END-POINT BIAS IN LARGE TN10-CATALYZED INVERSIONS IN SALMONELLA-TYPHIMURIUM

A genetic strategy identified Salmonella typhimurium strains carrying large (>40 kb) Tn10-catalyzed inversions; the inverted segments were c haracterized by XbaI digestion and pulsed field gel electrophoresis. T wo size classes of large inversions were found. More than half of the inversions extended 40-80 kb either clockwise or counterclockwise of t he original Tn10 site. The remaining inversions extended up to 1620 kb (33% of the genome), but the distal endpoints of these inversions wer e not randomly scattered throughout the chromosome. Rather, each Tn10 repeatedly yielded similar (though not identical) inversions. The bias ed endpoint selection may reflect the limited search for target DNA se quences by the Tn10 transposase, and the spatial proximity of the dono r and target regions in the folded S. typhimurium nucleoid. Using this interpretation, the data suggest that DNA sequences 40-80 kb clockwis e and counterclockwise of the insertion site are in spatial proximity with the insertion, perhaps reflecting the organization of DNA into si milar to 120-kb nucleoid domains. In addition, the data predict the sp atial proximity of several distant DNA regions, including DNA sequence s equidistant from the origin of DNA replication.