22Sequencing of native RNA and corresponding cDNA was performed using Oxford Nanopore 23Technology. The % Error of Specific Bases (%ESB) was higher for native RNA than for 24 cDNA, which enabled detection of ribonucleotide modification sites. Based on %ESB 25 differences of the two templates, a bioinformatic tool ELIGOS was developed and applied to 26 rRNAs of E. coli, yeast and human cells. ELIGOS captured 91%, 95%, ~75%, respectively, 27 of the known variety of RNA methylation sites in these rRNAs. Yeast transcriptomes from 28 different growth conditions were also compared, which identified an association between 29 metabolic adaptation and inferred RNA modifications. ELIGOS was further applied to human 30 transcriptome datasets, which identified the well-known DRACH motif containing N6-31 methyadenine being located close to 3'-untranslated regions of mRNA. Moreover, the RNA 32 G-quadruplex motif was uncovered by ELIGOS. In summary, we have developed an 33 experimental method coupled with bioinformatic software to uncover native RNA 34 modifications and secondary-structures within transcripts. 35 36 37
MAIN TEXT 38The transcriptome is the collection of all RNA molecules present in a given cell that can be 39 determined by high-throughput techniques, such as microarray analysis or RNA sequencing 40 (RNA-seq) methods 1 . RNA-seq using next-generation sequencing (NGS) techniques has 41 been replacing microarray analysis, since the former is able to detect novel or unknown 42 transcripts. Further, NGS enables transcriptome analysis with a higher dynamic range of 43 expression levels than for microarrays 2 . With improved sample preparation methods and 44 reduced sequencing costs, RNA-seq by NGS has become the method of choice to study 45 transcriptomes. 46The length of sequence reads generated with most NGS platforms range from 35 nt up 47 to about 500 nt, so that single reads rarely cover a complete transcript. Accurate alignment 48 and assembly of such short sequences depends on availability of a reference genome, and the 49 identification of spliced isoforms or gene-fusion transcripts remains a challenge 3 . Further, 50 methods depending on reverse transcription (RT) of RNA and amplification may introduce 51 biases and artifacts 4 . These shortcomings can be overcome by directly sequencing native 52RNA molecules using technologies such as the Oxford Nanopore Technologies (ONT) 53 platform. Direct RNA sequencing without amplification (dRNA-seq) is able to generate long 54 reads, typically covering the full length of a transcript 5 . The method can accurately quantify 55 transcripts in order to analyze differential gene expression with a dynamic range comparable 56 to traditional RNA-seq derived from short read sequencing, while it enables accurate 57 identification of the structure and boundaries of transcripts including spliced products 6 . 58An additional advantage of dRNA-seq is the detection of transcriptional modifications 59 inferred from the current signal as the RNA molecule passes a nanopore: modified RN...