The “Fifth” RNA Nucleotide: A Role for Ribosomal RNA Pseudouridylation in Control of Gene Expression at the Translational Level - Biophysical Approaches to Translational Control of Gene Expression - page 272

Acidosis, and Sideroblastic Anemia (MLASA) harbor missense mutations in the
gene encoding the pseudouridine synthase 1 (PUS1) (Bykhovskaya et al. 2004 ) . In
both yeast and mouse, PUS1 has been demonstrated to be a tRNA Y synthase.
Pathological features of this mitochondrial disorder include myopathy, sideroblastic
anemia, lactic acidosis, and mental retardation, among other severe symptoms.
Importantly, loss of both cytoplasmic and mitochondrial tRNA pseudouridylation is
evident in lymphoblasts from MLASA patients (Patton et al. 2005 ) . Altogether,
these findings support a role for reduced pseudouridylation in different species of
RNA in human diseases.

13.6 Conclusions and Perspectives

Decades of research have provided a multitude of information regarding the unique
chemical properties of Y residues. We have uncovered the position of Y residues
within the ribosome, characterized the components involved in rRNA pseudouridy-
lation, and provided insights into how Y modifications control gene expression at
the translational level. Numerous multidisciplinary approaches, in particular the
combination of biochemical analyses with X-ray crystallography, NMR spectros-
copy, mass spectrometry, and in vivo model systems, have altogether determined an
important contribution of rRNA pseudouridylation towards ribosome biogenesis
and control of protein synthesis. However, several outstanding questions still remain
to be addressed regarding the precise catalytic mechanisms of pseudouridylation,
the kinetics of pseudouridylation, and the specific mechanisms by which Y
modifications modulate translational control.
It will be important to determine if defective rRNA pseudouridylation may trig-
ger an rRNA quality control mechanism. For example, nonfunctional rRNA decay
(NRD) has been described in yeast as a mechanism to monitor mature rRNA in fully
assembled ribosomal subunits (LaRiviere et al. 2006 ) . Whether the NRD pathway
may also eliminate ribosomal subunits lacking rRNA Y levels below a critical
threshold, which would render these ribosomes completely inactive, remains to be
investigated. Future directions in the field should also include studies aimed at
determining if rRNA pseudouridylation is regulated in cells and if so, whether this
can occur both temporally and spatially at the organismal level. Indeed, emerging
data highlighting distinct expression patterns of H/ACA snoRNAs suggest that
pseudouridylation of rRNA may be modulated in a tissue-specific manner.
Additionally, understanding how rRNA pseudouridylation impinges on the struc-
ture of the ribosome remains to be further explored. It seems possible that rRNA
modifications may affect ribosome composition and function by influencing how
ribosomes bind to distinct proteins such as ITAFs, for example, which modulate
translational specificity. Furthermore, it remains unknown whether rRNA Y
modifications may control the engagement of ribosomal subunits to highly struc-
tured cis -acting regulatory elements in the 5¢ and 3¢ UTRs of distinct mRNAs, in
addition to IRES elements and frameshift signals (Fig. 13.5 ). Similarly, it can also