7SL

7SL RNA is a non-coding RNA component of the signal recognition particle (SRP) found in eukaryotes and some archaea. It is approximately 300 nucleotides long and folds into a structure that serves as the RNA scaffold for the SRP protein complex. The SRP binds to signal sequences on nascent polypeptides as they emerge from ribosomes and directs the ribosome–nascent chain to the endoplasmic reticulum membrane by interacting with the SRP receptor, thereby pausing translation and enabling co-translational targeting and translocation of the protein.

Genomic origin and expression: The mature 7SL RNA is transcribed by RNA polymerase III from RN7SL gene copies. Vertebrate genomes typically contain multiple RN7SL gene copies, with some functional and others present as pseudogenes. The RNA associates with SRP proteins to form the functional SRP particle in the cytosol, contributing to protein targeting to the ER across diverse eukaryotes.

Evolution and related sequences: The 7SL RNA sequence is the progenitor of the Alu family of short interspersed elements (SINEs) in primates. Alu elements arose from reverse transcription of 7SL RNA and have since proliferated to hundreds of thousands to over a million copies, influencing genome structure, regulation, and evolution, while most copies are nonfunctional.

Relation to other SRP RNAs: In bacteria, the SRP is organized differently, typically featuring a distinct RNA component (4.5S RNA) and different protein partners; 7SL RNA represents the eukaryotic/archaeal form of the SRP RNA component.