The 3′ untranslated region of the two cytosolic glutamine synthetase (GS1) genes in alfalfa (Medicago sativa) regulates transcript stability in response to glutamine

Glutamine synthetase (GS) catalyzes the ATP-dependent condensation of ammonia with glutamate to produce glutamine. The GS enzyme is located either in the chloroplast (GS2) or in the cytoplasm (GS1). GS1 is encoded by a small gene family and the members exhibit differential expression pattern mostly attributed to transcriptional regulation. Based on our recent finding that a soybean GS1 gene, Gmglnβ1 is subject to its 3′UTR-mediated post-transcriptional regulation as a transgene in alfalfa (Medicago sativa) we have raised the question of whether the 3′UTR-mediated transcript destabilization is a more universal phenomenon. Gene constructs consisting of the CaMV35S promoter driving the reporter gene, GUS, followed by the 3′UTRs of the two alfalfa GS1 genes, MsGSa and MsGSb, were introduced into alfalfa and tobacco. The analysis of these transformants suggests that while both the 3′UTRs promote transcript turnover, the MsGSb 3′UTR is more effective than the MsGSa 3′UTR. However, both the 3′UTRs along with Gmglnβ1 3′UTR respond to nitrate as a trigger in transcript turnover. More detailed analysis points to glutamine rather than nitrate as the mediator of transcript turnover. Our data suggests that the 3′UTR-mediated regulation of GS1 genes at the level of transcript turnover is probably universal and is used for fine-tuning the expression in keeping with the availability of the substrates.