutations conferring streptomycin-dependence and of ancillary mutations in the rpsL gene of Escherichia coli: implications for the clustering (hypermutation) hypothesis for spontaneous mutation

Colonies of newly arising streptomycin-dependent (SmD) mutants frequently contain a high proportion of cells with additional mutations (ancillary mutations) in the same gene (rpsL). The ancillary mutations appear to have arisen at a rate greatly above expectation. To better estimate this rate it is necessary to allow for any selective advantage conferred by the ancillary mutations. We have previously measured their effect on growth rate of established SmD strains in the presence of streptomycin. In the present work a pair of single and double mutant alleles (rpsL832 and rpsL852 respectively) has been employed together with the wild-type allele to model the situation soon after such mutations first arise, i.e. when the cell still contains wild-type S12 protein (the rpsL gene product). When these alleles, under the control of an IPTG-inducible promoter and carried on a plasmid, were expressed in the presence of a chromosomal wild-type allele, the double mutant allele permitted much faster cell growth than the single mutant allele. In the presence of streptomycin, and with rpsL+ on a plasmid, bacteria with a double mutant chromosomal gene grew faster than those with a single mutant chromosomal gene. If these results can be extrapolated to a bacterial cell in which an SmD mutation has just occurred, the ancillary mutation should be able to confer a selective advantage during a limited period when wild-type S12 protein is still present, both in the absence and in the presence of streptomycin.(ABSTRACT TRUNCATED AT 250 WORDS)