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J Bacteriol. 1973 June; 114(3): 1328-1335
Copyright © 1973 American Society for Microbiology. All Rights Reserved.

Molecular Nature of R-Factor Deoxyribonucleic Acid Isolated from Salmonella typhimurium Minicells

^a The University of Tennessee-Oak Ridge Graduate School of Biomedical Sciences and the Biology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830

ABSTRACT

In earlier reports it was shown that a variety of extrachromosomal elements harbored in the Escherichia coli minicell producer segregate into the minicells. We show in this report that the fi⁺ R factor R-100-1 (derepressed derivative of R-100) similarly segregates into minicells produced by a Salmonella typhimurium strain. Four distinct classes of covalently closed circular deoxyribonucleic acid molecules are found in minicells derived from the R⁺Salmonella minicell producer. The sum of the average molecular weights or contour lengths of the circular molecules in two of the classes is equal to the average molecular weight or contour length of those in a third class. The data suggest that R-100-1 dissociates into resistance determinants (i.e., genes that specify the molecules that confer resistance) and the resistance transfer factor (i.e., genes responsible for the transferability of the R factor to a recipient). In contrast, only one molecular species is found in minicells derived from the R⁺ (R-100-1) Escherichia coli minicell producer. The fourth size class consists of small covalently closed circles (minicircles), which were originally found in the R^–Salmonella minicell producer and are shown in this report to be enhanced in number in R⁺Salmonella minicells.

¹ Present address: Department of Microbiology, The Public Health Research Institute of New York, 455 First Avenue, New York, N. Y. 10009.

² Present address: Department of Microbiology, The University of Alabama in Birmingham, Birmingham, Ala. 35294.