The NMR-derived p270 structure was reported earlier this year (15) and compared directly with the MRF2 structure (17). The structure indicates that Helix 5 of p270 lies within the major groove. However, these authors determined, by assessment of changes in the dynamics of the complex, that the shorter Loop 2 of p270 is less flexible than the corresponding loop in MRF2. This suggested a ‘folding upon binding’ mechanism of sequence recognition, in which the shorter length and/or less flexible composition of Loop 2 of p270 in comparison to MRF2 and Dri affects orientation of the major groove contact residues, and is thus responsible for lack of sequence-specific contact.
