Figure 7.

Model for recognition of the ITS2-proximal stem by the pre-rRNA processing machinery. The ITS2-proximal stem in yeast is an imperfect 15 bp helix (inset) long enough to produce ∼1.5 helical turns. This would place the unpaired U nucleotide in 25S on the same face of the helix as the C in 5.8S. The distal/medial machinery (shaded oval) recognizes this region of the stem within very strict length and structural parameters. The proximal recognition machinery (hexagon) is more flexible to small increases in length. This machinery may or may not be the same complex that is responsible for the sequence-specific recognition of nucleotides that reside structurally opposite of the mature termini (upper case black nucleotides) that are required to promote accurate and efficient processing of the large subunit rRNAs.