Mapping of the 5′ and 3′ extremities of the 35S(P) and 32S transcripts. A, Structure of early pre-rRNA intermediates identified (in shaded box) by two pairs of primers: 32P1 and 32P2. Forward and reverse PCR primers for cDNA amplification are marked in red and blue, respectively. For each fragment, the number of clones obtained is indicated on the right. The number of clones with additional sequences at the 3′ end is marked in parentheses. B, The 32S and 35S(P) pre-rRNAs were determined in gel by cRT-PCR with primers 32P1 (18L/25R) and 32P2 (p23/25R). C and D, DNA sequencing results for 32S (C) and 35S(P) precursors (D). The 32S pre-rRNAs were validated by sequencing of 20 independent clones (D). The 35S(P) pre-rRNAs were validated by sequencing of 25 independent clones (D). The ITS1 and ITS2 locus matched by the 5′ and 3′ ends of these DNA sequences, respectively, are indicated by black triangles and the number of clones. Additional sequences in the 3′ extremities of these clones are marked in red lowercase letters. The number of identical clones is indicated to the right of each fragment.