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50S, roughly equivalent to the 60S ribosomal subunit in eukaryotic cells, is the larger subunit of the 70S ribosome of prokaryotes. The 50S subunit is primarily composed of proteins but also contains single-stranded RNA known as ribosomal RNA (rRNA). rRNA forms secondary and tertiary structures to maintain the structure and carry out the catalytic functions of the ribosome.
The 30S subunit is an integral part of mRNA translation.It binds three prokaryotic initiation factors: IF-1, IF-2, and IF-3. [3]A portion of the 30S subunit (the 16S rRNA) guides the initiating start codon (5′)-AUG-(3′) of mRNA into position by recognizing the Shine-Dalgarno sequence, a complementary binding site about 8 base pairs upstream from the start codon. [4]
A ribosome is made from complexes of RNAs and proteins and is therefore a ribonucleoprotein complex. In prokaryotes each ribosome is composed of small (30S) and large (50S) components, called subunits, which are bound to each other: (30S) has mainly a decoding function and is also bound to the mRNA
Quinupristin binds to a nearby site on the 50S ribosomal subunit and prevents elongation of the polypeptide, [9] as well as causing incomplete chains to be released. [9] Geneticin, also called G418, inhibits the elongation step in both prokaryotic and eukaryotic ribosomes. [10]
In the small (30S) subunit of E. coli ribosomes, the proteins denoted uS4, uS7, uS8, uS15, uS17, bS20 bind independently to 16S rRNA. After assembly of these primary binding proteins, uS5, bS6, uS9, uS12, uS13, bS16, bS18, and uS19 bind to the growing ribosome. These proteins also potentiate the addition of uS2, uS3, uS10, uS11, uS14, and bS21.
The initiation phase is completed once a 50S subunit joins the 30S subunit, forming an active 70S ribosome. [11] Termination of the polypeptide occurs when the A site of the ribosome is occupied by a stop codon (UAA, UAG, or UGA) on the mRNA, creating the primary structure of a protein. tRNA usually cannot recognize or bind to stop codons.
Ribosome recycling step is responsible for the disassembly of the post-termination ribosomal complex. [14] Once the nascent protein is released in termination, Ribosome Recycling Factor and Elongation Factor G (EF-G) function to release mRNA and tRNAs from ribosomes and dissociate the 70S ribosome into the 30S and 50S subunits. IF3 then ...
[1] [2] Eukaryotic ribosomes are also known as 80S ribosomes, referring to their sedimentation coefficients in Svedberg units, because they sediment faster than the prokaryotic ribosomes. Eukaryotic ribosomes have two unequal subunits, designated small subunit (40S) and large subunit (60S) according to their sedimentation coefficients.