The organization of the 7SL RNA in the signal recognition particle

Digestion of the signal recognition particle (SRP) of dog pancreas with micrococcal nuclease results in the stepwise cleavage of the 300 nucleotide 7SL RNA moiety producing five major fragments approximately 220 (1), 150 (2), 72 (3), 62 (4) and 45 (5) nucleotides long. The RNA molecule is initially cut once yielding fragments 1 and 3. Further degradation releases fragments 2, 4 and 5. The introduction of the first nick into the 7SL RNA does not alter the structure nor the function of the SRP. Further degradation of the RNA results in disruption and loss of activity of the particle. The sequence of the RNA fragments shows that the nuclease causes discrete cuts in the RNA with minimal nibbling indicating that only few sites are accessible to the action of the enzyme. The five major products of nuclease digestion together span almost the entire length of the 7SL RNA. Nicking occurs mainly around the boundary region between the central S sequence and the flanking Alu sequences constituting the 7SL RNA (1). The S fragment is bound to the four largest polypeptides while the 5' and 3' Alu fragments are associated with the two smallest protein constituents of the SRP.