REGULATION OF THE EXPRESSION OF FERREDOXIN GLUTAMATE SYNTHASE IN BARLEY

We have investigated the regulation of ferredoxin-glutamate synthase ( Fd-GOGAT) in leaves of barley (Hordeum vulgar L. cv. Maris Mink) at th e mRNA, protein and enzyme activity levels. Studies of the changes in Fd-GOGAT during plant development showed that the activity in shoots i ncreases rapidly after germination to reach a maximum (on a fresh-weig ht basis) at day 10 and then declines markedly to less than 50% of the maximal activity by day 30, this decline being correlated with an equ ivalent loss of Fd-GOGAT protein. Growing the plants in darkness reduc ed the maximum activity attained in the shoots, but did not affect the overall pattern of the changes or their timing. The activity of Fd-GO GAT increased two-to three-fold within 48 when etiolated leaves were e xposed to light, and Northern blots indicated that the induction occur red at the mRNA level. However, whilst a carbon source could at least partially substitute for light in the induction of nitrate reductase a ctivity, no induction of Fd-GOGAT activity was seen when etiolated lea ves were treated with either sucrose or glucose. Interestingly, the le vels of Fd-GOGAT mRNA and activity remained high up to a period of 16 h or 72 h darkness, respectively. Compared with plants grown in N-free medium, light-grown plants supplied with nitrate had almost two-fold higher Fd-GOGAT activities and increased Fd-GOGAT mRNA levels, but nit rate had no effect on the abundance of the enzyme or its mRNA in etiol ated plants, indicating that light is required for nitrate induction o f barley Fd-GOGAT.