Isolation and characterization of plasmid-deletion derivatives from an Hfr made with Rts1 plasmid carrying a chromosomal mutation, plt

The region controlling the thermosensitive effect of plasmid Rts1 on host growth is separate from the Rts1 replication region

Rts1 is a high-molecular-weight (126 x 10(6)) plasmid encoding resistance to kanamycin. It expresses unusual temperature-sensitive phenotypes, which affect plasmid maintenance and replication, as well as host cell growth. We have cloned the essential replication region of Rts1 from pAK8, a smaller derivative which is phenotypically similar to Rts1. Restriction endonuclease digests of isolated pAK8 deoxyribonucleic acid were allowed to "self-ligate" (ligation without an additional cloning vector) and subsequently were used to transform Escherichia coli strain 20SO to kanamycin resistance. Screening of these strains for the phenotypes of thermosensitive host growth and temperature-dependent plasmid elimination demonstrated that these two properties were expressed independently. Furthermore, it was shown that the Rts1 replication locus per se is not necessarily responsible for altered host growth at the nonpermissive temperature. The kanamycin resistance fragment of pAK8 was also cloned into pBR322. Electrophoretic analysis of BamHI restriction enzyme digests of this plasmid and similar digests of an Rts1 miniplasmid has allowed the identification of an 18.6-megadalton fragment carrying the replication locus and a 14.1-megadalton fragment carrying the kanamycin resistance gene.

Mating due to loss of surface exclusion as a cause for thermosensitive growth of bacteria containing the Rtsl plasmid

At 25 degrees C, Rtsl+ bacteria grow to about 5 X 10(9) bacterial/ml before leveling off, whereas at 42 degrees C they grow from 2.6 X 10(8) bacteria/ml for only 2-3 generations after temperature shift before the growth is inhibited with a zig-zag pattern at the plateau. When diluted, Rtsl+ bacteria grow rapidly at 42 degrees C, until the concentration reaches as high as the undiluted 42 degrees C culture when growth measured by colony counts stops and the zig-zag pattern again appears. This density-dependent growth inhibition is not due to the presence of stable growth inhibitor(s). Mating experiments show that at 42 degrees C, Rtsl+ bacteria retain good donor ability; at the same time, they become good recipients in mating with Hfr (Rtsl) bacteria. SDS-PAGE reveals that membranes are altered at 42 degrees C. Examination of DNA synthesis indicates that chromosomal DNA is synthesized at both 25 degrees C and 42 degrees C at high bacterial concentration, but that of the Rtsl plasmid is slowed down at 42 degrees C. The labeling experiments suggest that in 2 h there are 2 rounds of plasmid replication at 25 degrees C, 3.5 rounds at 42 degrees C when bacteria are diluted, and 0.6 rounds at 42 degrees C when bacteria are not diluted. These results suggest that the growth inhibition of Rtsl+ bacteria at 42 degrees C is probably the consequence of mating initiated due to loss of surface exclusion.