Betaine Aldehyde Dehydrogenase

In some higher plants, glycinebetaine is synthesized and accumulated in cells in response to salt stress. Glycinebetaine is a compatible solute and acts as an excellent osmoprotectant. Glycinebetaine of high levels is present in leaves of diverse families of dicotyledons19

and of some monocotyledons.20 The pathway of glycinebetaine biosynthesis in higher plants is as follows: Choline^betaine aldehyde^ glycinebetaine. The enzyme which catalyzes the first step is choline monooxygenase (CMO), recently purified and partially characterized.21 The final step is catalyzed by betaine aldehyde dehydrogenase (BADH), which has been well characterized.22-24 Existence of a BADH isozyme was suggested in spinach and sorghum,25,26 and we reported that all monocotyledonous BADHs have a C-terminal tripeptide SKL that is known to be a signal for targeting preproteins to microbodies.27

We cloned two types of BADH cDNA (BBADH1 and BBADH2) and determined their nucleotide sequence. On the basis of an analysis of nucleotide sequence of cDNAs which were cloned into a plasmid, the insert was judged to encode BADH. BBADH1 had the signal sequence (SKL) targeting to micro-bodies. However, BBADH2 did not have a SKL signal. Predicted amino acid sequence identity between BBADH1 and BBADH2 was 70%. The BBADH2 gene was more similar to the BADH gene of dicotyledons (spinach, sugar beet) than those of monocotyledons (barley, sorghum, rice) previously reported (Table 21.2) A small amount of the 1.9 kb BBADH1 mRNA was expressed under normal growth conditions. Under salinity conditions, BBADH1 gene is expressed, still at a low level. On the other hand, BBADH2 gene expressed at a very low level under non-stress conditions. However, the quantitative analysis revealed that the level of BBADH2 mRNA

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