The far-UV CD spectrum of XD ( Fig. 5A -D, gray line) is typical of a structured protein with a predominant ahelical content, as indicated by the positive ellipticity between 185 nm and 200 nm, and by the two minima at 208 nm and 222 nm. After mixing N TAILHNFC with different molar excesses of XD, the observed CD spectra differed from the corresponding theoretical average curves calculated from the individual spectra. Since the theoretical average curves correspond to the spectra that would be expected if no structural variations occur, deviations from these curves indicate structural transitions. The results obtained in the presence of a threefold molar excess of XD indicate a random coil to a-helix transition, as judged by the much more pronounced minima at 208 nm and 222 nm, and by the higher ellipticity at 190 nm of the experimentally observed spectrum compared to the corresponding theoretical average curve (see Fig. 5A ). Although even more dramatic structural transitions of full-length N TAIL have been previously observed with a twofold molar excess of XD , the results obtained with a threefold molar excess have been selected for presentation in order to allow direct comparison with the N TAIL deletion proteins (see below). Figure 5 , panels B-D, show the results obtained for the three N TAIL deletion proteins in the presence of a threefold molar excess of XD, a condition leading to the most dramatic structural transitions. Removal of Box3 significantly reduces, but does not abrogate, the folding ability of N TAIL . Indeed, the experimental CD spectrum does not significantly deviate from the average curve in the 200-260 nm region, but it considerably deviates from the average curve in the 185-195 region (58% mean increase of ellipticity) (Fig. 5B ), thus supporting partial folding ability of N TAILD3 in the presence of XD. Further removal of Box2 results in a truncated N TAIL form, which has completely lost its ability to fold in the presence of XD, as indicated by the good superimposition between the experimental and the average spectra (Fig. 5C) . Conversely, removal of Box1 does not affect the folding ability of N TAIL , as the deviations from the average spectrum are similar to those observed with the full-length form (Fig. 5, cf. D and A) . The mean increase in ellipticity in the 185-195 nm region (77%) and the decrease in the ellipticity value at 220 nm (46%) observed with N TAILD1 , are comparable to the corresponding values observed with N TAILHNFC (70% and 46%, respectively).