  Evolving biomolecular networks have to combine the stability against
perturbations with flexibility allowing their constituents to assume new roles
in the cell. Gene duplication followed by functional divergence of associated
proteins is a major force shaping molecular networks in living organisms.
Recent availability of system-wide data for yeast S. Cerevisiae allow us to
access the effects of gene duplication on robustness and plasticity of
molecular networks. We demonstrate that the upstream transcriptional regulation
of duplicated genes diverges fast, losing on average 4% of their common
transcription factors for every 1% divergence of their amino acid sequences. In
contrast, the set of physical interaction partners of their protein products
changes much slower. The relative stability of downstream functions of
duplicated genes, is further corroborated by their ability to substitute for
each other in gene knockout experiments. We believe that the combination of the
upstream plasticity and the downstream robustness is a general feature
determining the evolvability of molecular networks.
