LEAKINESS OF GENETIC-MARKERS AND SUSCEPTIBILITY TO POST-PLATING MUTAGENESIS IN ESCHERICHIA-COLI

When Escherichia coli strain AB1157 is subjected to starvation for thr eonine or leucine on solid media, threonine-independent or leucine-ind ependent colonies continue to emerge for several days after plating. T his process is strongly streptomycin dependent. Under identical condit ions arginine-independent colonies do not arise when arginine starvati on is imposed. Since the thrI and leuB6 alleles of AB1157 could be cla ssified as 'leaky' while the argE3 allele cannot be so classified, the re seems to be a correlation between leakiness of mutant genetic marke rs and post-plating mutagenesis which counters the effect of the mutat ions. 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, l exA-independent manner. I show that these variants harbour a mutation, tentatively named adi (adaptation inducer), at around 72 min on the g enetic map, and that the adi mutation increases the intrinsic leakines s of a lacZ (ochre) mutation, perhaps by enhanced translational error. These observations are discussed in relation to the phenomenon of 'ad aptive' mutagenesis. its possible mechanism, and its specificity.