The ribosomal density along different parts of the coding regions of the
mRNA molecule affect various fundamental intracellular phenomena including:
protein production rates, global ribosome allocation and organismal fitness,
ribosomal drop off, co-translational protein folding, mRNA degradation, and
more. Thus, regulating translation in order to obtain a desired
ribosomal profile along the mRNA molecule is an important biological
problem.
We study this problem using a dynamical model for mRNA translation,
called the ribosome flow model (RFM). In the RFM, the mRNA molecule is modeled
as chain of n sites. The n state-variables
describe the ribosomal density profile along the mRNA molecule, whereas the
transition rates from each site to the next are controlled by n
+ 1 positive constants. To study the problem of controlling the density
profile, we consider some or all of the transition rates as time-varying
controls.
We consider the following problem: given an initial and a desired
ribosomal density profile in the RFM, determine the time-varying values of the
transition rates that steer the system to this density profile, if they exist.
More specifically, we consider two control problems. In the first, all
transition rates can be regulated and the goal is to steer the ribosomal density
profile and the protein production rate from a given initial value to a desired
value. In the second, a single transition rate is controlled
and the goal is to steer the production rate to a desired value. In the first
case, we show that the system is controllable, i.e. the control is powerful
enough to steer the system to any desired value, and we provide simple
closed-form expressions for constant control functions (or
transition rates) that asymptotically steer the system to the desired value. In
the second case, we show that we can steer the production rate to any desired
value in a feasible region determined by the other constant transition rates. We
discuss some of the biological implications of these results.