Leakiness of genetic markers and susceptibility to post-plating mutagenesis in Escherichia coli

When Escherichia coli strain AB1157 is subjected to starvation for threonine or leucine on solid media, threonine-independent or leucine-independent colonies continue to emerge for several days after plating. This process is strongly streptomycin dependent. Under identical conditions arginine-independent colonies do not arise when arginine starvation is imposed. Since the thrI and leuB6 alleles of AB1157 could be classified as 'leaky' while the argE3 allele cannot be so classified, there seems to be a correlation between leakiness of mutant genetic markers and post-plating mutagenesis which counters the effect of the mutations. Some of the threonine-independent variants acquired the ability to increase the leakiness of otherwise nonleaky markers such as argE3 and permit development of arginine independence in a recA-dependent, lexA-independent manner. I show that these variants harbour a mutation, tentatively named adi (adaptation inducer), at around 72 min on the genetic map, and that the adi mutation increases the intrinsic leakiness of a lacZ (ochre) mutation, perhaps by enhanced translational error. These observations are discussed in relation to the phenomenon of 'adaptive' mutagenesis. its possible mechanism, and its specificity.