The hmr19 gene was cloned from Streptomyces hygroscopicus subsp.yingchengensis strain 10-22,a bacterium strain producing agricultural antibiotics.Sequence similarity comparison indicates that hmr19gene may encode a predicted protein with 14 putative transmembrane α-helical spanners,belonging to the drug:H+ antiporter-2 family of the major facilitator superfamily.The expression ofhmr19 in the mycelium of strain 10-22 was detected by Weste blotting analysis.Gene replacement technology was employed to construct an hmr19 disruption mutant.The growth inhibition test against different antibiotics indicated that the mutant strain was 5-20 fold more susceptible to tetracycline,vancomycin and mitomycin C than the parental wild type strain.The mutant took up tetracycline much faster and accumulated more antibiotics than the wild type strain 10-22.While with the addition of an energy uncoupler,carbonyl cyanide mchlorophenylhydrazone,the characteristics of the accumulation of [3H]tetracycline in these two strains were almost the same.It was thus concluded that hmr19 encoded a multidrug resistance efflux protein.