In vivo transcription of R-plasmid deoxyribonucleic acid in Escherichia coli strains with altered antibiotic resistance levels and/or conjugal proficiency

Abstract

The amounts of plasmid DNA and the levels of the in vivo transcription of the E. coli plasmids R538-1 (repressed for conjugal transfer) and R538-1drd (derepressed for transfer) were determined by DNA-DNA hybridization and DNA-RNA hybridization, respectively. The level of plasmid transcription is increased by 2-fold in the strain carrying the derepressed plasmid, compared to an isogenic strain carrying the repressed plasmid and the amount of plasmid DNA is approximately the same, suggesting that the transfer genes are under transcriptional control. Levels of plasmid DNA, plasmid DNA transcription and chloramphenicol acetyltransferase activity were also compared in a mutant strain that carried the R538-1drd plasmid and was resistant to high levels of antibiotics. This strain produces about 13 copies of plasmid DNA/chromosome compared to 5 copies for the parent strain. The level of transcription of plasmid DNA was 2-fold higher in the high-level resistant strain and the level of chloramphenicol acetyltransferase activity was increased by 10-fold. The levels of plasmid DNA transcription and chloramphenicol acetyltransferase activity in the high-level resistant strain further increased by the presence of high levels of chloramphenicol in the growth medium. The amount of plasmid DNA remained constant under these conditions, indicating that high levels of chloramphenicol can stimulate the expression of plasmid genes at the level of transcription in this strain.