Triple hydroxylation of tetracenomycin-a2 to tetracenomycin-c in streptomyces-glaucescens - overexpression of the tcmg gene in streptomyces-lividans and characterization of the tetracenomycin-a2 oxygenase
Nucleotide sequence analysis of the tcmG gene has suggested that the TcmG protein is responsible for the triple-hydroxylation of tetracenomycin (Tcm) A2 to Tcm C in Streptomyces glaucescens (Decker, H., Motamedi, H., and Hutchinson, C.R. (1993) J. Bacteriol. 175, 3876-3886). The heterologous expression of the tcmG gene in Streptomyces lividans and the purification and characterization of TcmG protein, which we have named Tcm A2 oxygenase, are described here. NH2-terminal amino acid analysis of the purified enzyme led to the revision of the translational start site of tcmG to a TTG, 33 base pairs downstream of the GTG site assigned initially on the basis of nucleotide sequence analysis. Tcm A2 oxygenase is a monomeric protein in solution and contains 1 mol of non-covalently bound FAD; the apoenzyme can be partially reconstituted in vitro by addition of FAD. Tcm A2 oxygenase exhibits an optimal pH of 9.0-9.5 and prefers NADPH over NADH as an electron donor. The apparent K'm of the enzyme for Tcm A2, NADH, and NADPH are 1.81 +/- 0.38, 260 +/- 19, and 82.1 +/- 17 microM, respectively, and the apparent V'max for the reaction is 14.7 +/- 1.1 nmol Tcm C/min.mg. Purification and characterization of Tcm A2 oxygenase provide direct evidence to support the notion that the angular hydroxy groups of naphthacenequinones like Tcm C are introduced from 18O2 via a mono- or dioxygenase process.