And the results told me that the USS is bent at the T-tracts.
Above is the gel photo. I tested seven different versions of the USS sequence, each embedded in an otherwise-identical 200 bp fragment. The white bands in the gel are the positions that the DNAs migrated to during the electrophoresis (6% acrylamide in 0.5 x Tris-Borate buffer, run at 60-70 volts for about 5 hours). 'S' indicates DNAs that I scored as running slower, and 'F' DNAs that ran faster.
Below is the key to the differences between the DNAs:
Most of the other DNA fragments ran with the same mobility as the USS, but DNAs 6 and 7 ran faster, like the randomized-sequence DNA. These are the only two DNAs whose T-tracts are changed: 6 has one T-G substitution in each T-tract, and 7 has these plus the same two outer-core changes as DNA 4.
You may wonder why I didn't run the DNAs for longer. to better resolve the migration differences. But I expected the DNA to have run much further; the Molecular Cloning manual I was using as a guide said that xylene cyanol (the upper turquoise dye band, labeled 'xc') would migrate at the same rate as a 260 bp DNA fragment in a 5% polyacrylamide gel, and at the same rate as a 160 bp fragment in an 8% gel, so I expected my DNAs to coincide with the xc band.
The only explanation I can think of is that I put 10 mM MgCl2 into the gel buffer but forgot to also put it in the running buffer in the tanks. The manual says that even minor differences in ionic strength 'can greatly distort the migration of DNA'. So I should probably repeat the gel with the correct buffer. Maybe I'll also try running it in the cold room.