Science Magazine - 2 November 2012

ncRNAs across the yeast genome. Further-
more, by focusing on pairs of tandem genes
where the promoter of the downstream gene
is far from the termination region of the
upstream gene, the authors established that
enhanced synthesis of ncRNAs originates
from the downstream bidirectional pro-
moter, rather than from the upstream poly-
adenylation site (see the figure). Enhanced
expression of CUTs, previously detected
in the ∆rrp6 deletion mutant, as well
as an extensive set of new CUTs called
SRTs (Ssu72-restricted transcripts), were
observed in the ssu72-2 mutant. However,
enhanced synthesis of these ncRNAs was
not specific to the ssu72-2 mutation; other
mutations that block looping—including
sua7-1 (which encodes a mutated form of
TFIIB) and defects in the Pta1, Rna14, and
Rna15 components of the 3′-end pre-mRNA
processing machinery—exhibited similar
enhancement of CUTs and SRTs. Accord-
ingly, Tan-Wong et al. define a new function
for gene loops: repression of ncRNA synthe-
sis from bidirectional promoters.
It remains to be determined how gene
loops repress upstream transcription. One
scenario is that bidirectional transcription is
mutually exclusive: Formation of one initia-
tion complex precludes formation of an adja-
cent complex of opposite polarity. In this
case, a promoter-terminator loop would sim-
ply repress ncRNA transcription by default.
This possibility is consistent with the small
decrease in mRNA production associated
with the ssu72-2 mutant observed by Tan-
Wong et al. Alternatively, gene loops might
actively block ncRNA synthesis, perhaps by
localized recruitment of a repressive histone
deacetylase complex.
Gene loops are not unique to yeast. For
example, the HIV provirus forms a transcrip-
tion-dependent gene loop between the 5′
long terminal repeat (LTR) promoter and the
3′ LTR polyadenylation site (14). Interest-
ingly, proper 3′-end formation of mammalian
β-globin mRNA stimulates transcription ini-
tiation of the β-globin gene —an effect that
is most easily explained by looping-mediated
recycling of Pol II (15). The extent to which
PERSPECTIVES
gene loops might be a general feature of Pol
II transcription awaits further investigation.
References and Notes
1. J. M. O’Sullivan et al., Nat. Genet. 36, 1014 (2004).
2. A. Ansari, M. Hampsey, Genes Dev. 19, 2969 (2005).
3. S. M. Tan-Wong et al., Science 338, 671 (2012); 10.1126/
science.1224350.
4. M. Hampsey, B. N. Singh, A. Ansari, J. P. Lainé, S. Krish-
namurthy, Adv. Enzyme Regul. 51, 118 (2011).
5. A. R. Bataille et al., Mol. Cell 45, 158 (2012).
6. D. W. Zhang et al., J. Biol. Chem. 287, 8541 (2012).
7. J. N. Kuehner, E. L. Pearson, C. Moore, Nat. Rev. Mol. Cell
Biol. 12, 283 (2011).
8. J. Dekker, K. Rippe, M. Dekker, N. Kleckner, Science 295,
1306 (2002).
9. S. Medler et al., J. Biol. Chem. 286, 33709 (2011).
10. S. M. Tan-Wong, H. D. Wijayatilake, N. J. Proudfoot, Genes
Dev. 23, 2610 (2009).
11. J. P. Lainé, B. N. Singh, S. Krishnamurthy, M. Hampsey,
Genes Dev. 23, 2604 (2009).
12. P. Preker et al., Science 322, 1851 (2008).
13. Z. Xu et al., Nature 457, 1033 (2009).
14. K. J. Perkins, M. Lusic, I. Mitar, M. Giacca, N. J. Proudfoot,
Mol. Cell 29, 56 (2008).
15. C. K. Mapendano, S. Lykke-Andersen, J. Kjems,
E. Bertrand, T. H. Jensen, Mol. Cell 40, 410 (2010).
Acknowledgments: M. H. is supported by NIH grant R01
GM039484.
10.1126/science.1230576
MATERIALS SCIENCE
Getting Moore from Solar Cells
Exploring alternative device designs and
component materials will be crucial in
improving solar cell performance.

David J. Norris1 and Eray S. Aydil2

Biological organisms, when faced with a difficult environment, take advantage of the process of muta-
tion. Beneficial mutations can help to opti-
mize a known survival strategy or even
reveal a new one. In solar cell research, a
similar process is occurring. Solar cells
must continue to improve in efficiency and
cost if they are to thrive as a viable energy
technology. Through the implementation of
variations on known device architectures,
“mutant” solar cells are leading not only to
important incremental improvements but
also to surprising new approaches. On page
643 of this issue, Lee et al. (1) demonstrate
an example of the latter, introducing a new
species of solar cell for study.
Photovoltaic solar cells convert sunlight
into usable electrical power. After decades
of research, several device archetypes have
been established. These include long-stand-
ing approaches, such as the common silicon
1Optical Materials Engineering Laboratory, ETH Zürich,
8092 Zürich, Switzerland. 2Department of Chemical Engi-
neering and Materials Science, University of Minnesota,
Minneapolis, MN 55455, USA. E-mail: dnorris@ethz.ch;
aydil@umn.edu
solar cell (2, 3), and newer alternatives at
various stages in their development ( 4). One
example is the dye-sensitized solar cell (5),
first reported in 1991. Unlike the silicon cell,
it is a hybrid device made of both inorganic
and organic components. A porous film of
inorganic titania particles is deposited on an
electrode. When a single layer of an organic
dye molecule decorates the surface of these
particles, sunlight is absorbed by the dye,
generating electrons. If extremely small
(nanometer-scale) titania particles are used,
even a thin film contains sufficient surface
area that most of the sunlight is absorbed by
the dye. A voltage can then be established
between the titania-coated electrode and a
counterelectrode when a liquid electrolyte
is placed in between. In this configuration,
the device produces power when the film of
titania particles performs several functions
simultaneously: It provides a scaffold for
the dye, collects the generated electrons, and
transports these charges to the electrode.
Initially, dye-sensitized solar cells con-
verted full sunlight into electricity with an
efficiency of 7.1% (5). By comparison, crys-
talline silicon solar cells convert full sun-
light into electricity with an efficiency of
25%. However, because titania particles are
inexpensive to produce and spread on a sur-
face, dye-sensitized solar cells were pursued
as a low-cost alternative to silicon. More-
over, through optimization of the dye and the
electrolyte, their efficiency now stands at an
impressive 12.3% (6).

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