We have previously cloned a 3·5 kb fragment from the multiresistance plasmid pSK1 which carries the determinant responsible for linked resistance to acriflavine (Ac), ethidium bromide (Eb), quaternary ammonium compounds (Qa), propamidinc isethionate (Pi), and diamidinodiphenylamine dihydrochloride (Dd). This report presents a biochemical and physical analysis of and shows the widespread carriage of this gene on resistance plasmids. Tn insertion mutagenesis defined the extent of to within 2·40 kb of pSK1 DNA. Examination of the expression of insertion and deletion mutants of the cloned sequences in both maxicells and minicells led to the association of a 50 kDa protein, designated QacA, with the Ac Eb Qa Pi Dd phenotype. Based on fluorimetric and isotopic assays used to determine the extent of accumulation of ethidium bromide by strains harbouring pSK1, we propose that the basis of Ac Eb Qa Pi Dd in is a -mediated efflux system.


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