group ii self-splicing introns -pre-rrna of fungal and plant mitochondria -majority of chloroplast...
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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
5’ exon 3’ exon
Domain Structure of a Group II Intron
A
AOH2'
A
OH
A OH
step one
step two
Chemistry of Group II Self-Splicing
ON
OHO
O
P
O
OO
ON
OHO
-
OA
O
O
HO
P
OO
ON
OHO
O
-
OA
O
O
O
ON
OHOH
O
1st step
P
OO
ON
OHO
O
-
ON
OH
O
O
ON
OHOH
O
P
OO
ON
OHO
O
-
ON
OHOH
OO
N
OH
O
O
2nd step
lariat
electron micrograph of Group II lariatGenes VII
Group II Self-Splicing
Lariat Formation
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Science 2008 320 77 - 82
Group II X-ray
Group II Metal Sites
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“Homing” Endonucleases
exon exonintron target site
transcriptionsplicing
excised intron
translation
Endonuclease/Reverse Transcriptase
cleaves
target DNA
intron incorporation
-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 subunitassembly
-U22 snoRNA host gene (UHG)-exon sequences not conserved between species, introns conserved
exon exon exonintron intron
function degraded
exon exon exonintron intron
degraded function
(a) (b)
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)
G C C A5' 3' G C C A5' 3'
M1
.
5'
3'
P1
P15
P18
P2
P3P5
P16
P4
P13
P14
P12
P11
P10P9
P8
P6
P17
P7
J13/14
.........................
Domain 1
Domain 2
RNase P Model
Proc Natl Acad Sci (2005) 102, 13392
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RNase P X-ray
E P A E P A
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