RESUMEN
A gene from the thermophilic Gram-negative bacterium Rhodothermus marinus JCM9785, encoding a dye-linked D-amino acid dehydrogenase homologue, was overexpressed in Escherichia coli, and its product was purified and characterized. The expressed enzyme was a highly thermostable dye-linked D-amino acid dehydrogenase that retained more than 80% of its activity after incubation for 10 min at up to 70 °C. When enzyme-catalyzed dehydrogenation of several D-amino acids was carried out using 2,6-dichloroindophenol as the electron acceptor, D-phenylalanine was the most preferable substrate among the D-amino acids tested. Immediately upstream of the dye-linked D-amino acid dehydrogenase gene (dadh) was a gene encoding a 4-hydroxyproline 2-epimerase homologue (hypE). That gene was successfully expressed in E. coli, and the gene product exhibited strong 4-hydroxyproline 2-epimerase activity. Reverse transcription PCR and quantitative real-time PCR showed that the six genes containing the dadh and hypE genes were arranged in an operon and were required for catabolism of trans-4-hydroxy-L-proline in R. marinus. This is the first description of a dye-linked D-amino acid dehydrogenase (Dye-DADH) with broad substrate specificity involved in trans-4-hydroxy-L-proline catabolism.