FIGURE 6.

Cytosine in the position +4 [C(+4)] is required for RNase MRP cleavage at the A3 site. (A) Cleavage of a 56-nt-long substrate containing the A3 site. (Arrow) The location of the cleavage site. (B) Cleavage of the same substrate as in A, but with C(+4) replaced with a random mix of G, A, and U. (Lanes 1,5) Alkaline hydrolysis (markers); (lanes 2,6) untreated RNA substrates; (lanes 3,4,7,8) RNase MRP digests. All substrates were 5′-end-labeled with ³²P and separated on a 6% denaturing polyacrylamide gel.