Cellular RNAs: Varied Roles

Abstract About 85% of the human genome is transcribed into RNA. RNAs play essential roles in numerous cellular processes. Less than 2% of all transcripts are coding RNAs (messenger RNA). The remaining vast majority of RNAs do not encode protein and are collectively referred as noncoding RNA (ncRNA). Based on their biological functions, ncRNAs can be grouped into two categories: infrastructural and regulatory ncRNAs. Infrastructural ncRNAs mainly include ribosomal RNAs (rRNAs), transfer RNAs (tRNAs), small nuclear RNAs (snRNAs), small nucleolar RNAs (snoRNAs) and telomerase RNAs. The best‐characterised regulatory ncRNAs are microRNAs (miRNAs) and long noncoding RNAs (lncRNAs). Other regulatory ncRNAs include small inhibitory RNAs (siRNAs), Piwi‐interacting RNAs (piRNAs) and circular RNAs (circRNA). This article will summarise the structure and function of major RNA categories. Three RNA types are essential for protein synthesis: mRNA carries genetic code; tRNA transfers protein codon; and rRNA makes up the ribosomes in which translation takes place. About 85% of the human genome is transcribed into RNA. Only <2% of RNAs are coding RNAs (mRNAs) and the remaining RNAs are noncoding RNAs (ncRNAs). Noncoding RNAs can be grouped into two categories: infrastructural and regulatory ncRNAs. Infrastructural ncRNAs mainly include rRNAs, tRNAs, snRNAs, and snoRNAs. The major regulatory ncRNAs include microRNAs (miRNAs) and long noncoding RNAs (lncRNAs). SnRNAs are often rich in uridylic acid and a group of U‐snRNAs and associated proteins form spliceosome and are responsible for pre‐mRNA splicing. SnoRNAs can be classified into two large subfamilies: C/D‐box (SNORA) and H/ACA‐box (SNORD). The main functions of snoRNAs are in the processing and maturation of pre‐rRNAs and posttranscriptional modification (methylation and pseudouridylation) of rRNA. Three small ncRNAs, siRNA, miRNA and piRNA, form RNA‐induced silencing complex (RISC) to exert posttranscriptional gene regulation by binding to 3′ UTR of target mRNAs and either inducing mRNA degradation (perfect match) or impeding translation (imperfect match). miRNAs are the major type of small ncRNAs. The biogenesis of miRNAs is a multistep process, from long primary miRNA transcripts (pri‐miRNAs) to pre‐miRNAs of ∼70 nucleotides long and to mature miRNA of ∼22 nucleotides. piRNAs mainly function in the germ line of animals to silence transposons and other repetitive elements and maintain genomic stability. lncRNAs are >200 nucleotides long. There are thousands of lncRNAs and the majority have not been functionally characterised. But many have been shown to posttranscriptionally regulate gene expression through various mechanisms. Unlike the better‐known linear RNA, circRNAs is a type of single‐stranded RNA that are covalently closed with no 5′ end caps or 3′ poly(A) tails. Some circRNAs can serve as miRNA and protein sponges to sequester miRNA/protein and regulate gene expression.