EXPRESSION OF ANTIBIOTIC-RESISTANCE GENES IN THE INTEGRATED CASSETTESOF INTEGRONS

Plasmids containing cloned integron fragments which differ only with r espect to either the sequence of the promoter(s) or the number and ord er of inserted cassettes were used to examine the expression of resist ance genes encoded in integron-associated gene cassettes. All transcri pts detected commenced at the common promoter P-ant, and alterations i n the sequence of P-ant affected the level of resistance expressed by cassette genes. When both P-ant and the secondary promoter P2 were pre sent, transcription from both promoters was detected. When more than o ne cassette was present, the position of the cassette in the array inf luenced the level of antibiotic resistance expressed by the cassette g ene. In all cases, the resistance level was highest when the gene was in the first cassette, i.e., closest to P-ant, and was reduced to diff erent extents by the presence of individual upstream cassettes. In Nor thern (RNA) blots, multiple discrete transcripts originating at P-ant were detected, and only the longer transcripts contained the distal ge nes. Together, these data suggest that premature transcription termina tion occurs within the cassettes. The most abundant transcripts appear ed to contain one or more complete cassettes, and is possible that the 59-base elements found at the end of the cassettes (31 to the coding region) not only function as recombination sites but mag also function as transcription terminators.