RNA editing and regulation ofDrosophila 4f-rnpexpression bysas-10antisense readthrough mRNA transcripts

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Abstract
We have previously described an example of extensively A-to-G edited cDNA derived from adult heads of the fruitflyDrosophila melanogaster. In that study, the source of the predicted antisense RNA pairing strand for template recognition by dADAR editase was not identified, and the biological significance of the observed hyperediting was not known. Here, we address each of these questions.4f-rnpandsas-10are closely adjacent X-linked genes located on opposite DNA strands that produce convergent transcripts. We show that developmentally regulated antisensesas-10readthrough mRNA arises by activation of an upstream promoter P2 during the late embryo stage of fly development. Thesas-10readthrough transcripts pair with4f-rnpmRNA to form double-stranded molecules, as indicated by A-to-G editing observed in both RNA strands. It would be predicted that perfect RNA duplexes would be targeted for modification/degradation by enzyme pathways that recognize double-stranded RNAs, leading to decline in4f-rnpmRNA levels, and this is what we observe. The observation using quantitative RT-PCR thatsas-10readthrough and4f-rnptranscript levels are inversely related suggests a role for the antisense RNA in posttranscriptional regulation of4f-rnpgene expression during development. Potential molecular mechanisms that could lead to this result are discussed, one of which is targeted transcript degradation via the RNAi pathway. Insofar as the dADAR editase and RNAi pathways are known to be constitutive in this system, it is likely that control of antisense RNA transcription is the rate-limiting factor. The results provide insight into roles of naturally occurring antisense RNAs in regulation of eukaryotic gene expression.

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