The repression machinery controlling <i>ZAM</i> and <i>Idefix</i> acts post-transcriptionally, before translation. DessetSophie BuchonNicolas MeigninCarine CoiffetMichael VauryChantal 2013 <p>A: Transcripts from the pGFP-ZU transgene were examined in northern blot experiments. A typical result is shown in A. GFP transcripts revealed by a riboprobe complementary to GFP mRNAs are detected in the U line and not in the S line. Actin is used as a loading control. B: Northern blots and quantification based on three northern blot experiments performed on flies containing pGFP-IdU and pGFP-IdUAS transgenes. Their structures are presented above the graph. No GFP transcripts synthesized from the pGFP-IdU transgene are detected by the GFP riboprobe in an S background, whereas their amount is high in a U background. An even higher amount of GFP transcripts is observed in an S or U background when the <i>Idefix</i> fragment is inserted in the opposite orientation (pGFP-IdUAS transgenes). C and D- RNase protection assays reveal the presence of small RNAs (20 to 30 nt long) that are homologous to <i>ZAM</i> and <i>Idefix.</i> These RNAs are detected in S lines and, at a much lower level, in the U line. Small RNAs homologous to the antisense strand of the 5′UTR of <i>ZAM</i> are presented in C. 20 to 30 nt long antisense strand RNAs (−) homologous to the 5′UTR or the <i>gag</i> gene of <i>Idefix</i> are detected. Sense strands (+) are absent or present in very small amounts. A typical experiment is presented in D. Signs (+) and (−) indicate respectively sense-strand and anti-sense strand RNAs of ZAM or Idefix revealed by the riboprobes.</p>