What is small RNA?

Small RNAs, as the name suggests, are RNA molecules, short in length (from around 18 to 30 nucleotides). They are non-coding and have been considered as “junk” sequences until more advanced tools and technologies, such as RNA sequencing became available. Now, the main function of small RNAs includes regulation of expression of target genes. They are believed to be crucial in developmental processes, immunity, and stem cell biology.

The major classes of small RNAs include miRNAs, siRNAs, piRNAs, and tsRNAs. Each of them has a distinct biogenesis process and function. We will analyze the function of each of the major small RNA categories separately.

• MicroRNA (miRNA) is usually 22 nucleotides long RNA molecule. It is the most known category of small RNAs. It contains a complementary sequence to mRNAs and allows miRNA to participate in transcriptional regulation. The main role of miRNA is RNA silencing by cleaving mRNA, inhibiting translation, or shortening mRNA’s poly(A) tail. When it comes to small RNA sequencing, miRNA is the first thing that most of us think of.
• In addition, the other three categories of small RNAs are important. For example, small interfering RNA (siRNA) is generally 18-26 nucleotides in length. It mediates the degradation and methylation of target genes and regulates physiological and biochemical reactions such as growth, development, and the occurrence of immune diseases.
• The next one – piRNA – stands for PIWI-interacting RNA. These RNAs are 24-31 nucleotides long. PIWI proteins are responsible for stem and germline cell differentiation and piRNAs ensure fertility by silencing transposable elements and regulating other genetic structures within germ cells. piRNAs form a complex with PIWI proteins and guide them. PIWI proteins are capable of cleaving RNAs and mediating transcriptional silencing. The main function of piRNAs is silencing RNAs and decreasing the expression of transposons. piRNAs are crucial for fertility and they are mostly active in the testes of mammals. It is also thought that piRNAs are related to epigenetic processes.
• Finally, tsRNAs stand for tRNA-derived small RNAs, which are usually 18-40 nucleotides long. They include three classes: precursor tsRNAs, mature tRNA-derived fragments, and tRNA-derived halves. The main functions of tsRNAs are thought to be in the regulation of homeostasis, ribosome biogenesis, translational regulation, etc. The exact functions are unknown, but during some diseases, the cells display high levels of tsRNAs.

As a throughput research approach, small RNA sequencing technology and improved bioinformatics methods, such as advanced genome mapping, shed light on novel processes and functions of small RNAs. The major groups of small RNAs are miRNAs, siRNAs, piRNAs, and tsRNAs, and each of them differs by biogenesis process and average length. With library selections at different lengths of insert, small RNA sequencing distinguishes sequencing targets and reveals research importance as gene function regulators and even potential diagnostic biomarkers.