                 SEGMENT-GROUP POSITIONING

    Activating the 'Segment-group Position' item of  the  3D
'Edit'  pulldown  menu  allows one to interactively define a
group of segments which may then be translated  and  rotated
as a rigid body relative to a Cartesian frame located at the
group's center of mass or at the backbone atom  (C1')  of  a
user-specified nucleotide.

   The purpose of this type of editing is for taking care of
cases  not  handled by the 'Segment Position' or 'Stem-group
Position' editing functions.  For example, it  is  sometimes
necessary  to  try  changing  the position of a stem region.
This is  not  possible  with  the  'Stem-group  Positioning'
function  because  any  stem within the chosen group will be
moved as a whole, that is, all of its regions constitute one
rigid  body.  Nor is it possible with the 'Segment Position'
function because it requires two segments to specify a  stem
region.

   Needless to say,  the  'Segment-group  Position'  editing
feature   is   only   for   achieving   alternate   starting
conformations.  As in the unedited case, these conformations
should  be  regarded as first-order approximations to stable
ones, and hence the need for subsequent refinement.

                          THE END



