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Group II Self-Splicing Introns
-pre-rRNA of fungal and plant mitochondria
-majority of chloroplast introns
-several classes
-require Mg2+
-no cofactor
Domain Structure of a Group II Intron
A
5’ exon
3’ exon
Chemistry of Group II Self-Splicing
1st step
2nd step
O
O
N
O
O
O
-O
O
O P
-
OH
OH
O
O O
O P
OH
N
O
N
O
N
OH
O
N
O
O
HO
O
OH
O
O
O
OH
A
O
N
O
O
N
O
O
A
O
OH
-O
OH
O
-
O O
O P
O
N
O
OH
OH
lariat
O
O
OH
P
O
N
O
O
N
O
O
O
OH
OH
Group II Self-Splicing
Lariat Formation
electron micrograph of Group II lariat
Genes VII
Group II X-ray
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Science 2008 320 77 - 82
Group II Metal Sites
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“Homing” Endonucleases
-some Group II introns contain ORF for DNA endonuclease
activity and reverse transcriptase (RT)activity
“When the junk isn’t junk”
Nature (1996) 379, 402-3
-snoRNAs (small nucleolar) involved in RNA transcription
-nucleoli are sites of rRNA transcription, processing , ribosomal subunit
assembly
-U22 snoRNA host gene (UHG)
-exon sequences not conserved between species, introns conserved
(a)
(b)
exon
degraded
intron
exon
intron
exon
function
RNase P
E. coli RNase P - the activity responsible for the maturation of
the 5’ end of pre-tRNAs contains a protein component (C5) and an
RNA component (M1)
RNase P Model
P12
P13
J13/14
P11
P14
.........................................
Domain 1
3'
P9
P10
P7
Domain 2
5'
P1
P8
.........................
P2
P4
P18
P5
P3
P6
P17
P16
P15
RNase P X-ray
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QuickTime™ and a
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Proc Natl Acad Sci (2005) 102, 13392
Protein Synthesis
E. coli Ribosome
-70S particle, MW ~2.5 x 106
-dissociable into small (30S) and large (50S) subunits
-30S contains 16S RNA, 21 polypeptides
-50S contains 5S, 23S RNA + 31 polypeptides
“Although the ribosome has been crystallized…it is such
a complex entity that it will be many years before its
structure is known in molecular detail”
- Voet and Voet, Biochemistry 1995
Chemistry of Peptidyl Transfer
B
B
+
B
Ribosome X-ray Structure
Science (2000) 289:920-30