Fig. (2A) Analysis by MALDI mass spectrometry of rps (L7/L12)2 or (L7)4. (A): MALDI mass spectrum of the control recombinant rp (L7/L12)2. The molecular weights measured are 13228 Da (L12 chain) and 13270 Da (acetylated L7 chain). The 42 Da difference between the two masses represents the acetyl group. The peaks at 13285 Da and 13327 Da correspond respectively to the acetylated L7 chain (MW 13270 Da) modified with a methyl group (MW 15 Da) or with both a methyl group and a supplementary acetyl group (total mass increase of 57 Da). (B): MALDI mass spectrum of the fully acetylated rp (L7)4. The peak of mass 13274 Da represents the acetylated L7 chain. The peak of mass 13499 Da showing a difference of 225 Da represents the L7 chain attached to additional acetyl and methyl groups. Both (L7/L12)2 and (L7)4 forms were eluted from the Superdex column, and the fractions of interest were pooled and evaporated prior to MALDI mass spectrometric analyses. For details, see Materials and Methods.