Similarly, Nop4 and other factors are required in vivo for A3 cleavage (Berg??s et al, 1994; Sun and Woolford, 1994), even though this site can be cleaved by RNase MRP alone in vitro (Lygerou et al, 1996).Rat1 showed strong pre-rRNA binding over several different regions, reflecting its diverse targets for processing and degradation.