Escherichia coli RecBC pseudorevertants lacking chi recombinational hotspot activity

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Abstract
Pseudorevertants of an E. coli exonuclease V (RecBC enzyme)-negative mutant were isolated after ethyl methane sulfonate mutagenesis of a recC73 (presumed missense) mutant. The remedial mutations in each of the 4 pseudorevertants studied in detail map and complement as recC mutations. By several criteria, such as recombination proficiency, support of phage growth, RecBC nuclease activity and cell viability, the pseudorevertants appear to have regained partially or completely various aspects of RecBC activity. Chi recombinational hotspots, which stimulate exclusively the RecBC pathway of recombination, have no detectable activity in .lambda. vegetative crosses in the pseudorevertants. The properties of these mutants, in which the RecBC pathway of recombination is active yet in which chi is not active, are consistent with the hypothesis that wild-type RecBC enzyme directly interacts with chi sites; alternatively, the mutants may block or bypass the productive interaction of another recombinational enzyme with chi.