The Genetic Control of Membrane Transport

Overexpression of GmAAP6a enhances tolerance to low nitrogen and improves seed nitrogen status by optimizing amino acid partitioning in soybean

Amino acid transport via phloem is one of the major source-to-sink nitrogen translocation pathways in most plant species. Amino acid permeases (AAPs) play essential roles in amino acid transport between plant cells and subsequent phloem or seed loading. In this study, a soybean AAP gene, annotated as GmAAP6a, was cloned and demonstrated to be significantly induced by nitrogen starvation. Histochemical staining of GmAAP6a:GmAAP6a-GUS transgenic soybean revealed that GmAAP6a is predominantly expressed in phloem and xylem parenchyma cells. Growth and transport studies using toxic amino acid analogs or single amino acids as a sole nitrogen source suggest that GmAAP6a can selectively absorb and transport neutral and acidic amino acids. Overexpression of GmAAP6a in Arabidopsis and soybean resulted in elevated tolerance to nitrogen limitation. Furthermore, the source-to-sink transfer of amino acids in the transgenic soybean was markedly improved under low nitrogen conditions. At the vegetative stage, GmAAP6a-overexpressing soybean showed significantly increased nitrogen export from source cotyledons and simultaneously enhanced nitrogen import into sink primary leaves. At the reproductive stage, nitrogen import into seeds was greatly enhanced under both sufficient and limited nitrogen conditions. Collectively, our results imply that overexpression of GmAAP6a enhances nitrogen stress tolerance and source-to-sink transport and improves seed quality in soybean. Co-expression of GmAAP6a with genes specialized in source nitrogen recycling and seed loading may represent an interesting application potential in breeding.

Sulfate uptake in Saccharomyces cerevisiae: biochemical and genetic study

Sulfate uptake is the first step of the sulfate assimilation pathway, which has been shown in our laboratory to be part of the methionine biosynthetic pathway. Kinetic study of sulfate uptake has shown a biphasic curve in a Lineweaver-Burk plot. The analysis of this plot indicates that two enzymes participate in sulfate uptake. One (permease I) has a high affinity for the substrate (K(m) = 0.005 mM); the other (permease II) shows a much lower affinity for sulfate (K(m) = 0.35 mM). Regulation of the synthesis of both permeases is under the control of exogenous methionine or S-adenosylmethionine. It was shown, moreover, that synthesis of sulfate permeases is coordinated with the synthesis of the other methionine biosynthetic enzymes thus far studied in our laboratory. An additional specific regulation of sulfate permeases by inhibition of their activity by endogenous sulfate and adenosyl phosphosulfate (an intermediate metabolite in sulfate assimilation) has been shown. A mutant unable to concentrate sulfate has been selected. This strain carried mutations in two independent genes. These two mutations, separated in two different strains, lead to modified kinetics of sulfate uptake. The study of these strains leads us to postulate that there is an interaction in situ between the products of these two genes.

Regulation of methionine transport activity in Escherichia coli

Methionine transport activity in cells of Escherichia coli K-12 is regulated by the level of the internal methionine pool. Transport activity is depressed either in cells grown in the presence of methionine or in cells exposed to methionine immediately prior to harvest. Alpha-Keto-gamma-methiol-butyrate, D-methionine, or methionine sulfoxide have little effect on the initial rate of uptake of L-methionine when they are added simultaneously with the substrate. However, methionine transport is markedly reduced in cells exposed to these sources of L-methionine before the addition of substrate. This reduction is prevented if the cells are treated with amino oxyacetic acid. The initial rate of uptake into L-methionine-loaded cells was lower than that into unloaded cells. This inhibition affected both methionine transport systems and the inhibition by the internal pool appeared to be non-competitive with the external methionine concentration. Two classes of mutants with increased methionine pools have decreased rates of uptake. Conversely, starvation for methionine in a methionine auxotroph with high rates of methionine degradation resulted in a substantial increase in the rate of methionine transport. Thus, these transport systems are subject to regulation by the internal pool size and possibly by repression.