Northern blot analysis of SAT5 small RNAs. Bulk SAT5 small RNAs that are 24-nt in length were detected using a probe derived from the SAT5 consensus sequence. These small RNAs are present in wild-type plants (WT) as well as an rdr6 mutant, which is defective in post-transcriptional gene silencing involving 21-nt small RNAs. The SAT5 24-nt small RNAs are undetectable in mutants defective in the Pol IV pathway (nrpd1, rdr2, dcl3). The blot was re-probed with a probe for a trans-acting small RNA (tasi255), which requires RDR6 for its biogenesis and is independent of Pol IV pathway components. The major RNA on the ethidium bromide stained gel is shown as a loading control (bottom).