Temperature sensitive modification and restriction phenotypes of an Escherichia coli dnaD mutant

A mutant of E. coli temperature sensitive for DNA synthesis, dnaD, was found to have temperature sensitive modification and restriction phenotypes. In contrast to the original observation by Carl, the mutant could support the growth of λ phage at 41C. However, the λ phages thus produced were able to form plaques with normal plating efficiency only on E. coli C, a restriction less strain, but not on E. coli K. Since the λ phages produced in the mutant at 20C could form plaques equally well on both E. coli strains, it was concluded that the dnaD mutant has a temperature sensitive modification phenotype. Furthermore, since the dnaD mutant allowed some growth of unmodified λ (C) phages at 41C but less at 30C, the mutant is also temperature sensitive in restriction. The relationship, if any, between temperature sensitive DNA synthesis and temperature sensitive modification restriction in the dnaD mutant is not known. Similar experiments were done with 3 dnaC mutants and one dnaA mutant. Two dnaC mutants were found to have altered restriction phenotypes at 41C, but none of the mutants were defective in modification.