INTERNAL GLN GLU RATIO AS A POTENTIAL REGULATORY PARAMETER FOR THE EXPRESSION OF A CYTOSOLIC GLUTAMINE-SYNTHETASE GENE OF RADISH IN CULTURED-CELLS/

The radish Gln1;1 gene encodes cytosolic glutamine synthetase, and is strongly expressed during senescence of cotyledons [Watanabe et al. (1 994) Plant Mol. Biol. 26: 1807]. We showed previously that nitrogenous compounds exogenously applied to the cotyledons significantly affecte d the transcript level of the gene during senescence, suggesting that the gene is possibly under metabolic control by cellular nitrogen stat us. To investigate such metabolic regulation of the gene, we tested th e response of the gene to the changes in cellular nitrogen status, usi ng radish cultured cells. Northern blotting and measurement of free am ino acids in the cells showed that the radish cells accumulating gluta mine at high levels exhibited contrasting low levels of Gln1;1 transcr ipts. Further and detailed study provided evidences suggesting that th e transcript level was regulated in response to the ratio of cellular glutamine to glutamate, but not to the glutamine level alone.