Aberrant pre-rRNA processing in atprmt3. (A) Northern blots identified aberrant accumulation of 32S, 27S (27SA or 27SB), pre-18S (21S or 20S), and pre-5.8S (7S, 5′-5.8S and 6S) rRNAs in atprmt3 mutants. apum23-1 is a positive control accumulating 35S pre-rRNA and the P-A3 fragment. Methylene blue staining (MB stain) of a membrane is shown as a loading control. (B) Diagram illustrating the various pre-rRNA processing intermediates detected by Northern blots with specific probes, which are indicated by horizontal arrows. Black vertical arrows above the diagram indicate endonucleolytic cleavage sites relevant to this study. Red arrows indicate accumulated rRNA intermediates detected by Northern blot assays.