where the weighting factor j is dependent on the
relative consumption rates of the two sugars. The
weighting factors for glucose and xylose uptakes are
specified as j1 and j2, respectively. An important assumption
of the model is that the sum of the weighting
factors for glucose and xylose uptake is unity. This
is based on the assumption that both glucose and xylose
compete for uptake via a common and unchanged
sugar transport system in Z. mobilis. This system has
been reported previously as the glucose facilitated
transport system (mediated by the Glf gene) (DiMarco
& Romano 1985, Parker et al. 1995, Weisser et al.
1995, 1996). Other authors have reported simultaneous
glucose and xylose uptake by recombinant Z.
mobilis (Zhang et al. 1995, Krishnan et al. 2000, Lawford
& Rousseau 1999, 2000, Lawford et al. 2000). It
is evident from our earlier kinetic data also that both
glucose and xylose can be taken up simultaneously,
with xylose at a considerably slower rate.
As a result of this assumption, we can write: