RecA-mediated 4-stranded exchange.
One of the simplest models for the study of in vitro RecA-mediated 4-
strand exchange is shown in part (A) of the figure. In this case ds linear
DNA2 has complete homology with the continuous strand within the gapped circle
, except that it lacks a short section at its proximal end. Three-
strand exchange initiates within the ss region of the gapped circle and
continues as 4-strand exchange once the strand exchange junction meets the
ds section of the gapped circle. The resulting Holliday junction can now
traverse the circle and eventually, when the ds linear ends are reached,
products are formed as shown in the figure.
Section (B) shows a similar reaction, but now the ds linear
substrate also lacks an additional section of homology at its distal end. (the
region on the circle that is missing on the ds linear molecule is shown as the
thicker lines in the circle). In this case once the Holliday junction has
traversed the circle and the distal end of the ds linear molecule is reached
there is still a region that cannot be exchanged because of the lack of
homology. One might therefore expect that the Holliday junction would stall at
this point. Remarkably, even if this region is 835bp long, the products shown
in (B) are produced; the ds region is somehow unwound in this reaction. 0028a.jpg shows an example of such
a gapped circular product: the measured
lengths of the gap in these molecules show them to be product rather than
substrate molecules. 0028b.jpg shows an
intermediate in this reaction in
which the Holliday junction has not yet reached the distal end of DNA2. These
intermediates are characterized by a ss segment within the circular component
and a ss region on the distal end of the tail.
Kerry MacFarland, Inman and Cox. JBC 272 17675-17685 (1997)
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