Two cDNAs (AtPT1 and AtPT2) encoding plant phosphate transporters have been isolated from a library prepared with mRNA extracted from phosphatestarved Arabidopsis thaliana roots. The encoded polypeptides are 78% identical to each other and show high degree of amino acid sequence similarity with high-affinity phosphate transporters of Saccharomyces cerevisiae, Neurospora crassa, and the mycorrhizal fungus Glomus versiforme. The AtPT1 and AtPT2 polypeptides are integral membrane proteins predicted to contain 12 membrane-spanning domains separated into two groups of six by a large charged hydrophilic region. Upon expression, both AtPT1 and AtPT2 were able to complement thepho84 mutant phenotype of yeast strain NS219 lacking the high-affinity phosphate transport activity. AtPT1 and AtPT2 are representatives of two distinct, small gene families in A. thaliana. The transcripts of both genes are expressed in roots and are not detectable in leaves. The steady-state level of their mRNAs increases in response to phosphate starvation.

Phosphorus is a major nutrient acquired by roots via high-affinity inorganic phosphate (Pi) transporters. In this paper, we describe the tissue-specific regulation of tomato (Lycopersicon esculentum L.) Pi-transporter genes by Pi. The encoded peptides of the LePT1 and LePT2 genes belong to a family of 12 membrane-spanning domain proteins and show a high degree of sequence identity to known high-affinity Pi transporters. Both genes are highly expressed in roots, although there is some expression of LePT1 in leaves. Their expression is markedly induced by Pi starvation but not by starvation of nitrogen, potassium, or iron. The transcripts are primarily localized in root epidermis under Pi starvation. Accumulation of LePT1 message was also observed in palisade parenchyma cells of Pi-starved leaves. Our data suggest that the epidermally localized Pi transporters may play a significant role in acquiring the nutrient under natural conditions. Divided root-system studies support the hypothesis that signal(s) for the Pi-starvation response may arise internally because of the changes in cellular concentration of phosphorus.

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Phosphorus is acquired by plant roots primarily via the high-affinity inorganic phosphate (P i ) transporters. The transcripts for P i transporters are highly inducible upon P i starvation, which also results in enhanced P i uptake when P i is resupplied. Using antibodies specific to one of the tomato P i transporters (encoded by LePT1), we show that an increase in the LePT1 transcript under P i starvation leads to a concurrent increase in the transporter protein, suggesting a transcriptional regulation for P i acquisition. LePT1 protein accumulates rapidly in tomato roots in response to P i starvation. The level of transporter protein accumulation depends on the P i concentration in the medium, and it is reversible upon resupply of P i . LePT1 protein accumulates all along the roots under P i starvation and is localized primarily in the plasma membranes. These results clearly demonstrate that plants increase their capacity for P i uptake during P i starvation by synthesis of additional transporter molecules.Phosphorus availability is considered one of the major factors that limits growth of plants in natural ecosystems. The concentration of available phosphorus is generally in the micromolar range, which is below that of many micronutrients (1). Consequently plants have developed several adaptive mechanisms to overcome P i deficiency. These include changes in root growth and architecture, increased production of phosphatases and RNases, altered activity of several enzymes of the glycolytic pathway (2, 3), and an increased P i uptake rate of roots (4). The ultimate consequences of these modifications are increased P i availability in the rhizosphere and enhanced uptake.Phosphorus is acquired by the plant roots in an energymediated cotransport process driven by a proton gradient generated by plasma membrane H ϩ -ATPases (5). The kinetic characterization of the P i -uptake system by whole plants and cultured cells indicates a high-affinity transport activity operating at low concentrations (micromolar range) and a lowaffinity activity operating at higher concentrations. The very low concentration (micromolar) of P i in soil solution suggests that high-affinity transporters are primarily involved in P i uptake by plants (3). The low-affinity system is apparently expressed constitutively, whereas the high-affinity system is induced under P i deficiency (6). The induction process appears to involve de novo protein synthesis, since inhibitors of protein synthesis drastically reduce the induction of high-affinity P i transport. The increased synthesis of a high-affinity carrier system has been proposed to be responsible for enhanced P i uptake observed under P i -deficiency conditions (6).High-affinity P i transporter genes have been cloned and characterized from fungi and from several plant species, including Arabidopsis, tomato, potato, Medicago, and Catharanthus (7). All the cloned P i transporters are integral membrane proteins containing 12 membrane-spanning regions, separated into two groups of 6 by a lar...

Bispecific antibodies based on full-length antibody structures are more optimal than fragment-based formats because they benefit from the favorable properties of the Fc region. However, the homodimeric nature of Fc effectively imposes bivalent binding on all current full-length bispecific antibodies, an attribute that can result in nonspecific activation of cross-linked receptors. We engineered a novel bispecific format, referred to as mAb-Fv, that utilizes a heterodimeric Fc region to enable monovalent co-engagement of a second target antigen in a full-length context. mAb-Fv constructs co-targeting CD16 and CD3 were expressed and purified as heterodimeric species, bound selectively to their co-target antigens, and mediated potent cytotoxic activity by NK cells and T cells, respectively. The capacity to co-engage distinct target antigens simultaneously with different valencies is an improved feature for bispecific antibodies with promising therapeutic implications.