A temperature sensitive strain of E. coli K12 has been isolated in which residual
DNA synthesis occurs at the 40 degrees C restrictive temperature; syntheses of
RNA,
protein and
DNA precursors are not directly affected. The mutation has been designated dna-325 and is located at 89 min on the E. coli map in the same region where the dnaC locus is found. dnaC mutants are considered to be defective in
DNA initiation. Some of the data are consistent with the view that the dna-325 mutation is temperature sensitive in the process of
DNA initiation rather than
DNA chain elongation: (1) more than two cell divisions occur after a shift to 40 degrees C; (2) upon a shift down to 30 degrees C, cell division occurs again only after the
DNA content of the cells has doubled; (3) 80% more
DNA is made at 30 degrees C in the presence of
chloramphenicol after prior inhibition of
DNA synthesis at 40 degrees C. These three observations indicate that rounds of DNA replication were completed at 40 degrees C. Also (4) infective lambda particles can be made at 40 degrees C long after
bacterial DNA replication has ceased. It appears however that some
DNA initiation can occur at 40 degrees C since (1) a limited amount of
DNA synthesis does occur at 40 degrees C after prior alignment of the chromosomes by
amino acid starvation at 30 degrees C, and (2) after incubation in
bromouracil at the restrictive temperature, heavy
DNA is found with both strands containing
bromouracil.