An aldose reductase homologous gene from barley: regulation and function.

The expression of a barley gene homologous to aldose reductase and aldehyde reductase is restricted to the embryo and temporally correlated with its acquisition of desiccation tolerance. In the work presented, two aspects of this barley gene were investigated: its transcriptional regulation and the initial characterization of the enzymatic function. The transcriptional regulation of the gene was studied in transgenic tobacco by analysing the expression of chimeric genes containing 5' sequences of the barley gene transcriptionally fused to the GUS reporter gene. This functional analysis of the promoter revealed that a 1364 bp 5' fragment confers the appropriate pattern of expression to the reporter gene in tobacco and that a short promoter fragment (-114 to +75) containing the sequence TACGTGGC, homologous to plant G-box elements, is sufficient for developmental expression during embryogenesis. To investigate the enzymatic properties of the gene product the wild-type protein and a mutant carrying a lysine 259 to methionine substitution were overexpressed in a procaryotic system and purified to homogeneity. The wild-type protein exhibits aldose reductase activity in the reduction of DL-glyceraldehyde and D-erythrose specifically using NADPH as co-factor whereas the mutant shows markedly reduced activity. However, the barley protein possesses some properties different to those of animal aldose and aldehyde reductases and its biological target still needs to be identified.