melRNA-seq for Expression Analysis of SINE RNAs and Other Medium-Length Non-Coding RNAs.

Background Small interspersed elements (SINEs) are transcribed by RNA polymerase III (Pol III) to produce RNAs typically 100-500 nucleotides in length. Although their RNA abundance can be evaluated by Northern blotting and primer extension, the nature (sequence, exact length, and genomic origin) of these RNAs cannot be revealed by these methods. Moreover, mRNA sequencing (mRNA-seq) is not able to distinguish bona fide SINE RNAs or SINE sequences present in longer transcripts. Results To elucidate the abundance, source loci, and sequence nature of SINE RNAs, we established a deep sequencing method, designated as melRNA-seq (medium-length RNA-seq), which can determine whole-length RNA sequences. Total RNA samples were treated with 5' pyrophosphohydrolase (RppH), which allowed ligation of an RNA adaptor to the 5' end of intact SINE RNAs. Similarly, another adaptor was ligated to the 3' end, followed by reverse transcription, PCR amplification, size selection, and single-end deep sequencing. The analysis of two biological replicates of RNAs from mouse spermatogonia showed high reproducibility of SINE expression data both at family and locus levels. Conclusions This new method can be used for quantification and detailed sequence analysis of medium-length non-coding RNAs, such as rRNA, snRNA, tRNAs, and SINE RNAs. Further, its dynamic range is much wider than Northern blotting and primer extension.