Mutations in bglY, the structural gene for the DNA-binding protein H1 of Escherichia coli, increase the expression of the kanamycin resistance gene carried by plasmid pGR71.

bglY mutants of Escherichia coli K12 which show higher levels of kanamycin resistance (Kmr) in the presence of plasmid pGR71 have been previously described. In this work, we show that this increased resistance to an aminoglycoside antibiotic is not due either to low drug uptake or to alteration of its target, the ribosome. The copy number of plasmid pGR71 is not modified. The fact that increased antibiotic resistance is observed with only some of the Kmr determinants used in this study suggests a specific role for the bglY gene product. Moreover, for one such determinant, a higher level of resistance was observed when it was inserted in the chromosome but not when harbored by a plasmid. This discrepancy can be explained by the twin transcriptional-loop model, which proposes that transcription can lead to local variation in topology. A kan-lacZ fusion was constructed from the Kmr gene of plasmid pGR71 and inserted into a low copy number vector. Assay of beta-galactosidase in wild-type and mutant strains showed that expression of the antibiotic resistance gene was directly affected by H1 protein, the bglY gene product.