Biomedical Engineering Reference
In-Depth Information
Ribose and base methylations occur in highly conserved regions of rRNA and in
many cases cluster near pseudouridine residues (Bachellerie and Cavaille
1997
;
Decatur and Fournier
2002
). Methyltransfer reactions to RNA nucleotides are cata-
lyzed by RNA-methyltransferases, which use
S
-adenosylmethonine (SAM) as a
methyl group donor. Groundbreaking analyses by Maden and colleagues since the
early 1970s have significantly contributed to our current understanding of methyl
sites. By employing radioactive labeling, T1 RNase digestion and separation of
rRNA they demonstrated that methylation occurs rapidly upon transcription of
rRNA in the nucleolus (Maden et al.
1972
; Salim et al.
1970
) . Subsequently, a
detailed list of methyl modifications in
Saccharomyces cerevisiae
provided further
evidence that both ribose and base methyl modifications are present in precursor
rRNA (pre-rRNA) and mature rRNA species, with some additional base methyla-
tions occurring in mature rRNA species (Brand et al.
1977
) . More recent studies
reveal that methylation residues are clustered in functionally important regions of
the ribosome, including the peptidyl transferase center (PTC) of the large ribosomal
subunit (LSU) and the decoding center of the small subunit (SSU) (Bachellerie and
Cavaille
1997
; Decatur and Fournier
2002
), suggesting that rRNA methylation may
play an important role within ribosomes. Importantly, in bacteria, rRNA methyla-
tions promote resistance to ribosome-targeted antibiotics, thus indicating that
modifications greatly influence bacterial ribosomes (Doi and Arakawa
2007
; Long
et al.
2006
). In yeast, rRNA methylations in functionally important regions of the
ribosome are important for maintaining global rates of protein synthesis (Liang
et al.
2009
) and a very recent study provides evidence that methylation of rRNA
may play a role in translational specificity in mammals (Basu et al.
2011
) . Indeed,
the importance of rRNA methylation in mammals is further highlighted by findings
that the rRNA methyltransferase, fibrillarin, is essential for development (Newton
et al.
2003
) .
A variety of biophysical approaches used to study the components responsible
for modifications of rRNA, including electron microscopy, X-ray crystallography,
NMR spectroscopy, mass spectrometry, and in vivo model systems, have shed
significant light on our current understanding of the molecular events that guide dif-
ferent types of rRNA methylation. Stand-alone enzymes carry out the majority of
base methylations; however, RNA methyltransferases catalyzing ribose methylation
reactions rely on specific guide snoRNAs to select RNA substrates (Cavaille et al.
1996
; Kiss-Laszlo et al.
1996
) . In eukaryotes, 2 ¢ -
O
-methylation of rRNA requires
the box C/D small nucleolar ribonucleoprotein (snoRNP) complex, which consists
of four core protein components, fibrillarin (Nop1 in yeast), NOP58 (Nop58 in
yeast), NOP56 (Nop56 in yeast), and 15.5 kDa protein (Snu13 in yeast), and a box
C/D snoRNA (Lafontaine and Tollervey
1999
; Lyman et al.
1999
; Reichow et al.
2007
; Schimmang et al.
1989
; Watkins et al.
2000
) . C/D snoRNAs speci fi cally
guide the snoRNP complex to the target site of 2¢ -
O
-methylation (Cavaille et al.
1996
; Kiss-Laszlo et al.
1996
), whereas the enzymatic activity is initiated by the
highly conserved RNA methyltransferase fibrillarin, one of the most abundant pro-
teins in the fibrillar region of the nucleolus, the site of pre-rRNA synthesis and
processing (Tollervey et al.
1991,
1993
). For a more detailed overview of the C/D
Search WWH ::
Custom Search