have been isolated under low ionic strength conditions: the actively translatable 15 S mRNP isolated from polyribosomes and the in vivo and in vitro translationally repressed 20 S free cyto- plasmic mRNP [l-4]. A specific group of proteins (possibly equivalent to initiation factor eIF-3) is removed from 15 S polyribosomal mRNP when puri- fied in a 0.5 M KC1 containing buffer. Furthermore, the 20 S free mRNP is split into several sub-particles at this ionic concentration. Two of these still transla- tionally repressed subparticles, namely the 16 S and 13 S mRNPs, contain both (Y- and /3-globin mRNA [I] and differ partially in their protein composition [2]. The protein composition of these two free globin mRNP particles is also clearly different from that of the translatable 15 S polyribosomal globin mRNP, suggesting a relationship between protein composition of mRNP and mRNA function [3,5,6]. We have shown that proteins do not interact at random with mRNA within mRNPparticles, but rather specifically. Indeed, specific segments of the mRNA are protected in the RNP against staphylococcal nuclease action [7,8]. To gain further insight into the structure/function rela- tionship of the active and repressed globin mRNP complexes, we pursued our comparative studies on the structure of cytoplasmic-free and polyribosomai duck globin mRNPs and investigated: (i) Whether in the different forms of mRNP particles, i.e., 16 S, 13 S and 15 S, different proteins inter- act with the same region of the mRNA sequence; We show here that the specific sets of mRNP proteins induce specific structural alterations in the globin mRNA chain which may be related to translational activity or repression. 2. MateriaIs and methods RNase-free sucrose was from Merck. [-Y-~‘P] ATP (3000 Ci/mmol) was from the Radiochemical Center (Amersham).