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A genetic strategy identified Salmonella typhimurium strains carrying large (> 40 kb) Tn 10-catalyzed inversions; the inverted segments were characterized by XbaI digestion and pulsed field gel electrophoresis. Two size classes of large inversions were found. More than half of the inversions extended 40-80 kb either clockwise or counterclockwise of the original Tn10 site. The remaining inversions extended up to 1620 kb (33% of the genome), but the distal endpoints of these inversions were not randomly scattered throughout the chromosome. Rather, each Tn10 repeatedly yielded similar (though not identical) inversions. The biased endpoint selection may reflect the limited search for target DNA sequences by the Tn10 transposase, and the spatial proximity of the donor and target regions in the folded S. typhimurium nucleoid. Using this interpretation, the data suggest that DNA sequences 40-80 kb clockwise and counterclockwise of the insertion site are in spatial proximity with the insertion, perhaps reflecting the organization of DNA into approximately 120-kb nucleoid domains. In addition, the data predict the spatial proximity of several distant DNA regions, including DNA sequences equidistant from the origin of DNA replication.

Mouse microtubule-associated protein 4 (MAP4) is a protein that co-locates with microtubules in vivo. It is encoded by a single-copy gene that expresses multiple transcripts in most cell types [West et al. J. Biol. Chem. 266 (1991) 21886–21896]. This report describes the identification of two distinct 3′-untranslated regions (UTR) for MAP4 transcripts. The 3′-UTRs of the transcripts are identical up to the site of polyadenylation of the shorter mRNA. The longer transcript contains an additional 775 nucleotides after the first polyadenylation site. Both poly(A) tails follow the canonical polyadenylation site motif, AAUAAA. These data show that two different UTRs arise as a result of alternative polyadenylation site usage. Northern blots of RNA from different tissues probed with coding sequence show hybridization to the common 5.5- and 6.5-kb transcripts, whereas blots probed with sequence unique to the longer 3′-UTR show hybridization only to the 6.5-kb band. Both transcripts are found within the same cell type. In addition, muscle contains additional transcripts of 8 and 9 kb, of which only the 9-kb transcript hybridizes to the longer 3′-UTR probe.