Eukaryotic, prokaryotic translation. Regulation of translation.

Prokaryote Translation:

The bacterial ribosome, particularly from E. coli, is a ribonucleoprotein complex composed of a small 30S and a large 50S subunit, together forming a 70S ribosome. The 30S subunit includes 16S rRNA and 21 proteins (S1–S21), while the 50S contains 23S and 5S rRNA and 34 proteins (L1–L34). These components are arranged to facilitate binding, catalysis, and regulation of protein synthesis. Translation initiation in bacteria begins with the assembly of the initiation complex involving the 30S subunit, mRNA, and a formylmethionine tRNA (fMet–tRNAᵢᴹᵉᵗ). This process is assisted by initiation factors IF1, IF2 (GTP-bound), and IF3. The Shine-Dalgarno sequence on the mRNA aligns with the 16S rRNA, ensuring correct positioning. Upon joining of the 50S subunit and release of initiation factors, translation proceeds with the fMet–tRNAᵢᴹᵉᵗ occupying the P site, and the next aminoacyl-tRNA entering the A site.

Elongation of the peptide involves three steps: delivery of the next aa–tRNA by EF–Tu (a G-protein), formation of the peptide bond catalyzed by the 50S subunit’s peptidyltransferase, and translocation of the ribosome assisted by EF–G. Translation ends when a stop codon (UAA, UAG, or UGA) is recognized by release factors (RF1 or RF2), triggering the hydrolytic release of the polypeptide. Ribosome subunits then dissociate, ready for another cycle. Multiple ribosomes can simultaneously translate a single mRNA, forming a polyribosome or polysome, enhancing efficiency. Several antibiotics inhibit bacterial translation at specific stages—for example, streptomycin prevents initiation, tetracycline blocks A-site tRNA binding, and chloramphenicol inhibits peptidyltransferase. Others, like erythromycin and puromycin, halt elongation by interfering with the ribosome or mimicking tRNA, respectively.

Eukaryotic Trannslation:

Eukaryotic cytoplasmic ribosomes are larger than bacterial ones, consisting of a small 40S and a large 60S subunit that together form the 80S ribosome. The 40S subunit contains 18S rRNA and about 30 proteins, while the 60S subunit includes 28S, 5.8S (or 7S), and 5S rRNA, along with 40 or more proteins depending on the species. Unlike prokaryotes, eukaryotic 5S rRNA is synthesized outside the nucleolus by RNA polymerase III. All other rRNA molecules originate from a common pre-45S rRNA precursor made in the nucleolus by RNA polymerase I. Ribosomal proteins are made in the cytoplasm and transported into the nucleolus to assemble with rRNA. Ribosomal subunits then mature and move to the cytoplasm. Notably, mitochondria have their own distinct protein synthesis machinery, including mitochondrial ribosomes.

Initiation of translation in eukaryotes is more complex than in bacteria and requires additional regulatory proteins. A cap-binding protein (CBP) attaches to the 5' cap of mRNA to unwind it, allowing the 40S subunit and associated initiation factors (eIFs) to bind. The initiator Met–tRNAᵢᴹᵉᵗ is not formylated. Complex formation involves multiple eIFs (e.g., eIF2, eIF3, eIF4C) and hydrolysis of ATP as the 40S subunit scans the mRNA for the AUG start codon. Once found, eIF5 releases other factors, allowing the 60S subunit to join and form the functional 80S ribosome. Elongation and termination proceed similarly to bacteria, using EF-1 and EF-2 for aminoacyl-tRNA delivery and translocation. Termination requires only one factor, eRF, which recognizes all stop codons. Translation occurs on polysomes for efficiency. Inhibitors of eukaryotic translation include cycloheximide, which blocks peptidyltransferase, and diphtheria toxin, which inactivates EF-2 through ADP-ribosylation, halting protein synthesis.

Regulation:

is a crucial control point in gene expression, allowing cells to rapidly respond to internal and external stimuli by modulating protein synthesis. It can occur at several stages, but is most commonly regulated during initiation. In eukaryotes, key factors like eIF2 play a central role—its activity depends on binding GTP, and it must be reactivated by the exchange factor eIF2B after GTP hydrolysis. Under stress conditions, specific kinases can phosphorylate the α-subunit of eIF2, preventing its interaction with eIF2B and effectively halting translation initiation. Other mechanisms include regulation via RNA-binding proteins or microRNAs that bind to mRNA and either block translation or promote mRNA degradation. Additionally, certain mRNAs contain upstream open reading frames (uORFs) or internal ribosome entry sites (IRES), which influence how ribosomes initiate translation. These diverse regulatory strategies allow cells to prioritize the synthesis of proteins necessary for survival, adaptation, and growth.

Source: https://www.wikiskripta.eu/w/Translace_u_prokaryot https://www.wikiskripta.eu/w/Translace_u_eukaryot