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

A mutant of Escherichia coli temperature-sensitive for deoxyribonucleic acid synthesis, dnaD, was found to have temperature-sensitive modification and restriction phenotypes. In contrast to the original observation by Carl (1970), the mutant could support the growth of lambda phage at 41 C. However, the lambda 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 lambda phages produced in the mutant at 30 C 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 lambda.C phages at 41 C but less at 30 C, the mutant is also temperature sensitive in restriction. The relationship, if any, between temperature-sensitive deoxyribonucleic acid synthesis and temperature-sensitive modification-restriction in the dnaD mutant is not known. Similar experiments were done with three dnaC mutants and one dnaA mutant. Two dnaC mutants were found to have altered restriction phenotypes at 41 C, but none of the mutants were defective in modification.