Control of replication and segregation of R plasmid Rts1

Abstract

A mutant plasmid, pTW2, which was derived from the integrated Rts1 genome in the Escherichia coli chromosome, was studied as to its mode of replication at 30.degree. C. When Proteus mirabilis Pm17 harboring pTW2 was grown in broth at 30.degree. C, a considerable number of R- segregants (approximately 40%) were consistently observed. pTW2 is unstable even at the permissive temperature for the replication of Rts1. The pTW2+ cells in a culture were heterogeneous with respect to the level of kanamycin resistance, ranging from 500-4000 .mu.g of the drug/ml. The amount of pTW2 DNA relative to the Pm17 chromosomal DNA was about 5-fold as large as that of Rts1 DNA in an exponentially growing culture. pTW2 in P. mirabilis continued to replicate after the chromosome ceased to replicate, which was shown in the study of the inhibition of protein synthesis. Contrary to pTW2, the parent plasmid Rts1 is highly stable, and the relative percent Rts1 DNA is maintained at approximately 7% in any cultural conditions at a permissive temperature. Copies of pTW2 may not segregate evenly into the host progeny upon cell division and the replication of pTW2 apparently does not coordinate with that of the chromosome. A remarkable instability of pTW2 and an increase in the relative precent pTW2 DNA was also shown when E. coli were used as the host cells. There may be a gene or a gene cluster on the Rts1 genome responsible for the control of replication and segregation of Rts1.