Chapter 19. Ribosomes and Transfer RNA. Ribosome composition. Prokaryotes 30S S1-S21 + 16S rRNA 50S L1-L33 (L34 not visible) + 5S rRNA and 23S rRNA Eukaryotes 40S About 30 proteins + 18S rRNA 60S About 40 proteins + 5S, 5.8S, and 28S rRNA. Fig. 19.5. Polysomes - P625.
Ribosomes and Transfer RNA
Both prokaryotes and eukaryotes have polysomes
Most mRNA are translated by more than one ribosome at a time; the result, a structure in which many ribosomes translate an mRNA in tandem, is called a polysome.
In eukaryotes, polysomes are found in the cytoplasm. In prokaryotes, transcription of a gene and translation of the resulting mRNA occur simultaneously. Therefore, many polysomes are found associated with an active gene.
74 ribosome translating
19.2 Transfer RNA
Mix RNA with pH5 enzyme,
ATP and [14C]Leucine
Charging of tRNA with an amino acid
Mix [14C]Leucine-charged pH 5
RNA with microsome
Incorporation of leucine from leucyl-tRNA in to the
nascent protein on ribosome
Recognition of tRNAs by aminoacyl-tRNA synthetase:
The second genetic code
It is the nature of the tRNA that matters
Given that the secondary and tertiary structures of all tRNA are essentially the same, what base sequences in tRNA do the synthetases recognize when they are selecting one tRNA out of a pool of over 20?
Proofreading and editing by aminoacyl-tRNA synthetases
1958, Pauling used thermodynamics and found:
Ile and val only differ in –CH2 group and one-fifth Val-tRNAile would be made
In fact, 1/150 amino acid is activated by IleRS to make Val and 1/3,000 aminoacyl-tRNA is Val-tRNAile
How does isoleucyl-tRNA synthetase prevent
formation of Val-tRNAIle?
Fersht in 1977
The space between Trp232 and Tyr386 is just big enough to
accommodate valine but not Ile (too big)
Abolish this region could abolish editing but not activation activity