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More Precise Mapping of the Replication Origin in Escherichia coli K-12

^a Département de Biologie Moléculaire, Université de Genève, 1211 Geneva 4, Switzerland

ABSTRACT

The origin of replication in Escherichia coli K-12 was mapped by determining the rate of marker replication during a synchronous round of replication. Four isogenic strains were made lysogenic for ind^– and for phage Mu-1, with Mu-1 integrated into a different chromosomal location in each strain. Cultures were starved for amino acids to allow completion of chromosome replication cycles and then starved for thymine in the presence of amino acids, and a synchronous cycle of replication was initiated by the addition of thymine. Samples were exposed to radioactive thymidine at intervals, deoxyribonucleic acid was extracted, and the rate of marker replication was determined by deoxyribonucleic acid-deoxyribonucleic acid hybridization to filters containing Mu-1, , and E. coli deoxyribonucleic acid. The results confirm that the origin of replication is near ilv. The travel times of the replication forks, calculated from the data obtained for cultures with doubling times of approximately 40 and 61 min, are 40 and 52 min, respectively.

¹ Present address: Centre de Recherche de Biochimie et de Génétique cellulaire du CNRS, 31—Toulouse, France.

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• Howe, M., Bade, E. (1975). Molecular biology of bacteriophage mu. Science 190: 624-632