The Yeast cell cycle network and its neutral mutants

The functioning of a living cell is largely determined by the
structure of its regulatory network, comprising complex, typically
not pairwise interactions between regulatory genes. An important
factor for the stability and evolvability of such regulatory
systems is neutrality --- typically a large number of alternative
structures exhibit the same dynamical behaviour.

Here we study the regulatory network for the cell cycle of budding
yeast (Saccharomyces cerevisiae). A coarse-grained Boolean
approach (Li et al, PNAS, 2004) allows us to abstract from
biochemical details such as precise binding constants and
degradation rates. We use a sampling algorithm of Wang-Landau type
to estimate the number of different yeast networks fulfilling the
task of cell division. We also study the reachability of the
alternative networks from the wild type. This neutral space of
regulatory networks is fragmented under point mutations which
establish or delete single interactions.