A method for the construction of equalized directional cDNA libraries from hydrolyzed total RNA

BACKGROUND: The transcribed sequences of a cell, the transcriptome, represent the trans-acting fraction of the genetic information, yet eukaryotic cDNA libraries are typically made from only the poly-adenylated fraction. The non-coding or translated but non-polyadenylated RNAs are therefore not represented. The goal of this study was to develop a method that would more completely represent the transcriptome in a useful format, avoiding over-representation of some of the abundant, but low-complexity non-translated transcripts. RESULTS: We developed a combination of self-subtraction and directional cloning procedures for this purpose. Libraries were prepared from partially degraded (hydrolyzed) total RNA from three different species. A restriction endonuclease site was added to the 3' end during first-strand synthesis using a directional random-priming technique. The abundant non-polyadenylated rRNA and tRNA sequences were largely removed by using self-subtraction to equalize the representation of the various RNA species. Sequencing random clones from the libraries showed that 87% of clones were in the forward orientation with respect to known or predicted transcripts. 70% matched identified or predicted translated RNAs in the sequence databases. Abundant mRNAs were less frequent in the self-subtracted libraries compared to a non-subtracted mRNA library. 3% of the sequences were from known or hypothesized ncRNA loci, including five matches to miRNA loci. CONCLUSION: We describe a simple method for making high-quality, directional, random-primed, cDNA libraries from small amounts of degraded total RNA. This technique is advantageous in situations where a cDNA library with complete but equalized representation of transcribed sequences, whether polyadenylated or not, is desired.