The flexibility of alternating dA-dT sequences.

The flexibility of alternating poly (dA-dT) has been investigated by the technique of transient electric dichroism. Rotational relaxation times, which are very sensitive to changes in the end-to-end length of flexible polymers, are determined from the field free dichroism decay curves of four, well defined fragments of poly (dA-dT) ranging in size from 136 to 270 base pairs. Persistence lengths, calculated from the results of Hagerman and Zimm (Biopolymers (1981) 29, 1481-1502), are in the range 200-250 A. This makes alternating dA-dT sequences about twice as flexible as naturally occurring, "random" sequence DNA. Considering a bend around a nucleosome, for example, this difference in persistence length translates to an energy difference between poly (dA-dT) and random sequence DNA of 0.17 kT/base pair or 1 kcal per 10 base pair stretch. This energy difference is sufficiently large to suggest that dA-dT sequences could serve as markers in DNA packaging, for example, at sites where DNA must tightly bend to accommodate structures.