Instructions
This Java applet (If
you don't see anything above, please install Java 1.4) models
interference
between two promoters, through RNA polymerase (RNAP) traffic.
Each polymerase occupies 70bp when sitting on the promoter, and 35 bp
when elongating. The interference imply that each of the promoters will
be
less active due to effect from the opposing promoter. This reduced
activity
is due to a number of effects associated to collisions between moving
RNAP's, occlusion of one promoter by the passage of an RNAP from the
opposing promoter,
as well as ejecting of RNAP on one promoter due to collision with a
moving
RNAP. Finally the applet also opens for simulation of road block
effect,
where a sitting duck also eject a elongating RNAP (with probability set
by
roadblock parameter).
The interference (Ifactor) measure activity of promoter without
opposing promoter, divided with activity in current setting.
The approximate Ifactor is based on an analytic estimate, which will
be
correct when opposing promoter is much stronger, or if both promoters
are
weak. It basically counts reduction of activity of given promoter,
assuming that
the opposing promoter is not perturbed.
Realistic strength of
promoters covers a wide interval. For the very
strong
promoter PL in lambda phage 1/K=4.5sec correspond to one firing per 4.5
second. Other typical numbers are 1/K=18sec for PR in phage 186,
interfering
with
the lysogen maintenance promoter promoter PL in 186 which have
1/K=180sec. The interference increases when as a promoter is
exposed to a
stronger
promoter (i.e. K(opposing)/K(observed) increases ). The aspect ratio
quantifies
the ratio between the on rate (kon) and the firing rate (kf) of an
undisturbed
promoter. For aspect ratio>>1, the on rate is high, and the
promoter
loads easily, but stays occupied for a long time. For aspect ratio
alpha<<1,
the promoter loads rarely, but fire instantly when it has loaded. PL in
186
have aspect ratio close to 1. pC from phage P2 on the contrary have
aspect
ratio of about 0.1. One will see that an aspect ratio of about 1
maximizes
the interference (try the applet).
The overall system
parameter also include distance N between
promoters, that could be varied fro 120 to 3000, with N<40
corresponding to overlapping or partially overlapping promoters. The
velocity is the assumed
velocity of elongating RNAP that experimentally (see paper
submitted
to JMB) is fitted to be between 40 and 50 bp/sec.
The applet also opens
for toying with the system, as clicking below or
above the DNA line in middle of screen, artificially initiate a
RNAP of corresponding
type on the DNA. Also one may view interference between distant
promoters by adjusting the distance button. For strong promoters with
similar strength
one then observe a system with possibility for very strong fluctuations.
More applets

Transcriptional Interference Model
K.Sneppen, I.B.
Dodd, K.E. Shearwin, A. Palmer,
R.A. Shubert, B. Callen
and J.B. Egan.
(A mathematical model
for transcriptional interference by RNA
polymerase
traffic in Escherichia coli)
The
model deals with the interference
between
the two promoters pA and pS as shown in the applet. The RNAPs are
injected
onto the DNA through binding and formation of sitting duck complexes at
the
respective promoters, followed by subsequent formation of elongating
complexes.
The traffic is simulated by a stochastic model that is adjusted to take
into
account all known details of the dynamics. The stochastic dynamics are
implemented
by updating at any time step [t,t+dt]= [t,t+1bp/v] the presence of any
RNAP
according to the basic processes (v is the velocity). That is, in time
step dt, a promoter forms an open complex with probability
kon dt unless it is occupied or occluded by other RNAPs. In
case
the promoter is occupied, it initiates elongation with a probability
k_f
dt except when an RNAP from the opposing promoter is positioned
such
that it will collide with the sitting duck in the time step dt. kon and
kf together defines the strength of the promoter, that is firing (going
from "sitting duck complex" to elongation) with a total
rate K which approximately is equal to kon *kf/(kon+kf).
Any elongating RNAP is
moved vdt step in the direction of elongation,
except
when it collides with an RNAP moving in opposite direction. When such a
collision
occurs the RNAP's are removed from the system. The transcription
activity
of any of the promoters is counted by the number of RNAPs which pass
the
opposing promoter. The interference is found by comparing this number
to
the number obtained when the opposing promoter is assumed to be silent.
The model, together
with carefull analysis and comparison to experiment
is
submitted to JMB.
References,
experimental:
Measurements of up to
30 fold interference between tandem promoters:
S. Adhya and M.
Gottesman (1982), Cell 29, 939944.
Measurements 210 fold
interference for various
convergent promoter
combinations:
B. P. Callen, I. Dodd,
K.E. Shearwin and J.B. Egan (2004), Mol. Cell
14, 647656.
