Comparison of the rep-38 and mmrA1 mutations of Escherichia coli

The rep-38 and mmrA1 mutations are located very close to each other (∼85 min), and have been suggested to be allelic. To address this question we have compared the phenotypes of the mmrA1 and rep-38 mutants. Both the mmrA1 and rep-38 mutations blocked the enhanced killing and inhibition of postreplication repair by rich growth medium that occurs in UV-irradiated Escherichia coli K-12 uvrA cells, i.e., the mmrA1 and rep-38 strains did not show minimal medium recovery (MMR). However, ΦX174 bacteriophage propagated well in mmrA1 strains, but not in rep-38 strains; a rep mutation sensitized a uvrA strain to UV irradiation, but a mmrA mutation did not. During chloramphenicol treatment, the rep-38 strain showed a larger amount of residual DNA synthesis than observed in the mmrA1 strain. The mmrA1 mutation appears to be a dominant mutation. This was determined by the failure of either plasmid pLC44-7 or episome F′KLF11, both of which carry the mmrA⁺ gene, to complement the Mmr⁻ phenotype of a uvrA mmrA strain. Plasmid pLC44-7 is known to complement the rep-38 mutation, suggesting that rep-38 is a recessive mutation. Although certain of the phenotypes of the rep and mmrA mutants are similar, a number are quite different. These differences suggest that these two mutations are not allelic.