The parB region of plasmid R1 encodes two genes, hok and sok, which are required for the plasmid‐stabilizing activity exerted by parB. The hok gene encodes a potent cell‐killing factor, and it is regulated by the sok gene product such that cells losing a parB‐carrying plasmid during cell division are rapidly killed. Coinciding with death of the host cell, a characteristic change in morphology is observed. Here we show that the killing factor encoded by the hok gene is a membrane‐associated polypeptide of 52 amino acids. A gene located in the Escherichia coli relB operon, designated relF, is shown to be homologous to the hok gene. The relF gene codes for a polypeptide of 51 amino acids, which is 40% homologous to the hok gene product. Induced overexpression of the hok and relF gene products results in the same phenomena: loss of cell membrane potential, arrest of respiration, death of the host cell and change in cell morphology. The parB region and the relB genes were cloned into unstably inherited oriC minichromosomes. Whereas the parB region also conferred a high degree of genetic stability to an oriC minichromosome, the relB operon (with relF) did not; therefore the latter does not appear to ‘stabilize’ its replicon (the chromosome). The function of the relF gene is not known.
Escherichia coli relB mutants react to amino acid starvation by several abnormal responses, including accumulation of a translational inhibitor. We have isolated a relB‐complementing plasmid from the Clarke and Carbon E. coli DNA library. From this plasmid we sequenced a 2140‐bp segment which included the relB gene by the following two criteria: (i) it complements chromosomal relB mutations, (ii) the corresponding DNA segment cloned from chromosomal DNA of three relB mutants was defective in relB complementation. All three mutations fell within an open reading frame of 79 amino acids. A polypeptide of 9 kd compatible with this open reading frame was synthesized in maxicells and is in all probability the product of the relB gene. By nuclease S1 mapping we have determined the transcription start and stop of an 870 base transcript of the relB gene.
The plasmids in 19 chloramphenicol-resistant Escherichia coli strains of three pig pathogenic antigen types were studied in conjugation and transduction experiments. The plasmids had identical resistance patterns: streptomycin, spectinomycin, sulfonamides, and chloramphenicol (Sm, Sp, Su, Cm) and belonged to IncFII. One plasmid carried ampicillin resistance in addition. Restriction enzyme analysis of the deoxyribonucleic acid from five of the plasmids originating from the same herd showed that their digestion patterns with EcoRI were indistinguishable. EcoRI cleaved the deoxyribonucleic acid of a sixth plasmid from the same herd and displayed nine of the ten bands of the other five plasmids plus an additional six. It appears that the five plasmids with identical restriction patterns have a common origin and may be copies of the same plasmid from which the sixth may have developed. Four strains carried two plasmids each. In two of these strains, a plasmid with a tetracycline marker (Tc), or possibly the tetracycline marker alone, recombined frequently with the Sm Sp Su Cm plasmid without destroying any known function of the latter. The possibility that Tc is carried on a translocation sequence is discussed.
A total of 359 hemolytic Escherichia coli strains, representing eight pig pathogenic serotypes and isolated from pigs with enteric disease, was tested for transferable resistance to eight antibiotics. The co-transfer of plasmids controlling hemolysin production (Hly) with antibiotic resistance plasmids (R-factors) was evaluated. Transferable resistance to tetracycline, streptomycin, and/or sulfonamides was found in 47% of the total number of strains and 80% of the resistant ones. Chloramphenicol resistance was seldom seen. Co-transfer of Hly with R-factors occurred in 22.5% of the strains, generally to a degree excluding a genetic linkage. Stable coexistence of two R-factors in a cell was indicated by transfer patterns in 29 of the strains.
SUMMARY. I have examined 20 plasmids conferring chloramphenicol resistance (Cm) in multiresistant strains of Escherichia coli pathogenic for man and piglets. In Denmark, one plasmid family, exemplar pHG33, is responsible for all chloramphenicol resistance in serotypes of E. coli found in diseased piglets. A closely related plasmid, pHG50, was identified in an enteropathogenic E. coli (EPEC) strain from an infant. The isolate was epidemiologically unrelated to the piglet isolates. The molecular relatedness of the plasmids was established by restriction enzyme analyses and Southern blots. Chloramphenicol resistance plasmids in E. coli from urinary tract infections, or in English EPEC strains, did not show the same close relatedness with the piglet plasmid pHG33. However, many were of the same incompatibility group and their restriction profiles displayed a number of common bands. The close relatedness of pHG5O and pHG33 suggests exchange of plasmids between pathogenic serotypes of E. coli from man and animals. The infant from whom the EPEC strain carrying plasmid pHG5O was isolated might have acquired it from piglets. Disease in human babies caused by EPEC strains is now rare in Denmark and no Cm-resistant strains are found. Possible reasons for the loss of Cm-resistance plasmids from human strains and their retention in piglet strains are discussed.
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