  Dynamical properties of the transcriptional regulatory network of {\it
Escherichia coli} and {\it Saccharomyces cerevisiae} are studied within the
framework of random Boolean functions. The dynamical response of these networks
to a single point mutation is characterized by the number of mutated elements
as a function of time and the distribution of the relaxation time to a new
stationary state, which turn out to be different in both networks. Comparison
with the behavior of randomized networks reveals relevant structural
characteristics other than the mean connectivity, namely the organization of
circuits and the functional form of the in-degree distribution. The abundance
of single-element circuits in {\it E. coli} and the power-law in-degree
distribution of {\it S. cerevisiae} shift their dynamics towards marginal
stability overcoming the restrictions imposed by their mean connectivities,
which is argued to be related to the simultaneous presence of robustness and
adaptivity in living organisms.
