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J Bacteriol. 1974 June; 118(3): 855-866
Copyright © 1974 American Society for Microbiology. All Rights Reserved.

Round of Replication Mutant of a Drug Resistance Factor

^a Laboratory of Molecular Biology and Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706

ABSTRACT

A derivative of the R factor NR1 (called R12) has been isolated which undergoes an increased number of rounds of replication each division cycle in Proteus mirabilis, Escherichia coli, and Salmonella typhimurium. The alteration resulting in the increased number of copies (round of replication mutation) is associated with the transfer factor component of the R factor. R12 has the same drug resistance pattern as NR1, is the same size as shown by sedimentation in a sucrose gradient and electron microscopy (63 x 10⁶ daltons), and has the same partial denaturation map. The level of the R factor gene product chloramphenicol acetyltransferase has been examined in P. mirabilis and was found to be consistent with gene dosage effects. The plasmid to chromosomal deoxyribonucleic acid ratio of NR1 increases several fold after entry into stationary phase, whereas this ratio for R12 remains approximately constant. Individual copies of R12 are selected at random for replication from a multicopy plasmid pool. A smaller percentage of R12 copies replicate during amino acid starvation than has previously been found for NR1 in similar experiments.

¹ Present Address: Department of Biochemical Sciences, Frick Chemical Laboratory, Princeton University, Princeton, N. J. 08540.

² Present Address: Department of Microbiology, Gunma University, School of Medicine, Maebashi, Japan.

³ Present Address: Department of Microbiology, State University of New York, Stony Brook, Long Island, N. Y. 11790.