srnas

Small RNAs, or sRNAs, are short, non-coding RNA molecules that regulate gene expression at the post-transcriptional level. They are found across bacteria, archaea, and eukaryotes and typically range from about 20 to a few hundred nucleotides in length. sRNAs can control gene expression by base pairing with target RNAs or by guiding protein complexes to transcripts, affecting translation initiation and mRNA stability or guiding chromatin modifications in some contexts.

Bacterial sRNAs often act with RNA chaperones such as Hfq or ProQ to stabilize RNA duplexes with target messages, influencing iron metabolism, stress responses, and virulence. Examples include RyhB, DsrA, and OxyS in Escherichia coli. In many bacteria, sRNA activity provides rapid, reversible regulation in response to environmental changes.

In eukaryotes, the best known sRNAs are microRNAs (miRNAs), small interfering RNAs (siRNAs), and piwi-interacting RNAs (piRNAs). miRNAs and siRNAs typically arise from Drosha-DGCR8 and Dicer processing of precursor transcripts, and are loaded into Argonaute proteins to form RNA-induced silencing complexes that repress translation or mediate mRNA cleavage. siRNAs can derive from exogenous double-stranded RNA and participate in antiviral defense. piRNAs, mostly in animals, arise from single-stranded precursors and interact with PIWI proteins to silence transposons and regulate gene expression, often through epigenetic mechanisms.

Biogenesis and function of sRNAs are context-dependent, and their study employs methods such as RNA sequencing, Northern blotting, and quantitative RT-PCR. sRNAs participate in development, immunity, stress responses, and disease, and have potential as therapeutic targets or tools in biotechnology.