•Ribosomes are the sites of protein synthesis in both prokaryotic and eukaryotic cells.
•First characterized as particles detected by ultracentrifugation of cell lysates
•ribosomes are usually designated according to their rates of sedimentation: 70S for bacterial ribosomes and 80S for the somewhat larger ribosomes of eukaryotic cells.
•Both prokaryotic and eukaryotic ribosomes are composed of two distinct subunits, each containing characteristic proteins and rRNAs
•E. coli, for example, contain about 20,000 ribosomes, which account for approximately 25% of the dry weight of the cell, and rapidly growing mammalian cells contain about 10 million ribosomes.
•The large subunit (designated 50S) of E. coli ribosomes consists of the 23S and 5S rRNAs and 34 proteins; the small subunit (30S) is composed of the 16S rRNA and 21 proteins
•The large subunit (60S) of eukaryotic ribosomes is composed of the 28S, 5.8S, and 5S rRNAs and 46 proteins; the small subunit (40S) contains the 18S rRNA and 33 proteins.
•Direct evidence for the catalytic activity of rRNA first came from experiments of Harry Noller and his colleagues in 1992.
•large ribosomal subunit is able to catalyze the formation of peptide bonds
•rRNA was responsible for catalyzing peptide bond formation
•large ribosomal subunit functions as a ribozyme, with the fundamental reaction of protein synthesis being catalyzed by RNA
•Rather than being the primary catalytic constituents of ribosomes, ribosomal proteins play a structural role
•The direct involvement of rRNA in the peptidyl transferase reaction has important evolutionary implications.
•The 5′ terminal portions of both prokaryotic and eukaryotic mRNAs are therefore noncoding sequences, referred to as 5′ untranslated regions (UTR)
•Eukaryotic mRNAs usually encode only a single polypeptide chain, but many prokaryotic mRNAs encode multiple polypeptides that are synthesized independently from distinct initiation sites
•Messenger RNAs that encode multiple polypeptides are called polycistronic
•monocistronic mRNAs encode a single polypeptide chain.
•In both prokaryotic and eukaryotic cells, translation always initiates with the amino acid methionine, usually encoded by AUG.
•In most bacteria, protein synthesis is initiated with a modified methionine residue (N formylmethionine), whereas unmodified methionines initiate protein synthesis in eukaryotes (except in mitochondria and chloroplasts, whose ribosomes resemble those of bacteria).
•Initiation codons in bacterial mRNAs are preceded by a specific sequence (called a Shine-Dalgarno sequence, after its discoverers, John Shine and Lynn Dalgarno) that aligns the mRNA on the ribosome for translation by base-pairing with a complementary sequence near the 3′ terminus of 16S rRNA
•ribosomes recognize most eukaryotic mRNAs by binding to the 7-methylguanosine cap at their 5′ terminus
•The ribosomes then scan downstream of the 5′ cap until they encounter the initiation codon (usually AUG)
•The group of ribosomes bound to an mRNA molecule is called a polyribosome, or polysome.
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