subunitmRNAs

Subunit mRNAs are messenger RNAs that encode the individual protein components of multiprotein complexes. In eukaryotes most subunit mRNAs arise from nuclear genes and are translated in the cytosol; their protein products assemble with other subunits to form functional complexes such as ribosomes, proteasomes, RNA polymerases, and signaling or transport complexes. In organelles such as mitochondria and chloroplasts, some subunits are encoded by organellar genomes, while others are nuclear-encoded subunits imported into the organelles, requiring intercompartmental coordination.

Expression of subunit mRNAs is tightly regulated to achieve proper stoichiometry. Transcriptional programs respond to growth and stress, and post-transcriptional control at the level of mRNA stability and translation influences subunit production. For example, mammalian ribosomal protein mRNAs often contain a 5' TOP motif that couples translation to growth signals. 3' UTRs and RNA-binding proteins shape localization, stability, and translation efficiency. Quality control systems preferentially degrade excess unassembled subunits to prevent proteotoxic stress.

By contrast, in bacteria, many subunits of multisubunit enzymes are encoded within operons as polycistronic mRNAs, yielding coexpression of multiple subunits from a single transcript.

Studied examples include ribosomal protein mRNAs, proteasome subunit mRNAs, and the subunits of mitochondrial respiratory-chain complexes; dysregulation of subunit mRNA expression is implicated in developmental disorders and cancer in humans, and in mitochondrial diseases when assembly is compromised.