Primary structure, sequence analysis, and expression of the thermostable D-hydantoinase from Bacillus stearothermophilus SD1

The gene coding for the thermostable D-hydantoinase from the thermophilic bacterium Bacillus stearothermophilus SD1 was cloned and its nucleotide sequence was completely determined. The D-hydantoinase protein showed considerable amino acid sequence homology (20-28%) with other hydantoinases and functionally related allantoinases and dihydroorotases. Strikingly the sequence of the enzyme from B. stearothermophilius SD1 exhibited greater than 89% identity with hydantoinases from thermophilic bacteria, Despite the extremely high amino acid homology among the hydantoinases from thermophiles. the C-terminal regions of the enzymes were completely different in both sequence and predicted secondary structure, implying that the C-terminal region plays an important role in determining the biochemical properties of the enzymes. Alignment of the sequence of the D-hydantoinase from B. stearothermophilus SD1 with those of other functionally related enzymes revealed four conserved regions, and five histidines and an acidic residue were found to be conserved, suggesting a close evolutionary relationship between all these enzymes.