EXPRESSION OF THE STRA-STRB STREPTOMYCIN RESISTANCE GENES IN PSEUDOMONAS-SYRINGAE AND XANTHOMONAS-CAMPESTRIS AND CHARACTERIZATION OF IS6100IN XANTHOMONAS-CAMPESTRIS

Expression of the strA-strB streptomycin resistance (Sm-r) genes was e xamined in Pseudomonas syringae pv. syringae and Xanthomonas campestri s pv. vesicatoria. The strA-strB genes in P. syringae and X. campestri s were encoded on elements closely related to Tn5393 from Erwinia amyl ovora and designated Tn5393a and Tn5393b, respectively. The putative r ecombination site (res) and resolvase-repressor (tnpR) genes of Tn5393 from E. amylovora, P. syringae, and X. campestris were identical; how ever, IS6100 mapped within tnpR in X. campestris, and IS1133 was previ ously located dowmstream of tnpR in E. amylovora (C.-S. Chiou and A. L . Jones, J. Bacteriol, 175:732-740, 1993). Transcriptional fusions (st rA-strB::uidA) indicated that a strong promoter sequence was located w ithin res in Tn5393a. Expression from this promoter sequence was reduc ed when the tnpR gene was present in a cis position relative to the pr omoter. In X. campestris pv. vesicatoria, analysis of promoter activit y with transcriptional fusions indicated that IS6100 increased the exp ression of strA-strB. Analysis of codon usage patterns and percent G+C in the third codon position indicated that IS6100 could have originat ed in a gram-negative bacterium. The data obtained in the present stud y help explain differences observed in the levels of Sm-r expressed by three genera which share common genes for resistance. Furthermore, th e widespread dissemination of Tn5393 and derivatives in phytopathogeni c prokaryotes confirms the importance of these bacteria as reservoirs of antibiotic resistance in the environment.