ICAM Workshop:
Chromatin Dynamics, Gene Regulation and Silencing
Aug 11-13, 2006
Snowmass, CO
Organizers: Jonathan Widom,
Northwestern University
Anirvan Sengupta,
Rutgers University
Scope:
Dynamic properties of the
chromatin play an essential role in mechanisms of eukaryotic gene expression
regulation. This workshop is intended for physical scientists who want to
understand the biophysical underpinnings of such mechanisms, both in the
context of local regulation of gene expression as well regulation of whole loci
by mechanisms such as chromatin silencing. The topics covered would include,
among other things, in vitro single molecule experiments on the fluctuations in
chromatin structure, efforts to model its three-dimensional structure as well
as mechanical properties and single cell measurements of epigenetic silencing
effects.
The
meeting is sponsored by ICAM. ICAM strongly encourages participation by junior members
of the scientific community. We have set aside 16-20 places for junior
participants-graduate students, postdocs, junior faculty members-from US
institutions, and can offer substantial travel and living expense support to
those willing to share a room. Moreover, junior participants coming from ICAM
branches abroad may apply for an I2CAM travel grant - go to the web site i2cam.org to apply.
Speakers:
Jim
Broach, Princeton
Brad Cairns, U. of Utah
Alex
Grosberg, U. of Minnesota Terrence
Hwa, UCSD
Wilma
Olson, Rutgers
Rob Phillips, Caltech
Vincenzo
Pirotta, Rutgers
Oliver Rando, Harvard
Jasper
Rine, Berkeley
Anirvan Sengupta, Rutgers
Boris
Shklovskii, U. of Minnesota Vasily Studitsky, UMDNJ
Jose Vilar, Sloan-Kettering Jonathan
Widom, Northwestern
Titles
and Abstracts:
As they come in, the titles and abstracts of the talks would
be found here.
Venue and Schedule
The workshop will take place
in the Silvertree Hotel. It would begin with a welcome dinner on Thursday
evening.
Formal talks start Friday morning 9.30AM and conclude before
Sunday lunch hours at 12.30PM.
According to the
current schedule (see below) talks would be over by Saturday, late evening.
This is due to a cancellation and to requests from some speakers.
Travel Info
Snowmass is very close to the
Aspen airport. For cheaper travel arrangements, one might also fly to Denver
and take a bus from there.
Hotel and Registration Information
Please make your reservations at the Silvertree Hotel (100 Elbert
Lane, Snowmass Village), Snowmass at (1-800-525-9402) or by fax at
(970-923-5494). The reservation block is under "ICAM Chromatin
Workshop"; please refer to the above title when making your reservations. The
rate is $99.00 single or $99.00 double occupancy excluding any taxes. The cut
off date for making these reservations is July 10, 2006. The hotel check in
time is 4:00 pm and the check out time is 10:00 am.
For further information about the Silvertree Hotel, you can go to
their website. (Click on
"Getting here" under the heading "About our hotel" for
travel instructions.)
In order to register for the meeting, please download the registration form, fill it out and return it
to Rose Romero at your earliest convenience at (rbromero@lanl.gov),
or by fax at (505) 665-9030.
There will be a registration fee of $175 for all workshop
attendees, payable by check on arrival, the funds from which will be used to
cover the cost of meals and beverages and the meeting rooms.
Schedule:
Friday
9.30AM Widom
10.30AM Rando
11.30AM Cairns
12.30-2.00PM
Lunch Break
2.00PM Vilar
3.00PM Hwa
Break 4.00-4.30PM
4.30PM Studitsky
5.30PM Grosberg
Saturday
9.30AM Rine
10.30AM Broach
11.30AM Sengupta
Lunch
12.30-2.00PM
2.00PM Olson
3.00PM Pirrotta
Break 4.00-4.30PM
4.30PM Shklovskii
5.30PM Phillips
Sunday
Nothing scheduled
Analysis of Heterchromatin Formation
and Inheritance by Single Cell Observations
Eugenia Y. Xu, Karl A. Zawadzki and James R. Broach
Princeton University
Analysis of transcriptional silencing in Saccharomyces has provided valuable insights into
heterochromatin formation and function. However, most of these studies revealed
only the average behaviors of populations of cells. We have examined transcriptional silencing by monitoring
individual yeast cells carrying distinguishable fluorescent reporter genes
inserted at two different silent loci.
These studies showed that two silent loci in a single cell behave
independently, demonstrating that heterochromatin formation is locus
autonomous. Furthermore, some
silencing mutants with an intermediate phenotype, such as sir1, consist of two distinct populations, one
repressed and one derepressed, while other mutants, such as those inactivating the
SAS-I histone H4 K16 acetylase, consist of cells all with an intermediate level
of expression. Finally, both
establishment and decay of silencing can be influenced by specific gene
activators, with establishment occurring stochastically over several generations. Thus, quantifying silencing in
individual cells reveals aspects of silencing not evident from population-wide
measurements.
DNA
translocation as a mechanism for nucleosome remodeling
Brad Cairns, HCI,
Utah
Proteins searching
for their targets on DNA
Alexander
Grosberg, Department of Physics, University of
Minnesota, 116
Church Street SE, Minneapolis, MN 55403, USA
Abstract:
Proteins locate their specific targets on DNA up to two orders of magnitude
faster than the Smoluchowski 3D diffusion rate. A widely accepted explanation of this fact is that proteins
are non-specifically adsorbed on DNA, and sliding along DNA provides for the
faster 1D search. Surprisingly, the role of DNA conformation was never
considered in this context. By explicitly addressing the relative role of 3D
diffusion and 1D sliding along coiled or globular DNA and the possibility of
correlated re-adsorption of desorbed proteins, we have identified a plethora of
new different scaling regimes and made quantitative predictions for the
macroscopic diffusion of proteins through a semi-dilute DNA system.
Quantitative
characterization of the Lac promoter -- cooperativity and sensitivity mediated
by DNA looping
Terrence Hwa,
Dept. of Physics, UCSD
It is the goal of
systems biology to understand the behavior of the whole in terms of the
knowledge of the parts. This is hard to achieve in many cases so far due to the
difficulty of characterizing the many constituents and their complex web of
interactions involved. The lac operon of E. coli, being one of the most extensively studied systems of molecular
biology, offers a possibility of confronting Òsystem-levelÓ properties of
transcriptional regulation with the known biochemistry of the molecular
constituents and their mutual interactions. Such confrontations can reveal
previously unknown constituents and interactions, as well as offering new
insight into how the components work together as a whole. Here we study the
combinatorial control of the lac promoter by the regulators LacR and CRP. A previous in vivo study [Setty et al, PNAS 100: 7702-7
(2003)] found gross disagreement between the observed promoter activities and
the expected behavior based on the known molecular mechanisms. We repeated the
study by identifying and removing several extraneous factors which
significantly modulated the expression of the lac promoter. Through quantitative,
systematic characterization of promoter activity for a number of key mutants
and guided by the thermodynamic model of transcriptional regulation, we are
able to account for the combinatorial control of the lac promoter quantitatively, in term of a
cooperative interaction between CRP and LacR-mediated DNA looping.
Specifically, our analysis indicates that the sensitivity of the inducer
response results from LacR-mediated DNA looping, which is significantly
enhanced by CRP. Thus CRP plays a triple role, in elevating the overall
transcription level through direct activation, and in increasing the
fold-change and sensitivity of repression by assisting LacR-mediated DNA
looping.
Targeting a Dosage
Compensation Complex to X Chromosomes (Cancelled)
Barbara J. Meyer, HHMI, Dept.
of Molecular & Cell Biology, U.C. Berkeley, CA 94720
Chromosomes must be properly
expressed, resolved, compacted, and segregated for genome stability. These diverse processes are controlled
by an interacting set of proteins and complexes. On set of such proteins, the condensin complex, is essential
for chromosome resolution and compaction during mitosis and meiosis. A homologous set of proteins is
essential for the X-chromosome-wide process of dosage compensation, which
ensures that males (XO) and hermaphrodites (XX) express equal levels of
X-linked gene products, despite their difference in X chromosome dose. This dosage compensation complex (DCC)
binds the entirety of both hermaphrodite X chromosomes to achieve
chromosome-wide reduction in gene expression. The DCC not only resembles mitotic condensin, it shares a
component with condensin, and DCC components also participate in other aspects
of chromosome segregation, for example the regulation of crossover interference
during meiosis. This talk will
focus on the connection between dosage compensation and chromosome segregation
and on the mechanism by which the DCC specifically recognizes and binds X
chromosomes.
A novel ÔRoll and SlideÕ mechanism of DNA
folding in chromatin: implications for nucleosome positioning
Michael Y. Tolstorukov
V.
Karazin Kharkov National University, Kharkov, Ukraine
Victor B. Zhurkin
National
Cancer Institute, National Institutes of Health, Bethesda, Maryland
Andrew V. Colasanti, David McCandlish, Wilma K.
Olson
Rutgers,
the State University of New Jersey, Piscataway, New Jersey
The bending of DNA in high-resolution nucleosome structures is accompanied
by large changes in Slide (the shearing of adjacent base pairs along their long
axes). These shear deformations play a
much more important role in the formation of nucleosomal DNA than heretofore
thought. Specifically, if Slide is set to zero at all base-pair steps, the
crystallographically observed DNA superhelical pathway is flattened into a
circle. Moreover, the large Slide deformations imposed on DNA by the histone
proteins at sites of sharp local bending into the minor groove (negative Roll)
appear to govern nucleosome positioning. The computed cost of deforming DNA on
the nucleosome increases substantially if the crystallized sequence is
displaced relative to its observed positioning. Furthermore, this positioning preference
disappears if the contribution from Slide is omitted from the calculations.
Other DNA sequences, which are experimentally known to position nucleosomes in
solution, show similar dips in the total deformation energy close to the
observed ÒnaturalÓ settings (1-3 bp). The computed scores are lowest when
the sequences are positioned such that the most easily deformed base-pair steps
(TA and CA:TG) occur at sites of large positive Slide and negative Roll.
Physics of Genome Management: from Viruses to
Nucleosomes
Rob Phillips, Caltech
Tightly bent DNA
is a fact of life. Whether we
consider how genomes are packaged into their hosts or how genes are
transcribed, there are many examples where the properties of DNA as a physical
object play a critical role in dictating biological function. In this talk, I will
describe several case studies in which the mechanics of DNA plays an intriguing
role in biological function. One case study will center on how bacterial
viruses both package and deliver their genomes and will focus on recent
experiments which measured these properties at the single-molecule level. The second case study in DNA packaging
will consider the accessibility of nucleosomal DNA. In particular, I will describe our recent attempts to
model the dynamics of site accessibility in nucleosomes. As a final related
example in which the mechanics of tightly bent DNA plays a role, I will
describe our efforts to export what we have learned about sequence-dependent
elasticity from nucleosomal studies to DNA looping in transcriptional
regulation.
Polycomb complexes,
Polycomb silencing and its genomic targets
Vincenzo Pirrotta
Department of Molecular
Biology & Biochemistry, Rutgers University, Piscataway, NJ.
Polycomb Group (PcG)
silencing mechanisms, first known for their role in homeotic gene regulation in
Drosophila, have now been found to have a fundamental role in the epigenetic
programming of genomic information controlling growth, differentiation and
self-renewal in Drosophila and in vertebrates. PcG proteins are recruited to
regulatory regions called Polycomb Response Elements (PREs) by multiple
DNA-binding proteins. Chromatin immunopreciptiation experiments analysed by
hybridization to Drosophila genomic tiling microarrays reveal that such presumptive
PREs are found at several hundred genomic loci, corresponding to key genes
controlling most differentiation, growth and pattern-formation processes. The
PREs bind PRC2 complex with histone H3 methyltransferase activity, as well as a
PRC1 complex containing a histone H2A ubiquitylating activity and a
chromodomain protein, PC. Although found only at the PRE, the PRC2 complex
methylates a large domain including the regulatory and transcriptional unit of
the target gene. The PRE itself is not methylated because its core region is
nucleosome free. We present models for the long-distance action of the PRE.
Genome-scale
studies on static and dynamics aspects of chromatin structure in yeast
Oliver Rando,
Bauer Center, Harvard
We have designed a
tiled oligonucleotide microarray to sequence the primary structure of half a
megabase of yeast chromatin. We
have found that yeast promoters exhibit remarkably stereotyped chromatin
architecture, with upstream regulatory DNA sequence found in a long nucleosome-free
region surrounded by two well-positioned nucleosomes with a characteristic
modification pattern. These
modifications associated with promoter nucleosomes comprise one of two groups
of correlated histone modifications, with the other group consisting of histone
modifications that occur over coding regions and whose levels vary with
transcription levels. In yeast,
histone modifications occur in few combinations, suggesting that much less
information is carried in
histone
modifications than is theoretically possible. Altogether, we suggest that the
dominant role of histone modification in transcriptional control is permissive,
rather than instructive, and hypothesize that one of the reasons for the
plethora of histone modifications observed is to allow for analog control of
gene expression. Finally, I will discuss recent results on dynamic aspects of
nucleosome occupancy and histone modification.
On the establishment,
containment and evolution of silencing in Saccharomyces
J. Babiarz J. Halley, J.
Gallagher, E. Osborn, B. Ozydin, L. Teytelman, O. Zill, and J. Rine
Dept. Molec. Cell Biologym UC
Berkeley, Berkeley Calif. 94720
Silencing at HM loci requires binding of the Sir proteins with
proteins at the silencer, and subsequent spreading of Sir proteins to barriers
that require histone H2A.Z. We
have re-evaluated the cell-cycle requirements for silencing using mating type
itself as the benchmark for whether silencing. We inject a single-cell dose of wild-type Sir proteins into sir-
mutants using yeast mating, and then
monitor the establishment of silencing at the single cell level by the
acquisition of sensitivity to mating pheromone. The single-cell studies reveal a lag between phenotype and
genotype, reflecting an increasing sub-population of cells that have
stochastically switched from On to Off at HML and HMR. The probability of switching from On to
Off increases with successive cell divisions, with evidence of patterns to the
switches in state, and increases in mutants lacking distinct histone
modifications, suggesting that the removal of specific histone modifications
increases the probability of switching to the silenced state. Histone variant H2A.Z, conserved from
yeast to man. We showed that S. cerevisiae H2A.Z is acetylated on four N-terminal lysines by a combination of
NuA4 and SAGA. Additionally, H2A.Z
deposition by SWR-Com was a pre-requisite for acetylation. H2A.Z acetylation is critical in
H2A.ZÕs function as a boundary to silent chromatin at the telomeres. Because bromo domains can bind to
acetylated lysines, we tested the contribution of the 10 yeast bromodomains to
heterochromatin boundary function. Two bromo domain proteins specifically bind
the acetylated H2A.Z amino
terminal tail in vitro and in vivo.
Synthetic lethal interactions with bromo domain mutants are leading to
deeper understanding of the post-translational marks. Curiously, the Rap1 binding site at the silencers is a
unusual variant of consensus found at most other locations, and the odd variant
is conserved among silencer is sensu stricto strains. Substituting the Rap1 consensus sequence in the silencer
creates a conditional silencer, underscoring the importance of the variant
binding site.
Sir1 is a target of
evolutionary change in Saccharomyces with
4 paralogs is some species. We
show that each contribute to silencing in S. bayanus, and have uncovered novel epigenetic phenomena
controlled by both SUM1 and by
mating type.
Modeling Epigenetic
Silencing
Mohammad Sedighi and Anirvan
Sengupta, Rutgers
We formulate a mathematical
version of the conventional model of maintenance of silencing in S.
cerevisiae and analyze the conditions
for bistability as well as for formation of stationary boundaries. Although the
model is perhaps too simple, the structure of the bifurcation diagram,
describing parameter regions with different kinds of qualitative behavior, is
likely to be more robust. We can place some of the known mutants in different
regions of this diagram. One interesting finding of this study is that, under
some conditions, the lowering of acetylation rates might have non-obvious
consequences for silencing. Possible improvements of the model are discussed at
the end.
Phase diagram of DNA with
shorter positive polycations.
Boris Shklovskii,
University of Minnesota
We study a water solution of
long polyanions (PA) such as DNA with shorter polycations (PC) such as
polylysine which is a model system for gene delivery compounds. We concentrate on
the equilibrium phase diagram of this solution in the plane of ratio x of total
charges of PC and PA and concentration of monovalent salt. Each PA adsorbs many
PCs and makes a complex. At x >1 complexes are positive and at x <1 they
are negative. At |x| very close to 1 complexes are weakly charged, their
Coulomb repulsion is weak, and their short range attraction leading to condensation into almost neutral
macroscopic droplets. At large |x-1| the Coulomb repulsion dominates and makes
all complexes stable. At each sign of x, between stable phase and condensed
phase there is domain, where PA disproportionate themselves between different
complexes. Some complexes become almost neutral and condense into liquid
droplets, others become stronger charged and stay stable. We show that with
increasing concentration of a monovalent salt the phase of macroscopic liquid
widens and the domain of disproprtionation vanishes. We discuss application of
this model to chromatine, where the role of PA is played by the net negative
chain of DNA with nucleosomes without H1 and the role of shorter PC is played
by H1. In this case x < 1, because chromatin is overcharged by DNA even with
H1. The stable (large 1-x) phase is identified then with 10 nm fiber and the
smaller 1-x condensed phase is thought of as 30 nm fiber and more condensed
phases of chromatine.
Mechanisms of communication
over a distance
Yury S. Polikanov, Mikhail A. Rubtsov, Vladimir A.
Bondarenko, Alexander Vologodskii, Vasily M. Studitsky
Department of Pharmacology,
UMDNJ, Piscataway, NJ 08854, USA
The majority of eukaryotic genes are
regulated by activator-binding DNA sequences (enhancers, E) that can
efficiently communicate and directly interact with their targets (promoters, P)
over a large distance (1). This interaction is accompanied by formation of
large loops including spacer DNA organized into chromatin structure (1). However it is unclear what mechanisms mediate
formation of large DNA loops with high efficiency and at a high rate.
Previously we have shown that DNA supercoiling greatly facilitates E-P
communication over a large (2.5 kb), but not over a short distance (2,3), most likely facilitating sliding (ÒslitheringÓ) of
intertwined DNA double helices (4). Based on the knowledge of the mechanism, a
rationally designed insulator-like elements that prevents E-P communication was
constructed (4). In more recent studies we have shown that the rate
of E-P communication on supercoiled DNA is not limited by diffusion of the DNA
sites one to another and that DNA supercoiling strongly increases the
probability of E-P juxtaposition. In contrast, the rate of communication on
linear or relaxed DNA most likely is diffusion-limited and is not affected by
the presence of the insulator-like elements. Action of the enhancer action on
DNA assembled in chromatin was also studied. Assembly of linear,
communication-deficient templates in chromatin or introduction of negative DNA
supercoiling results in efficient, quantitatively similar, but mechanistically
distinct enhancer action over a distance. The efficiency of enhancer action
over a large distance in chromatin does not depend on the level of
unconstrained DNA supercoiling.
Thus eukaryotic chromatin structure per se can support highly efficient
communication over a distance and functionally mimic the supercoiled state of
DNA characteristic for prokaryotes. Mechanisms increasing the efficiency of
communication over a distance are likely to be widely utilized in both pro- and
eukaryotic enhancer-dependent systems.
1. Bondarenko, V. A., Liu, Y. V., Jiang, Y.
I., and Studitsky, V. M. (2003) Biochem. Cell. Biol. 81, 241-251.
2. Liu, Y., Bondarenko, V., Ninfa, A., and
Studitsky, V. M. (2001) Proc. Natl. Acad. Sci. USA 98, 14883-14888.
3. Bondarenko, V., Liu, Y., Ninfa, A., and
Studitsky, V. M. (2002) Nucleic Acids Res. 30, 636-642.
4. Bondarenko, V. A., Jiang, Y. I., and
Studitsky, V. M. (2003) EMBO J. 22, 4728-4737.
Nucleosome dynamics and
positioning
Jonathan Widom
Dept. of
Biochemistry, Molecular Biology and Cell Biology, and Dept. of Chemistry,
Northwestern
University, 2153 Sheridan Road, Evanston, IL 60208-3500, USA
j-widom@northwestern.edu
The genomic DNA
of eukaryotes is tightly wrapped into chromosomes through a hierarchical series
of folding steps. In the lowest
level of compaction, short stretches of DNA are wrapped around small octameric
protein spools, forming structures known as nucleosomes. The structure of the nucleosome
occludes most of the wrapped DNA from interaction with the regulatory proteins
and enzymes that must act on it. I will discuss studies using enzymatic and
fluorescence probes that show nucleosomes to spontaneously undergo rapid,
large-scale, conformational fluctuations that facilitate the invasion of
nucleosomes by gene regulatory proteins.
I will then discuss studies on DNA sequence motifs that bias where
nucleosomes are placed along the DNA and control the stabilities of
nucleosomes. I will summarize our
progress identifying such nucleosome positioning DNA sequences, and our current
understanding of how such sequences function to attract and stabilize
nucleosomes. We have used this
information in a novel computational approach to construct and experimentally
validate a nucleosome-DNA interaction model, and to predict the genome-wide
organization of nucleosomes. Our results demonstrate that genomes encode an
intrinsic nucleosome organization, and that this intrinsic organization itself
can explain ~50% of the in vivo nucleosome positions. This nucleosome
positioning code may facilitate specific chromosome functions, including
transcription factor binding, transcription initiation, and even remodeling of
the nucleosomes themselves.
Macromolecular
assembly on looped DNA
Jose Vilar,
Memorial Sloan Kettering
The formation of
DNA loops by proteins and protein complexes is ubiquitous to many fundamental cellular
processes, including transcription, recombination and replication. I will
discuss recent advances in understanding the properties of DNA looping in its
natural context and how they propagate to the cellular behavior through gene
regulation. The results of connecting the molecular properties of DNA looping
with cellular physiology measurements indicate that looping of DNA in vivo is
much more complex and easier than predicted from current models, and reveals a
wealth of previously unappreciated details.