Activation of the cryptic aac(6 ')-Iy aminoglycoside resistance gene of Salmonella by a chromosomal deletion generating a transcriptional fusion

Salmonella enterica subsp, enterica serotype Enteritidis BM4361 and BM4362 were isolated from the same patient. BM4361 was susceptible to aminoglycosi des, whereas BM4362 was resistant to tobramycin owing to synthesis of a 6'- N-acetyltransferase type I [AAC(6')-I]. Comparative analysis of nucleotide sequences, pulsed-field gel electrophoresis patterns, and Southern hybridiz ations indicated that the chromosomal aac(6')-Iy genes for the enzyme in bo th strains were identical and that BM4362 derived from BM4361 following a c a, 60-kb deletion that occurred 1.5 kb upstream from the resistance gene. N orthern hybridizations showed that aac(6')-Iy was silent in BM4361 and high ly expressed in BM4362 due to a transcriptional fusion, Primer extension ma pping identified the transcriptional start site for aac(6')-Iy in BM4362: 5 bp downstream from the promoter of the nmpC gene. Study of the distributio n of aac(6')-Iy by PCR and Southern hybridization with a specific probe ind icated that the gene, although not found in S, enterica subsp. arizonae, wa s specific for Salmonella, In this bacterial genus, aac(6')-Iy was located downstream from a cluster of seven open reading frames analogous to an Esch erichia coil locus that encodes enzymes putatively involved in carbohydrate transport or metabolism. This genomic environment suggests a role in the c atabolism of a specific sugar for AAC(6')-Iy in Salmonella.