TABLE 1.

Distances and interhelical angles for different binding situations

	FRET-measurements	Modeled structure
	Distance	Interhelical angle	Distance	Interhelical angle
No binding site	6.5 ± 0.25 nm	180°	6.0 ± 0.95 nma	180°a
Native binding motif	5.6 ± 0.3 nm	76 ± 15°
Native binding motif + 15.5K protein	4.5 ± 0.2 nm	48 ± 15°	4.2 ± 1.1 nmb	53°b

^aA-RNA

^bCrystal structure

For both the linear RNA and the k-turn RNA/15.5K complex, good agreement was found for the measured and modeled distances. The free k-turn motif adopts an unfolded conformation that is clearly less kinked than the conformation found in the U4 snRNA k-turn/15.5K complex. The interhelical angles were calculated based on the FRET measured distances, assuming the helix to kink in the phosphate backbone of the donor strand between bases G9 and A10. These distances were measured in buffer A+.