Résumé
We isolated two new cDNAs, BCH3 and BCH4 (barley CRNAhomologues), which are members of the BCH multigene family from barley. BCH genes are thought to encode putative inducible high affinity nitrate transporters. BCH3 was isolated using RACEPCR with specific primers to the BCRNA fragment (Trueman et al., 1996), while BCH4 was isolated using RT-PCR. BCH3 and BCH4 are 1822 and 1705 bp, encode putative polypeptides of 507 amino acids, with a predicted m.w. of 54.6 kDa. Predicted BCH3 and BCH4 proteins are members of a nitrate/nitrite subfamily of major facilitator superfamily. These cDNAs are related to Aspergillus nidulans CRNA, Chlamydomonas rienhardtii NAR3 and NAR4, Hordeum vulgare BCHI and BCH2, and Nicotinia plumbaginifolia NRT2Np. In barley, southern analysis indicates 7–10 copies of BCH genes (Trueman et al., 1996). BCHI, BCH2, BCH3 and BCH4 are highly homologous (greater than 87% identity). In Northern blot analysis, we found that NO3− supply to N-deprived plants increased both the abundance of BCH transcripts and 13NO3− influx. We analyzed the expression pattern of the four BCH genes using oligonucleotides direct to the 3 ’UTR, and found that all four genes are co-ordinately up-regulated in response to NO3− treatment. We assayed the effect of various external concentrations of NO3− and found that 50 µM NO3− grown plants showed the highest BCH transcript abundance and 13NO3− influx. These results provide indirect evidence that the BCH genes encode proteins which are involved in inducible high-affinity transport in plants We analyzed the effect of amino acids feeding on BCH transcript levels, when co-supplied with nitrate, and found that asparagine, aspartate, glutamate and glutamine dramatically decrease transcript levels (>60%). Concurrently, we measured influx using 13NO3− and found a different pattern for the amino acid treatments. Glutamine and asparagine decreased influx by 40 and 20% respectively, while aspartate and glutamate decreased influx by 78 and 65%, respectively. The effect of inhibitors of N assimilation were assayed. Tungstate (an inhibitor of nitrate reductase) increased the level of expression of BCH transcripts, whereas there was a slight decrease for methionine sulfoximine (an inhibitor of glutamine synthetase), compared to control (NO3− treatment for 6h), but NO3− influx decreased by 50%. Azaserine (an inhibitor of glutamate synthase) decreased the levels of BCH transcript by 95% of the control, while NO3− influx decreased by >95%. Amino-oxyacetate (an aspartate amino-transferase and glutamate decarboxylase inhibitor) decreased both the BCH transcripts abundance and NO3− influx by 65%.