sRNA

Small RNA (sRNA) refers to short non-coding RNA molecules that regulate gene expression in bacteria, archaea, and eukaryotes. sRNAs vary in size and biogenesis across organisms, but many function by guiding effector proteins to target transcripts or chromatin to modulate expression. They can be encoded as independent transcripts or as antisense or intronic elements.

In bacteria, sRNAs are typically 50 to several hundred nucleotides long and regulate processes such as stress responses, metabolism, and virulence. They often base-pair with target mRNAs to block ribosome binding or recruit RNases that degrade the transcript. RNA chaperones such as Hfq or ProQ facilitate sRNA–mRNA interactions. Some sRNAs act as regulators of protein activity or as decoys for proteins or small molecules, and many are conserved across related species while others are species-specific.

In eukaryotes, the major classes are microRNAs (miRNAs) and small interfering RNAs (siRNAs), usually about 20 to 24 nucleotides, and Piwi-interacting RNAs (piRNAs) that function primarily in animals. miRNAs originate from hairpin precursors processed by Drosha in the nucleus and Dicer in the cytoplasm, then loaded into Argonaute-containing RNA-induced silencing complexes to repress translation or promote mRNA decay. siRNAs derive from long double-stranded RNA or bidirectional transcription and guide silencing of complementary transcripts. piRNAs associate with Piwi proteins to defend the genome against transposable elements, especially in the germline. In plants, some small RNAs direct RNA-directed DNA methylation and chromatin modifications.

sRNAs contribute to development, immune defense, and adaptation to environmental changes. Their study employs RNA sequencing, Northern blotting, and functional assays to validate targets and mechanisms. Dysregulation of sRNA pathways is linked to disease and impacts host–pathogen interactions and crop traits.