  In yeast (Saccharomyces cerevisiae) and plant roots (Arabidopsis thaliana)
zinc enters the cells via influx transporters of the ZIP family. Since zinc is
both essential for cell function and toxic at high concentrations, tight
regulation is essential for cell viability. We provide new insight into the
underlying mechanisms, starting from a general model based on ordinary
differential equations and adapting it to the specific cases of yeast and plant
root cells. In yeast, zinc is transported by the transporters ZRT1 and ZRT2,
which are both regulated by the zinc-responsive transcription factor ZAP1.
Using biological data, parameters were estimated and analyzed, confirming the
different affinities of ZRT1 and ZRT2 reported in the literature. Furthermore,
our model suggests that the positive feedback in ZAP1 production has a
stabilizing function at high influx rates. In plant roots, various ZIP
transporters are involved in zinc uptake. Their regulation is largely unknown,
but bZIP transcription factors are thought to be involved. We set up three
putative models: activator only, activator with dimerization and
activator/inhibitor. These were fitted to measurements and analyzed.
Simulations show that the activator/inhibitor model outperforms the other two
in providing robust and stable homeostasis at reasonable parameter ranges.
