Deoxyribonucleic acid synthesis in a temperature-sensitive Escherichia coli dnaH mutant, strain HF4704S

dnaA acts before dnaC in the initiation of DNA replication

We constructed a double mutant of Escherichia coli K-12 carrying dnaA(Ts) and dnaC(Cs) lesions. In this mutant DNA synthesis proeceeds normally at 32 degrees C and initiation is inhibited at both 41 and 20 degrees C. By shifting this culture grown at 32 degrees C to the two restrictive temperatures in different time sequences and assaying protein and DNA synthesis of cells growing at different temperatures, we found that dnaA and dnaC genes work independently with dnaA acting before dnaC. While preparing special strains for this work, we also showed that the order of genes in the neighborhood of dnaA is dnaA-tnaA-phoS-ilv.

A DNA fragment containing the origin of replication of the Escherichia coli chromosome

A 38 kilobase pair region of the Escherichia coli K12 chromosome containing the replication origin has been physically mapped with restriction endonucleases EcoRI and HindIII. Replication starts within or very near a 1.3 kilobase pair HindIII fragment in the middle of this region and proceeds outward in both directions with apparently equal speed. This pattern was observed in both dnaA and dnaC temperature-sensitive (ts) initiation mutants at the start of the synchronous round of replication which occurs after downshift from the nonpermissive to the permissive temperature.