The unmasking of 'junk' RNA reveals novel sRNAs: from processed RNA fragments to marooned riboswitches.

While the notion that RNAs can function as regulators dates back to early molecular studies of gene regulation of the lac operon, it is only over the last decade that the ubiquity and diversity of regulatory RNAs are being realized. Advancements in high throughput sequencing and the adoption of these approaches to rapidly sequence genomes and transcriptomes and to examine gene expression and RNA binding protein specificity have revealed an ever-expanding RNA world. In this review, we focus on recent studies revealing that RNA fragments cleaved from larger coding or noncoding RNAs can have regulatory functions. Additionally, we discuss examples of riboswitches that function in trans as mRNA or protein-binding sRNAs, upending the traditional thinking that these are exclusively cis-acting elements.