Two different amplification products, termed c1 and c2, showing a high similarity to glutamate dehydrogenase sequences from plants, were obtained from Asparagus officinalis using two degenerated primers and RT–PCR (reverse transcriptase polymerase chain reaction). The genes corresponding to these cDNA clones were designated aspGDHA and aspGDHB. Screening of a cDNA library resulted in the isolation of cDNA clones for aspGDHB only. Analysis of the deduced amino acid (aa) sequence from the full-length cDNA suggests that the gene product contains all regions associated with metabolic function of NAD glutamate dehydrogenase (NAD–GDH). A first phylogenetic analysis including only GDHs from plants suggested that the two GDH genes of A. officinalis arose by an ancient duplication event, pre-dating the divergence of monocots and dicots. Codon usage analysis showed a bias towards A/T ending codons. This tendency is likely due to the biased nucleotide composition of the asparagus genome, rather than to the translational selection for specific codons. Using principal coordinate analysis, the evolutionary relatedness of plant GDHs with homologous sequences from a large spectrum of organisms was investigated. The results showed a closer affinity of plant GDHs to GDHs of thermophilic archaebacterial and eubacterial species, when compared to those of unicellular eukaryotic fungi. Sequence analysis at specific amino acid signatures, known to affect the thermal stability of GDH, and assays of enzyme activity at non-physiological temperatures, showed a greater adaptation to heat stress conditions for the asparagus and tobacco enzymes compared with the Saccharomyces cerevisiae enzyme.
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