UNUSUAL CONFORMATION OF (DA)N.(DT)N-TRACTS AS REVEALED BY CYCLOBUTANE THYMINE THYMINE DIMER FORMATION

Cyclobutane dimer formation has been used to probe conformation of (dA)n.(dT)n-tracts cloned in plasmid DNA. The observed dimer probability patterns for (dA)n.(dT)n-tracts with n greater-than-or-equal-to 4 exhibit maximum intensity at the 3'-terminal TT site of T(n)-tract, whereas photoreactivity at all the other TT sites is inhibited. Both temperature and dimethyl sulfoxide increase dimer formation within T(n)-tracts and result in an even dimer pattern. The data obtained have been interpreted in terms of an unusual structure adopted by (dA)n.(dT)n-tracts. An influence of flanking base pairs, ethidium bromide binding and ionic strength has also been studied.