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

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.on July 16, 2020 by guest

Genetic loci responsible for incompatibility on a co-integrate plasmid, R100-1

“…In a previous paper (14), I reported a method for analyzing a plasmid, R100-1, integrated into the host chromosome by transduction to a dnaA-ts recipient. In accord with previous molecular studies in other laboratories (2,8,9,11), this plasmid was found to be a co-integrate in that it carried two independently operative replication genes, repA and repB.…”

“…This will be determined after completion of mapping of the tra cluster, which is now in progress in my laboratory. It should be noted that R100-1 has only a single gene or gene cluster responsible for incompatibility despite previous data suggesting that this plasmid is a co-integrate consisting of two replicons (2,8,9,11,14). For the classification of the conjugative plasmid by incompatibility tests to be reasonable, one plasmid, even if it is a co-integrate, should have one incompatibility gene or gene cluster specific for that plasmid.…”

Extrachromosomal deoxyribonucleic acid in R factor-harboring Enterobacteriaceae

“…Electron micrograph of a circular DNA molecule. Open circle, about 22 ,m, from an E. coli strain harboring plasmid pJM 3, which transfers linked resistance to sulfonamide and streptomycin.…”

Recombination between two IS/s flanking the r-determinant of R100-1: involvement of dor and recA gene functions in Salmonella typhimurium

“…In Escherichia coli, R100-1 exists almost exclusively in the form of a composite molecule. In Salmonella typhimurium, however, there is a frequent loss of the r-det component from the composite molecule (23,29,30).…”