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J. Bacteriol., 05 1996, 2825-2835, Vol 178, No. 10
NP Higgins, X Yang, Q Fu and JR Roth
A genetic system was developed to investigate the supercoil structure of
bacterial chromosomes. New res-carrying transposons were derived from
MudI1734 (MudJr1 and MudJr2) and Tn10 (Tn10dGn). The MudJr1 and MudJr2
elements each have a res site in opposite orientation so that when paired
with a Tn10dGn element in the same chromosome, one MudJr res site will be
ordered as a direct repeat. Deletion formation was studied in a
nonessential region (approximately 100 kb) that extends from the his operon
through the cob operon. Strains with a MudJr insertion in the cobT gene at
the 5' end of the cob operon plus a Tn10dGn insertion positioned either
clockwise or counterclockwise from cobT were exposed to a burst of RES
protein. Following a pulse of resolvase expression, deletion formation was
monitored by scoring the loss of the Lac+ phenotype or by loss of
tetracycline resistance. In exponentially growing populations, deletion
products appeared quickly in some cells (in 10 min) but also occurred more
than an hour after RES induction. The frequency of deletion (y) diminished
with increasing distance (x) between res sites. Results from 15 deletion
intervals fit the exponential equation y = 120 . 10(-0.02x). We found that
res sites can be plectonemically interwound over long distances ( > 100
kb) and that barriers to supercoil diffusion are placed stochastically
within the 43- to 45-min region of the chromosome.
Copyright © 1996, American Society for Microbiology
Surveying a supercoil domain by using the gamma delta resolution system in Salmonella typhimurium
Department of Biochemistry, University of Alabama at Birmingham, 35294- 2170, USA. nphiggins@bmg.bhs.uab.edu
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