Public health impact of long-term, low-level mixed element exposure in susceptible population strata

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At least one third of the world's population is thought to suffer from zinc deficiency. Dietary zinc deficiency is particularly prevalent in populations that rely extensively on cereals for calorific intake since the endosperm of cereal grains is poor in zinc.
We have identified and characterised a barley zinc transporter (HvMTP1) that is located at the vacuolar membrane and which is responsible for vacuolar zinc accumulation. The metal specificity determinants of HvMTP1 have been identified (a) by constructing chimeras between HvMTP1 (which transports both Zn and Co) and Arabidopsis MTP1 (which is selective for Zn); (b) through random mutagenesis followed by a yeast-based screen on different transition metals; (c) through site-directed mutagenesis. A five-residue domain in a cytosolically-located histidine-rich loop is critical in facilitating discrimination between Zn and Co, while residues in the third transmembrane domain are involved in encoding specificity for Zn over a number of other transition metals.
HvMTP1 is poorly expressed in endosperm, although in other regions of the seed (aleurone, endosperm) HvMTP1 transcript is abundant. This expression pattern correlates with the regions of cereal grains in which Zn is known to be accumulated. By expressing HvMTP1 on a D-hordein promoter - which is endosperm-specific - we are examining the hypothesis that the transporter plays a pivotal role in Zn accumulation in vivo. If this is indeed the case, modulation of MTP1 expression might provide a means to biofortify cereal grains with Zn.

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