Shifts in configuration of the 5'-noncoding region of a mouse messenger RNA under translational control

Several major mRNA species of mouse and other mammalian cells occur both as small untranslated ribonucleoprotein particles and as functional molecules associated with ribosomes in polysomes. One of these, that codes for a 21-kDa polypeptide, was analyzed with respect to distribution of sites accessible to RNase T1 in the 5'-noncoding region. This region, which is about 100 nucleotides long, contains several sites that are highly sensitive to the enzyme, as well as many G residues not susceptible to cleavage. The distribution of highly sensitive sites was compared in the active and inactive states of the P21 mRNA present in cytoplasmic extracts by subjecting the extract to limited nuclease digestion followed by separation of partially fragmented polysomes from free messenger ribonucleoprotein particles. The mRNA in polysomes contained two highly sensitive sites, one near the 5' terminus and the other in the middle of the region, next to a sequence potentially capable of Shine-Dalgarno interaction. The untranslated molecules lacked the 5'-proximal site but had several highly accessible sites not present in the active molecules. The initiation AUG showed little accessibility both in polysomes and in messenger ribonucleoproteins. Both forms were quite different from the deproteinized mRNA with respect to distribution of nuclease-sensitive sites. Our results indicate that interaction of the mRNA with cytoplasmic factors strongly affects its conformation in the 5'-noncoding region and that a particular conformation may be important for effective interaction with ribosomal particles during polypeptide chain initiation.