Weiss and colleagues have designed
an artificial cellular communications network in which
signaling among cells leads to the formation of various
supracellular patterns. Their system includes Escherichia
coli 'sender' cells genetically engineered to express the
LuxI enzyme, which catalyzes the synthesis of acyl-homoserine
lactone (AHL). AHL, in turn, diffuses into nearby 'receiver'
cells expressing LuxR, an AHL-dependent transcriptional
activator that regulates the expression of the lambda
repressor and the lac repressor, which in turn control the
expression of green fluorescent protein (GFP). The system is
such that GFP expression is inhibited by both high and low AHL
concentrations and activated by intermediate AHL
concentrations. As GFP expression depends on the amount of AHL
that reaches the receiver cells, the fluorescence output
varies with the proximity of sender and receiver cells. By
strategically arranging both cell types in dishes, the authors
induced the formation of several patterns, including a
bull's-eye, an ellipse, a heart and a clover. Such synthetic
systems may facilitate the study of developmental processes
and may also have applications in biosensing, biomaterial
fabrication and tissue engineering. (Nature434, 1130−1134, 2005) NC
Unraveling the mysteries of biofilms Biofilms—complex
communities of microorganisms—figure in the geochemistry of
natural formations as well as in some difficult-to-treat
infections in man. Although the bacteria found in biofilms are
often not easily cultured and studied, recently developed
techniques are allowing the molecular signatures of
microorganisms to be determined in situ. Using shotgun
mass spectrometry combined with partial genome databases,
Banfield and colleagues have identified over two thousand
proteins from a community of bacteria found in the acid-rich
environment of underground mines. They were able to discern
metabolic activities of the five major bacterial species in
their samples and to trace the partitioning of the proteins in
the biofilm's complex architecture. Interestingly, they
identified many novel proteins encoded by genes previously
labeled as hypothetical. They also found that certain classes
of proteins—those involved with amino acid metabolism and
protein refolding, for example—were overrepresented compared
with their representation in the genomes. This 'proteogenomic'
approach permits complex communities to be characterized with
only partial genome data and should open the door to studying
other microbial communities. (Science;
published online 5 May 2005; 10.1126/science.1109070,
2005) LD
Swapping partners Research aimed at
altering the substrate specificity of enzymes requires
appropriate screening tools to identify novel activities.
Iverson and colleagues have designed an assay with which the
specificity for both the original and the desired new
substrate can be determined simultaneously. The approach
consists of first displaying a collection of enzyme variants
on the cell surface of Escherichia coli and then
incubating the bacteria with the original substrate,
fluorescently labeled in one color, and with the desired new
substrate, labeled in a different color. As the enzymes cleave
either substrate, the products interact electrostatically with
the cells and stick to their surface. Using flow-cytometry,
the authors are then able to separate those cells that exhibit
the desired color. With this tool, they identify a variant of
the endopeptidase OmpT that has higher specificity for Ala-Arg
bonds than for the original Arg-Arg substrate, with no loss of
efficiency. The same screening concept should be easily
adaptable to other enzymes. (Proc. Natl. Acad. Sci. USA102, 6855−6860, 2005) GTO
Plants deliver double punch An international
team led by Christou and Gatehouse has developed a more toxic
and wider-spectrum Bt toxin to provide insect
resistance to transgenic crops. The new toxin, a fusion
protein combining the Bt toxin Cry1Ac and the nontoxic
ricin B-chain (RB), acts by binding not only to the Cry1Ac
receptors in an insect's midgut but also to the wide
repertoire of receptors with which RB interacts. Expression of
this Cry1Ac-RB fusion protein in transgenic maize and rice
plants rendered the plants substantially more toxic to
particular insects than Cry1Ac alone, and also increased the
range of insects to which the plants were resistant. This
approach represents a further step in the quest to develop
more complex toxins that make the emergence of resistance more
unlikely and should thus contribute to the design of better
strategies for sustainable agricultural pest control.
(Proc. Natl. Acad. Sci. USA102,
7812−7816, 2005) GTO
Rice pest
sequence gets blasted Scientists have sequenced the
genome of a major rice pest—Magnaporthe grisea, the
rice blast fungus. Greater than sevenfold sequence coverage
was achieved using a whole-genome shotgun approach. The rice
blast sequence is predicted to contain 11,109 genes—more than
are found in two related nonpathogenic species, Neurospora
crassa and Aspergillus nidulans. The genome
diverges in several ways from previously sequenced fungi,
which may explain why rice blast is such a successful
pathogen. M. grisea contains many G-protein-coupled
receptor (GPCR)-like genes, which may, like known GPCR genes,
encode proteins that sense the environment and defeat plant
immune systems. In contrast, N. crassa has only one
such gene, and A. nidulans has only two. The M.
grisea genome is also rich in secreted proteins, critical
for sensing the environment; 739 are predicted, more than
twice the number in N. crassa. All in all, the M.
grisea sequence is as different from the sequences of
closely related species as the human genome is from that of
Xenopus laevis. Knowledge of the sequence should
suggest ways of combatting the fungus, which yearly causes the
loss of enough rice to feed 60 million people. (Nature434, 980−986, 2005) TM
Research Highlights written by Nadia Cervoni,
Laura DeFrancesco, Teresa Moogan and Gaspar
Taroncher-Oldenburg.
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