Accumulation of replicative intermediates and catenated forms of the colicinogenic factor E 1 in E. coli during the replication at elevated temperatures

Thermostability and superhelicity of plasmid DNA in Bacillus stearothermophilus

The thermostability of the staphylococcal plasmids pC194 and pUB110 and their antibiotic-resistance determinants was examined upon transfer to Bacillus stearothermophilus CU21. Plasmid pGS13, a pUB110 derivative carrying the chloramphenicol acetyltransferase (CAT) gene of pC194, could be maintained up to the maximum growth temperature (68 degrees C) by selection for chloramphenicol resistance. In the absence of selective pressure, pGS13 was lost at temperatures above 60 degrees C. Segregational instability of pGS13 was accompanied by a progressive loss of negative superhelicity at elevated temperatures. Thermostable mutants of pGS13 were isolated by screening for expression of the antibiotic-resistance determinants after growth under non-selective conditions. These mutants were found to contain an insertion of a 1.7 kb DNA sequence derived from the cryptic B. stearothermophilus plasmid pBS02. Increased thermostability correlated with preservation of plasmid superhelicity at elevated temperatures.

Factors influencing the copy number of F-like plasmids in Escherichia coli and Salmonella typhimurium

The copy numbers of Flac, four F-like plasmids and pLT2 were estimated in two strains of Salmonella typhimurium and (for all except pLT2) one strain of Escherichia coli. For organisms grown in casamino acids minimal medium, the plasmids spanned a 7--8 fold range of copy number with ColB-K98 having the highest copy number in each strain and R124 the lowest. The copy number of ColB-K98 was substantially greater than 1 in each of the strains tested. There was no clear relation between the plasmid size and copy number, although the plasmids studied spanned only a narrow size range. The copy number of individual plasmids was slightly reduced or not affected at all by the presence of a second plasmid in the same strain. Derivatives harbouring each of the plasmids were grown in three different media to ascertain how plasmid copy number responds to changes in growth rate. For each plasmid, the copy number increased with decreasing growth rate. Extracts from each of the three strains harbouring ColB-K98 contained two distinct plasmid species. One appeared to be about twice as large as the other and both were absent from Col- segregants.

Replication of colicinogenic factor E1 DNA in plasmolysed Escherichia coli cells. Coupling of DNA replication and RNA synthesis

Plasmolysed chloramphenicol-treated Escherichia coli cells carrying the colicinogenic factor E1 utilize deoxynucleoside triphosphates for the semi-conservative synthesis of Col E1 DNA. Col E1 DNA replication in plasmolysed cells can be dissociated into two temporally separated processes: (a) a rifampicin-sensitive RNA synthesis, which is stimulated by adenosine 3':5'-monophosphate (cyclic AMP) and requires all four ribonucleoside triphosphates and (b) an ATP-dependent DNA synthesis, which is inhibited by arabinosylnucleoside triphosphates and sulfhydryl-blocking reagents. Thes two processes exhibit different sensitivities to inhibition by polyamines and actinomycin D.

[Nature of bacteriocins]

The bacteriocins comprise a large, heterogeneous group of bactericidal substances produced by bacteria from widely differing species. The best known bacteriocins are the colicins. Some aspects of research on colicins are discussed especially their chemical nature and mode of action, and their genetic determinants.