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Lecture PowerPoint to accompany
Molecular Biology
Fourth Edition
Robert F. Weaver
Chapter 19
Ribosomes and
Transfer RNA
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
19.1 Ribosomes
• E. coli ribosome is a two-part structure
with a sedimentation coefficient of 70S
• Two subunits of this structure:
– 30S is the small subunit that decodes mRNA
– 50S subunit links amino acids together
through peptide bonds
19-2
Fine Structure of 70S Ribosome
• T. thermophilus crystal structure of 70S
ribosome in complex with mRNA analog and 3
tRNAs shows:
– Positions and tertiary structures of all 3 rRNA and
most proteins can be determined
– Shapes and locations of tRNAs in A, P, and E sites
are evident
– Binding sites for tRNAs in ribosome are composed of
rRNA, not protein
– Anticodons of tRNAs in A and P sites approach each
other closely enough to base-pair with adjacent
codons bound to 30S subunit as mRNA kinks 45°
19-3
More Structural Detail
• Acceptor stems of tRNAs in A and P sites
also approach each other closely (5 Å) in
the peptidyl transferase pocket of 50S
subunit
• Consistent with need for 2 stems to
interact during peptide bond formation
• 12 contacts are seen between subunits,
most RNA-RNA interactions
19-4
E. coli Ribosome Structure
• Crystal structure of E. coli ribosome
contains 2 structures differing from each
other by rigid body motions of ribosome
domains relative to each other
– Head of 30S particle rotates by 6
– 12 rotation compared to T. thermophilus
ribosome
– Probably part of a ratchet action that occurs
during translocation
19-5
Eukaryotic Ribosomes
• Eukaryotic cytoplasmic ribosomes are:
– Larger
– More complex
• Eukaryotic organellar ribosomes are
smaller than prokaryotic ones
19-6
Ribosome Composition
• The E. coli 30 subunit contains
– 16S rRNA
– 21 proteins (S1 – S21)
• E. coli 50S subunit contains
– 5S rRNA
– 23S rRNA
– 34 proteins (L1 – L34)
• Eukaryotic cytoplasmic ribosomes are:
– Larger
– Contain more RNAs and proteins
19-7
Ribosome Assembly
• Assembly of the 30S ribosomal subunit in
vitro begins with 16S rRNA
• Proteins join sequentially and
cooperatively
– Proteins added early in the process
– Help later proteins to bind to the growing
particle
19-8
Fine Structure of the 30S
Subunit
• Sequence studies of 16S rRNA led to a
proposal for secondary structure of the
molecule
• X-ray crystallography studies have
confirmed the conclusions of these studies
– 30S subunit with extensively base-paired 16S
rRNA whose shape essentially outlines the
whole particle
– X-ray crystallography studies confirmed
locations of most of the 30S ribosomal
proteins
19-9
Interaction of the 30S Subunit
with Antibiotics
• 30S ribosomal subunit plays 2 roles
– Facilitates proper decoding between codons
and aminoacyl-tRNA anticodons
– Also participates in translocation
• Crystal structures of 30S subunits with 3
antibiotics interferring with these 2 roles
shed light on translocation and decoding
– Spectinomycin
– Streptomycin
– Paromomycin
19-10
Spectinomycin
• Spectinomycin binds to 30S subunit near
the neck
• At this site, binding interferes with
movement of the head
• Head movement is required for
translocation
19-11
Streptomycin
• Streptomycin binds near the A site of 30S
subunit
• Binding stabilizes the ram state of the
ribosomes
• Fidelity of translation is reduced:
– Allowing noncognate aminoacyl-tRNAs to
bind easily to the A site
– Preventing the shift to the restrictive state that
is necessary for proofreading
19-12
Paromomycin
• Paromomycin binds in the minor groove of 16S
rRNA H44 helix near the A site
• This binding flips out bases A1492 and A1493 to
stabilize base pairing between codon and anticodon
– Flipping out process normally requires energy
– Paromomycin forces it to occur and keeps the
stabilizing bases in place
• State of the A site stabilizes codon-anticodon
interaction, including interaction between
noncognate codons and anticodons with decline
in fidelity
19-13
Interaction of the 30S Subunit
with Initiation Factors
• X-ray crystal structure of IF1 bound to the 30S
ribosomal subunit shows IF1 binds to the A site
• In that position IF1:
– Blocks fMet-tRNA from binding to the A site
– May also actively promote fMet-tRNA binding to P site
through interaction between IF1 and IF2
• IF1 also interacts closely with helix H44 of the
30S subunit
• IF accelerates both association and dissociation
of the ribosomal subunits
19-14
Fine Structure of the 50S
Subunit
• Crystal structure of the 50S ribosomal
subunit has been determined to 2.4 Å
• Structure reveals relatively few proteins at
interface between ribosomal subunits
– No proteins within 18 Å of peptidyl transferase
active center tagged with a transition state
analog
– 2’-OH group of tRNA in the P site is very well
positioned to form a hydrogen bond to amino
group of aminoacyl-tRNA in A site
19-15
2‘-Hydroxyl Group Role
• 2’-OH group of tRNA in the P site
– Forms a hydrogen bond to amino group of aminoacyltRNA in A site
– Helps catalyze peptidyl transferase reaction
• Removal of this hydroxyl group eliminates
peptidyl transferase activity
• Removal of the 2’-OH of A2451 of the 23S rRNA
inhibits peptidyl transferase activity
• May also participate in catalysis by:
– Hydrogen bonding
– Helping to position reactants properly for catalysis
19-16
50S Exit Tunnel
Exit tunnel through the 50S subunit
– Just wide enough to allow a protein a-helix to
pass
– Walls of tunnel are made of RNA
– Hydrophobicity is likely to allow exposed
hydrophobic side chains of nascent
polypeptide to slide through easily
19-17
Polysomes
• Most mRNAs are translated by more than one
ribosome at at time
• A structure in which many ribosomes translate
mRNA in tandem is called a polysome
• Eukaryotic polysomes are found in the
cytoplasm
• In prokaryotes, transcription of a gene and
translation of the resulting mRNA occur
simultaneously
• Many polysomes are found associated with an
active gene
19-18
19.2 Transfer RNA
• An adaptor molecule was proposed that
could serve as a mediator between string
of nucleotides in DNA or RNA and the
string of amino acids in the corresponding
protein
• The adaptor contained 2 or 3 nucleotides
that could pair with nucleotides in codons
19-19
The Discovery of tRNA
• Transfer RNA was discovered as a small
species independent of ribosomes
• This small species could be charged with
an amino acid
• That species could then pass the amino
acid to a growing polypeptide
19-20
tRNA Structure
• All tRNAs share a common secondary
structure represented by a cloverleaf
• Four base-paired stems define three stemloops
– D loop
– Anticodon loop
– T loop
• The acceptor stem is the site to which
amino acids are added in the charging
step
19-21
tRNA Shape
• tRNAs share a common three-dimensional
shape resembling an inverted L
• This shape maximizes stability by lining up
the base pairs:
– In the D stem with those in the anticodon stem
– In the T stem with those in the acceptor stem
• Anticodon of the tRNA protrudes from the
side of the anticodon loop
– Anticodon is twisted into a shape that basepairs with corresponding codon in mRNA
19-22
Modified Nucleosides in tRNA
19-23
Recognition of tRNA Acceptor
Stem
• Biochemical and genetic experiments
have demonstrated the importance of the
acceptor stem in recognition of a tRNA by
its cognate aminoacyl-tRNA synthetase
• Changing one base pair in the acceptor
stem can change the charging specificity
19-24
The Anticodon
• Biochemical and genetic experiments
have shown that anticodon, like acceptor
stem, is an important element in charging
specificity
• Sometimes the anticodon can be the
absolute determinant of specificity
19-25
Structures of Synthetase-tRNA
Complexes
Crystallography has shown that synthetasetRNA interactions differ between the 2
classes of aminoacyl-tRNA synthetases
– Class I synthetases
• Pockets for acceptor stem and anticodon of their
cognate tRNA
• Approach the tRNAs from the D loop and acceptor
stem minor groove side
– Class II synthetases
• Also have pockets for acceptor stem and anticodon
• Approach tRNA from opposite including the variable
arm and the major groove of the acceptor stem
19-26
Proofreading and Editing
Amino acid selectivity of at least some aminoacyltRNA synthetases is controlled by a double-sieve
mechanism
– 1st sieve is coarse excluding amino acids too big
• Enzyme accomplishes this with an active site for
activation of amino acids just big enough to
accommodate the cognate amino acid, not larger amino
acids
– 2nd sieve is fine, degrades too small aminoacyl-AMPs
• Done with a second active site, the editing site, admits
small aminoacyl-AMPs and hydrolyzes them
• Cognate aminoacyl-AMP is too big to fit into the editing
site
• Enzyme transfers the activated amino acid to its cognate
19-27
tRNA