Flavin-dependent monooxygenases catalyze the incorporation of a single atom of molecular oxygen into organic substrates (1, 2). The enzymes have been classified into six classes according to their catalytic and structural properties. They have also been categorized into two major types according to their protein components: a single-component type, in which reduction of a flavin cofactor and oxygenation of an organic substrate occurs within the same single polypeptide chain; and a two-component type, in which each reaction occurs on separate proteins (1, 2). Single-component monooxygenases have been identified since the 1960s and have been found to be involved in the aerobic metabolism of aromatic and aliphatic compounds in various organisms (2-4). The well known prototype for single-component monooxygenases is p-hydroxybenzoate hydroxylase from Pseudomonas fluorescens (5, 6). The first enzyme identified as a two-component monooxygenase was bacterial luciferase (7). Nevertheless, most of the two-component monooxygenases have been identified only during the past decade and have increasingly emerged as common enzymes in nature that are involved in many important reactions in various microorganisms (1,8). Reactions catalyzed by two-component monooxygenases include oxygenation and halogenation of organic compounds such as p-hydroxyphenylacetate (9), phenol (10), trichorophenol (11, 12), p-nitrophenol (13), styrene (14 -16), alkane sulfonate (17, 18), and EDTA (19). Two-component monooxygenases are also involved in oxygenation and halogenation reactions in the biosynthetic pathways of actinorhodin (ActVA) (20), angucyclin (21), enediyne (SgcC) (22), rebeccamycin (RebH) (23), pyrrolnitrin (PrnA) (24), violacein (25), kutzneride (26), and differentiation-inducing factor-1 (27).All flavin-dependent monooxygenases perform oxygenation through the participation of a reactive intermediate, C4a-hydroperoxy-flavin, which has been well documented and detected by transient kinetics for the reactions of singlecomponent monooxygenases. These monooxygenases include p-hydroxybenzoate hydroxylase (PHBH) 3 (5, 6), phenol hydroxylase (28), melilotate hydroxylase (29), antranilate hydroxylase (30), Baeyer-Villiger monooxygenases (31, 32), 2-methyl-3-hydroxypyridine-5-carboxylic oxygenase * This work was supported by Grants BRG5180002 (to P. C.) and MRG5380240 (to J. S.) from the Thailand Research Fund and a grant from the Faculty of Science, Mahidol University (to P. C.