Chromosome structure of Escherichia coli mutants temperature sensitive for deoxyribonucleic acid replication

Folded chromosomes were isolated from Eschericia coli thermosensitive dnaA initiation mutants incubated at the nonpermissive temperature and were analyzed by neutral sucrose density gradient centrifugation. A chromosomal structure that sedimented at approximately 1,500S accumulated when the dnaA gene product was inactive. When the cells were returned to a permissive temperature, the folded chromosomes exhibited a decrease in sedimentation velocity to 1,300S but still retained their uniform structure. Very little deoxyribonucleic acid synthesis occurred during the period in which the chromosomes exhibited the reduction in sedimentation velocity. A dnaG elongation mutant showed no unique chromosome structure when the dnaG gene product was inactive.