  A dynamic mathematical model has been developed and validated to describe the
synthesis of pectate lyases (Pels), the major virulence factors in Dickeya
dadantii. This work focuses on the simultaneous modeling of the metabolic
degradation of pectin by Pel enzymes and the genetic regulation of pel genes by
2-keto-3-deoxygluconate (KDG), a catabolite product of pectin which inactivates
KdgR, one of the main repressors of pel genes. This modeling scheme takes into
account the fact that the system is composed of two time-varying compartments:
the extracellular medium, where Pel enzymes cleave pectin into oligomers, and
the bacterial cytoplasm where, after internalization, oligomers are converted
to KDG. Using the quasi-stationary state approximations, the model consists of
some nonlinear differential equations for which most of the parameters could be
estimated from the literature or from independent experiments. The few
remaining unknown parameters were obtained by fitting the model equations
against a set of Pel activity data. Model predictions were verified by
measuring the time courses of bacterial growth, Pel production, pel mRNA
accumulation and pectin consumption under various growth conditions. This work
reveals that pectin is almost totally consumed before the burst of Pel
production. This paradoxical behaviour can be interpreted as an evolutionary
strategy to control the diffusion process so that as soon as a small amount of
pectin is detected by the bacteria in its surroundings it anticipates more
pectin to come. The model also predicts the possibility of bistable steady
states in the presence of constant pectin compounds.
