SEQUENCE IDENTITY WITH TYPE-VIII AND ASSOCIATION WITH IS176 OF TYPE-IIIC DIHYDROFOLATE-REDUCTASE FROM SHIGELLA-SONNEI

An uncommon dihydrofolate reductase (DHFR), type IIIc, was coded for b y Shigella sonnei that harbors plasmid pBH70O and that was isolated in North Carolina. The trimethoprim resistance gene carried on pBH700 wa s subcloned and sequenced. The nucleotide sequence of the gene encodin g type IIIc DHFR was identical to the gene encoding type VIII DHFR. Th e type IIIc amino acid sequence was approximately 50% similar to those of DHFRs commonly found in enteric bacteria. Furthermore, this gene w as flanked by IS176 (IS26), an insertion sequence usually associated w ith those of aminoglycoside resistance genes. The gene for type IIIc D HFR was located by hybridization within a 1,993-bp PstI fragment in ea ch of eight conjugative plasmids from geographically diverse strains o f S. sonnei. Each plasmid also conferred resistance to ampicillin, str eptomycin, and sulfamethoxazole and belonged to incompatibility group M. Plasmids carrying this new trimethoprim resistance gene, which is u niquely associated with IS176, have disseminated throughout the United States.