To test whether exrB, a mutation in Escherichia coli B making this strain sensitive to ultraviolet radiation and unable to divide normally, is dominant to exrB+, merodiploids were constructed by crossing Hfr exrB metA+his × F−malB met A thr proA recA. metA+recombinants were fertile (F′) and sensitive to UV. One of these, F′ exrB proA, was crossed with an F−metA malB thr, selecting for metA+proA+. All such progeny acquiring malB+ were UV-sensitive, fertile and segregated met mal UV-resistant progeny. They were, therefore, meriodiploids, F′ exrB/exrB+. exrB is dominant to exrB+, in which respect it resembles exrA (lex).
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