Figure A3.—

The drug-in-drug assay for detecting duplications in recA mutant strains. The locus to be assayed carries an inserted Tet^R gene that is disrupted by a Kan^R cassette. The assay strain carries this compound allele and a plasmid that encodes the Red functions of phage lambda (Yu et al. 2003). The assay consists of a transformation cross in which a single-stranded donor fragment is electroporated into the recipient assay strain. This fragment displaces the Kan^R determinant and repairs the Tet^R determinant, thereby converting a Kan^R, Tet^S allele into a Kan^S, Tet^R allele. Recipients with a duplication of the test locus gain Tet^R (in one copy of the region) while retaining the Kan^R phenotype (provided by the other copy).