a1 Palo Alto Medical Research Foundation, Palo Alto, California
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).
(Received July 29 1974)
* This investigation was supported by Public Health Service Grant CA 05687–13 from the National Cancer Institute, the Santa Clara County United Fund and the Harvey Bassett Clarke Foundation.